Implement rseq(2)

PiperOrigin-RevId: 288342928

15 files changed, 598 insertions, 79 deletions

diff --git a/pkg/abi/linux/BUILD b/pkg/abi/linux/BUILD
index 9553f164d..716ff22d2 100644
--- a/pkg/abi/linux/BUILD
+++ b/pkg/abi/linux/BUILD
@@ -41,6 +41,7 @@ go_library(
         "poll.go",
         "prctl.go",
         "ptrace.go",
+        "rseq.go",
         "rusage.go",
         "sched.go",
         "seccomp.go",
diff --git a/pkg/abi/linux/rseq.go b/pkg/abi/linux/rseq.go
new file mode 100644
index 000000000..76253ba30
--- /dev/null
+++ b/pkg/abi/linux/rseq.go
@@ -0,0 +1,130 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package linux
+
+// Flags passed to rseq(2).
+//
+// Defined in include/uapi/linux/rseq.h.
+const (
+	// RSEQ_FLAG_UNREGISTER unregisters the current thread.
+	RSEQ_FLAG_UNREGISTER = 1 << 0
+)
+
+// Critical section flags used in RSeqCriticalSection.Flags and RSeq.Flags.
+//
+// Defined in include/uapi/linux/rseq.h.
+const (
+	// RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT inhibits restart on preemption.
+	RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT = 1 << 0
+
+	// RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL inhibits restart on signal
+	// delivery.
+	RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL = 1 << 1
+
+	// RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE inhibits restart on CPU
+	// migration.
+	RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE = 1 << 2
+)
+
+// RSeqCriticalSection describes a restartable sequences critical section. It
+// is equivalent to struct rseq_cs, defined in include/uapi/linux/rseq.h.
+//
+// In userspace, this structure is always aligned to 32 bytes.
+//
+// +marshal
+type RSeqCriticalSection struct {
+	// Version is the version of this structure. Version 0 is defined here.
+	Version uint32
+
+	// Flags are the critical section flags, defined above.
+	Flags uint32
+
+	// Start is the start address of the critical section.
+	Start uint64
+
+	// PostCommitOffset is the offset from Start of the first instruction
+	// outside of the critical section.
+	PostCommitOffset uint64
+
+	// Abort is the abort address. It must be outside the critical section,
+	// and the 4 bytes prior must match the abort signature.
+	Abort uint64
+}
+
+const (
+	// SizeOfRSeqCriticalSection is the size of RSeqCriticalSection.
+	SizeOfRSeqCriticalSection = 32
+
+	// SizeOfRSeqSignature is the size of the signature immediately
+	// preceding RSeqCriticalSection.Abort.
+	SizeOfRSeqSignature = 4
+)
+
+// Special values for RSeq.CPUID, defined in include/uapi/linux/rseq.h.
+const (
+	// RSEQ_CPU_ID_UNINITIALIZED indicates that this thread has not
+	// performed rseq initialization.
+	RSEQ_CPU_ID_UNINITIALIZED = ^uint32(0) // -1
+
+	// RSEQ_CPU_ID_REGISTRATION_FAILED indicates that rseq initialization
+	// failed.
+	RSEQ_CPU_ID_REGISTRATION_FAILED = ^uint32(1) // -2
+)
+
+// RSeq is the thread-local restartable sequences config/status. It
+// is equivalent to struct rseq, defined in include/uapi/linux/rseq.h.
+//
+// In userspace, this structure is always aligned to 32 bytes.
+type RSeq struct {
+	// CPUIDStart contains the current CPU ID if rseq is initialized.
+	//
+	// This field should only be read by the thread which registered this
+	// structure, and must be read atomically.
+	CPUIDStart uint32
+
+	// CPUID contains the current CPU ID or one of the CPU ID special
+	// values defined above.
+	//
+	// This field should only be read by the thread which registered this
+	// structure, and must be read atomically.
+	CPUID uint32
+
+	// RSeqCriticalSection is a pointer to the current RSeqCriticalSection
+	// block, or NULL. It is reset to NULL by the kernel on restart or
+	// non-restarting preempt/signal.
+	//
+	// This field should only be written by the thread which registered
+	// this structure, and must be written atomically.
+	RSeqCriticalSection uint64
+
+	// Flags are the critical section flags that apply to all critical
+	// sections on this thread, defined above.
+	Flags uint32
+}
+
+const (
+	// SizeOfRSeq is the size of RSeq.
+	//
+	// Note that RSeq is naively 24 bytes. However, it has 32-byte
+	// alignment, which in C increases sizeof to 32. That is the size that
+	// the Linux kernel uses.
+	SizeOfRSeq = 32
+
+	// AlignOfRSeq is the standard alignment of RSeq.
+	AlignOfRSeq = 32
+
+	// OffsetOfRSeqCriticalSection is the offset of RSeqCriticalSection in RSeq.
+	OffsetOfRSeqCriticalSection = 8
+)
diff --git a/pkg/sentry/arch/arch.go b/pkg/sentry/arch/arch.go
index 498ca4669..81ec98a77 100644
--- a/pkg/sentry/arch/arch.go
+++ b/pkg/sentry/arch/arch.go
@@ -125,9 +125,9 @@ type Context interface {
 	// SetTLS sets the current TLS pointer. Returns false if value is invalid.
 	SetTLS(value uintptr) bool
 
-	// SetRSEQInterruptedIP sets the register that contains the old IP when a
-	// restartable sequence is interrupted.
-	SetRSEQInterruptedIP(value uintptr)
+	// SetOldRSeqInterruptedIP sets the register that contains the old IP
+	// when an "old rseq" restartable sequence is interrupted.
+	SetOldRSeqInterruptedIP(value uintptr)
 
 	// StateData returns a pointer to underlying architecture state.
 	StateData() *State
diff --git a/pkg/sentry/arch/arch_amd64.go b/pkg/sentry/arch/arch_amd64.go
index 67daa6c24..2aa08b1a9 100644
--- a/pkg/sentry/arch/arch_amd64.go
+++ b/pkg/sentry/arch/arch_amd64.go
@@ -174,8 +174,8 @@ func (c *context64) SetTLS(value uintptr) bool {
 	return true
 }
 
-// SetRSEQInterruptedIP implements Context.SetRSEQInterruptedIP.
-func (c *context64) SetRSEQInterruptedIP(value uintptr) {
+// SetOldRSeqInterruptedIP implements Context.SetOldRSeqInterruptedIP.
+func (c *context64) SetOldRSeqInterruptedIP(value uintptr) {
 	c.Regs.R10 = uint64(value)
 }
 
diff --git a/pkg/sentry/kernel/rseq.go b/pkg/sentry/kernel/rseq.go
index 24ea002ba..b14429854 100644
--- a/pkg/sentry/kernel/rseq.go
+++ b/pkg/sentry/kernel/rseq.go
@@ -15,17 +15,29 @@
 package kernel
 
 import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/hostcpu"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
-// Restartable sequences, as described in https://lwn.net/Articles/650333/.
+// Restartable sequences.
+//
+// We support two different APIs for restartable sequences.
+//
+//  1. The upstream interface added in v4.18.
+//  2. The interface described in https://lwn.net/Articles/650333/.
+//
+// Throughout this file and other parts of the kernel, the latter is referred
+// to as "old rseq". This interface was never merged upstream, but is supported
+// for a limited set of applications that use it regardless.
 
-// RSEQCriticalRegion describes a restartable sequence critical region.
+// OldRSeqCriticalRegion describes an old rseq critical region.
 //
 // +stateify savable
-type RSEQCriticalRegion struct {
+type OldRSeqCriticalRegion struct {
 	// When a task in this thread group has its CPU preempted (as defined by
 	// platform.ErrContextCPUPreempted) or has a signal delivered to an
 	// application handler while its instruction pointer is in CriticalSection,
@@ -35,86 +47,359 @@ type RSEQCriticalRegion struct {
 	Restart         usermem.Addr
 }
 
-// RSEQAvailable returns true if t supports restartable sequences.
-func (t *Task) RSEQAvailable() bool {
+// RSeqAvailable returns true if t supports (old and new) restartable sequences.
+func (t *Task) RSeqAvailable() bool {
 	return t.k.useHostCores && t.k.Platform.DetectsCPUPreemption()
 }
 
-// RSEQCriticalRegion returns a copy of t's thread group's current restartable
-// sequence.
-func (t *Task) RSEQCriticalRegion() RSEQCriticalRegion {
-	return *t.tg.rscr.Load().(*RSEQCriticalRegion)
+// SetRSeq registers addr as this thread's rseq structure.
+//
+// Preconditions: The caller must be running on the task goroutine.
+func (t *Task) SetRSeq(addr usermem.Addr, length, signature uint32) error {
+	if t.rseqAddr != 0 {
+		if t.rseqAddr != addr {
+			return syserror.EINVAL
+		}
+		if t.rseqSignature != signature {
+			return syserror.EINVAL
+		}
+		return syserror.EBUSY
+	}
+
+	// rseq must be aligned and correctly sized.
+	if addr&(linux.AlignOfRSeq-1) != 0 {
+		return syserror.EINVAL
+	}
+	if length != linux.SizeOfRSeq {
+		return syserror.EINVAL
+	}
+	if _, ok := t.MemoryManager().CheckIORange(addr, linux.SizeOfRSeq); !ok {
+		return syserror.EFAULT
+	}
+
+	t.rseqAddr = addr
+	t.rseqSignature = signature
+
+	// Initialize the CPUID.
+	//
+	// Linux implicitly does this on return from userspace, where failure
+	// would cause SIGSEGV.
+	if err := t.rseqUpdateCPU(); err != nil {
+		t.rseqAddr = 0
+		t.rseqSignature = 0
+
+		t.Debugf("Failed to copy CPU to %#x for rseq: %v", t.rseqAddr, err)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return syserror.EFAULT
+	}
+
+	return nil
 }
 
-// SetRSEQCriticalRegion replaces t's thread group's restartable sequence.
+// ClearRSeq unregisters addr as this thread's rseq structure.
 //
-// Preconditions: t.RSEQAvailable() == true.
-func (t *Task) SetRSEQCriticalRegion(rscr RSEQCriticalRegion) error {
+// Preconditions: The caller must be running on the task goroutine.
+func (t *Task) ClearRSeq(addr usermem.Addr, length, signature uint32) error {
+	if t.rseqAddr == 0 {
+		return syserror.EINVAL
+	}
+	if t.rseqAddr != addr {
+		return syserror.EINVAL
+	}
+	if length != linux.SizeOfRSeq {
+		return syserror.EINVAL
+	}
+	if t.rseqSignature != signature {
+		return syserror.EPERM
+	}
+
+	if err := t.rseqClearCPU(); err != nil {
+		return err
+	}
+
+	t.rseqAddr = 0
+	t.rseqSignature = 0
+
+	if t.oldRSeqCPUAddr == 0 {
+		// rseqCPU no longer needed.
+		t.rseqCPU = -1
+	}
+
+	return nil
+}
+
+// OldRSeqCriticalRegion returns a copy of t's thread group's current
+// old restartable sequence.
+func (t *Task) OldRSeqCriticalRegion() OldRSeqCriticalRegion {
+	return *t.tg.oldRSeqCritical.Load().(*OldRSeqCriticalRegion)
+}
+
+// SetOldRSeqCriticalRegion replaces t's thread group's old restartable
+// sequence.
+//
+// Preconditions: t.RSeqAvailable() == true.
+func (t *Task) SetOldRSeqCriticalRegion(r OldRSeqCriticalRegion) error {
 	// These checks are somewhat more lenient than in Linux, which (bizarrely)
-	// requires rscr.CriticalSection to be non-empty and rscr.Restart to be
-	// outside of rscr.CriticalSection, even if rscr.CriticalSection.Start == 0
+	// requires r.CriticalSection to be non-empty and r.Restart to be
+	// outside of r.CriticalSection, even if r.CriticalSection.Start == 0
 	// (which disables the critical region).
-	if rscr.CriticalSection.Start == 0 {
-		rscr.CriticalSection.End = 0
-		rscr.Restart = 0
-		t.tg.rscr.Store(&rscr)
+	if r.CriticalSection.Start == 0 {
+		r.CriticalSection.End = 0
+		r.Restart = 0
+		t.tg.oldRSeqCritical.Store(&r)
 		return nil
 	}
-	if rscr.CriticalSection.Start >= rscr.CriticalSection.End {
+	if r.CriticalSection.Start >= r.CriticalSection.End {
 		return syserror.EINVAL
 	}
-	if rscr.CriticalSection.Contains(rscr.Restart) {
+	if r.CriticalSection.Contains(r.Restart) {
 		return syserror.EINVAL
 	}
-	// TODO(jamieliu): check that rscr.CriticalSection and rscr.Restart are in
-	// the application address range, for consistency with Linux
-	t.tg.rscr.Store(&rscr)
+	// TODO(jamieliu): check that r.CriticalSection and r.Restart are in
+	// the application address range, for consistency with Linux.
+	t.tg.oldRSeqCritical.Store(&r)
 	return nil
 }
 
-// RSEQCPUAddr returns the address that RSEQ will keep updated with t's CPU
-// number.
+// OldRSeqCPUAddr returns the address that old rseq will keep updated with t's
+// CPU number.
 //
 // Preconditions: The caller must be running on the task goroutine.
-func (t *Task) RSEQCPUAddr() usermem.Addr {
-	return t.rseqCPUAddr
+func (t *Task) OldRSeqCPUAddr() usermem.Addr {
+	return t.oldRSeqCPUAddr
 }
 
-// SetRSEQCPUAddr replaces the address that RSEQ will keep updated with t's CPU
-// number.
+// SetOldRSeqCPUAddr replaces the address that old rseq will keep updated with
+// t's CPU number.
 //
-// Preconditions: t.RSEQAvailable() == true. The caller must be running on the
+// Preconditions: t.RSeqAvailable() == true. The caller must be running on the
 // task goroutine. t's AddressSpace must be active.
-func (t *Task) SetRSEQCPUAddr(addr usermem.Addr) error {
-	t.rseqCPUAddr = addr
-	if addr != 0 {
-		t.rseqCPU = int32(hostcpu.GetCPU())
-		if err := t.rseqCopyOutCPU(); err != nil {
-			t.rseqCPUAddr = 0
-			t.rseqCPU = -1
-			return syserror.EINVAL // yes, EINVAL, not err or EFAULT
-		}
-	} else {
-		t.rseqCPU = -1
+func (t *Task) SetOldRSeqCPUAddr(addr usermem.Addr) error {
+	t.oldRSeqCPUAddr = addr
+
+	// Check that addr is writable.
+	//
+	// N.B. rseqUpdateCPU may fail on a bad t.rseqAddr as well. That's
+	// unfortunate, but unlikely in a correct program.
+	if err := t.rseqUpdateCPU(); err != nil {
+		t.oldRSeqCPUAddr = 0
+		return syserror.EINVAL // yes, EINVAL, not err or EFAULT
 	}
 	return nil
 }
 
 // Preconditions: The caller must be running on the task goroutine. t's
 // AddressSpace must be active.
-func (t *Task) rseqCopyOutCPU() error {
+func (t *Task) rseqUpdateCPU() error {
+	if t.rseqAddr == 0 && t.oldRSeqCPUAddr == 0 {
+		t.rseqCPU = -1
+		return nil
+	}
+
+	t.rseqCPU = int32(hostcpu.GetCPU())
+
+	// Update both CPUs, even if one fails.
+	rerr := t.rseqCopyOutCPU()
+	oerr := t.oldRSeqCopyOutCPU()
+
+	if rerr != nil {
+		return rerr
+	}
+	return oerr
+}
+
+// Preconditions: The caller must be running on the task goroutine. t's
+// AddressSpace must be active.
+func (t *Task) oldRSeqCopyOutCPU() error {
+	if t.oldRSeqCPUAddr == 0 {
+		return nil
+	}
+
 	buf := t.CopyScratchBuffer(4)
 	usermem.ByteOrder.PutUint32(buf, uint32(t.rseqCPU))
-	_, err := t.CopyOutBytes(t.rseqCPUAddr, buf)
+	_, err := t.CopyOutBytes(t.oldRSeqCPUAddr, buf)
+	return err
+}
+
+// Preconditions: The caller must be running on the task goroutine. t's
+// AddressSpace must be active.
+func (t *Task) rseqCopyOutCPU() error {
+	if t.rseqAddr == 0 {
+		return nil
+	}
+
+	buf := t.CopyScratchBuffer(8)
+	// CPUIDStart and CPUID are the first two fields in linux.RSeq.
+	usermem.ByteOrder.PutUint32(buf, uint32(t.rseqCPU))     // CPUIDStart
+	usermem.ByteOrder.PutUint32(buf[4:], uint32(t.rseqCPU)) // CPUID
+	// N.B. This write is not atomic, but since this occurs on the task
+	// goroutine then as long as userspace uses a single-instruction read
+	// it can't see an invalid value.
+	_, err := t.CopyOutBytes(t.rseqAddr, buf)
+	return err
+}
+
+// Preconditions: The caller must be running on the task goroutine. t's
+// AddressSpace must be active.
+func (t *Task) rseqClearCPU() error {
+	buf := t.CopyScratchBuffer(8)
+	// CPUIDStart and CPUID are the first two fields in linux.RSeq.
+	usermem.ByteOrder.PutUint32(buf, 0)                                   // CPUIDStart
+	usermem.ByteOrder.PutUint32(buf[4:], linux.RSEQ_CPU_ID_UNINITIALIZED) // CPUID
+	// N.B. This write is not atomic, but since this occurs on the task
+	// goroutine then as long as userspace uses a single-instruction read
+	// it can't see an invalid value.
+	_, err := t.CopyOutBytes(t.rseqAddr, buf)
 	return err
 }
 
+// rseqAddrInterrupt checks if IP is in a critical section, and aborts if so.
+//
+// This is a bit complex since both the RSeq and RSeqCriticalSection structs
+// are stored in userspace. So we must:
+//
+// 1. Copy in the address of RSeqCriticalSection from RSeq.
+// 2. Copy in RSeqCriticalSection itself.
+// 3. Validate critical section struct version, address range, abort address.
+// 4. Validate the abort signature (4 bytes preceding abort IP match expected
+//    signature).
+// 5. Clear address of RSeqCriticalSection from RSeq.
+// 6. Finally, conditionally abort.
+//
+// See kernel/rseq.c:rseq_ip_fixup for reference.
+//
+// Preconditions: The caller must be running on the task goroutine. t's
+// AddressSpace must be active.
+func (t *Task) rseqAddrInterrupt() {
+	if t.rseqAddr == 0 {
+		return
+	}
+
+	critAddrAddr, ok := t.rseqAddr.AddLength(linux.OffsetOfRSeqCriticalSection)
+	if !ok {
+		// SetRSeq should validate this.
+		panic(fmt.Sprintf("t.rseqAddr (%#x) not large enough", t.rseqAddr))
+	}
+
+	if t.Arch().Width() != 8 {
+		// We only handle 64-bit for now.
+		t.Debugf("Only 64-bit rseq supported.")
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	buf := t.CopyScratchBuffer(8)
+	if _, err := t.CopyInBytes(critAddrAddr, buf); err != nil {
+		t.Debugf("Failed to copy critical section address from %#x for rseq: %v", critAddrAddr, err)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	critAddr := usermem.Addr(usermem.ByteOrder.Uint64(buf))
+	if critAddr == 0 {
+		return
+	}
+
+	buf = t.CopyScratchBuffer(linux.SizeOfRSeqCriticalSection)
+	if _, err := t.CopyInBytes(critAddr, buf); err != nil {
+		t.Debugf("Failed to copy critical section from %#x for rseq: %v", critAddr, err)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	// Manually marshal RSeqCriticalSection as this is in the hot path when
+	// rseq is enabled. It must be as fast as possible.
+	//
+	// TODO(b/130243041): Replace with go_marshal.
+	cs := linux.RSeqCriticalSection{
+		Version:          usermem.ByteOrder.Uint32(buf[0:4]),
+		Flags:            usermem.ByteOrder.Uint32(buf[4:8]),
+		Start:            usermem.ByteOrder.Uint64(buf[8:16]),
+		PostCommitOffset: usermem.ByteOrder.Uint64(buf[16:24]),
+		Abort:            usermem.ByteOrder.Uint64(buf[24:32]),
+	}
+
+	if cs.Version != 0 {
+		t.Debugf("Unknown version in %+v", cs)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	start := usermem.Addr(cs.Start)
+	critRange, ok := start.ToRange(cs.PostCommitOffset)
+	if !ok {
+		t.Debugf("Invalid start and offset in %+v", cs)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	abort := usermem.Addr(cs.Abort)
+	if critRange.Contains(abort) {
+		t.Debugf("Abort in critical section in %+v", cs)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	// Verify signature.
+	sigAddr := abort - linux.SizeOfRSeqSignature
+
+	buf = t.CopyScratchBuffer(linux.SizeOfRSeqSignature)
+	if _, err := t.CopyInBytes(sigAddr, buf); err != nil {
+		t.Debugf("Failed to copy critical section signature from %#x for rseq: %v", sigAddr, err)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	sig := usermem.ByteOrder.Uint32(buf)
+	if sig != t.rseqSignature {
+		t.Debugf("Mismatched rseq signature %d != %d", sig, t.rseqSignature)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	// Clear the critical section address.
+	//
+	// NOTE(b/143949567): We don't support any rseq flags, so we always
+	// restart if we are in the critical section, and thus *always* clear
+	// critAddrAddr.
+	if _, err := t.MemoryManager().ZeroOut(t, critAddrAddr, int64(t.Arch().Width()), usermem.IOOpts{
+		AddressSpaceActive: true,
+	}); err != nil {
+		t.Debugf("Failed to clear critical section address from %#x for rseq: %v", critAddrAddr, err)
+		t.forceSignal(linux.SIGSEGV, false /* unconditional */)
+		t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+		return
+	}
+
+	// Finally we can actually decide whether or not to restart.
+	if !critRange.Contains(usermem.Addr(t.Arch().IP())) {
+		return
+	}
+
+	t.Arch().SetIP(uintptr(cs.Abort))
+}
+
 // Preconditions: The caller must be running on the task goroutine.
-func (t *Task) rseqInterrupt() {
-	rscr := t.tg.rscr.Load().(*RSEQCriticalRegion)
-	if ip := t.Arch().IP(); rscr.CriticalSection.Contains(usermem.Addr(ip)) {
-		t.Debugf("Interrupted RSEQ critical section at %#x; restarting at %#x", ip, rscr.Restart)
-		t.Arch().SetIP(uintptr(rscr.Restart))
-		t.Arch().SetRSEQInterruptedIP(ip)
+func (t *Task) oldRSeqInterrupt() {
+	r := t.tg.oldRSeqCritical.Load().(*OldRSeqCriticalRegion)
+	if ip := t.Arch().IP(); r.CriticalSection.Contains(usermem.Addr(ip)) {
+		t.Debugf("Interrupted rseq critical section at %#x; restarting at %#x", ip, r.Restart)
+		t.Arch().SetIP(uintptr(r.Restart))
+		t.Arch().SetOldRSeqInterruptedIP(ip)
 	}
 }
+
+// Preconditions: The caller must be running on the task goroutine.
+func (t *Task) rseqInterrupt() {
+	t.rseqAddrInterrupt()
+	t.oldRSeqInterrupt()
+}
diff --git a/pkg/sentry/kernel/task.go b/pkg/sentry/kernel/task.go
index ab0c6c4aa..d25a7903b 100644
--- a/pkg/sentry/kernel/task.go
+++ b/pkg/sentry/kernel/task.go
@@ -489,18 +489,43 @@ type Task struct {
 	// netns is protected by mu. netns is owned by the task goroutine.
 	netns bool
 
-	// If rseqPreempted is true, before the next call to p.Switch(), interrupt
-	// RSEQ critical regions as defined by tg.rseq and write the task
-	// goroutine's CPU number to rseqCPUAddr. rseqCPU is the last CPU number
-	// written to rseqCPUAddr.
+	// If rseqPreempted is true, before the next call to p.Switch(),
+	// interrupt rseq critical regions as defined by rseqAddr and
+	// tg.oldRSeqCritical and write the task goroutine's CPU number to
+	// rseqAddr/oldRSeqCPUAddr.
 	//
-	// If rseqCPUAddr is 0, rseqCPU is -1.
+	// We support two ABIs for restartable sequences:
 	//
-	// rseqCPUAddr, rseqCPU, and rseqPreempted are exclusive to the task
-	// goroutine.
+	//  1. The upstream interface added in v4.18,
+	//  2. An "old" interface never merged upstream. In the implementation,
+	//     this is referred to as "old rseq".
+	//
+	// rseqPreempted is exclusive to the task goroutine.
 	rseqPreempted bool `state:"nosave"`
-	rseqCPUAddr   usermem.Addr
-	rseqCPU       int32
+
+	// rseqCPU is the last CPU number written to rseqAddr/oldRSeqCPUAddr.
+	//
+	// If rseq is unused, rseqCPU is -1 for convenient use in
+	// platform.Context.Switch.
+	//
+	// rseqCPU is exclusive to the task goroutine.
+	rseqCPU int32
+
+	// oldRSeqCPUAddr is a pointer to the userspace old rseq CPU variable.
+	//
+	// oldRSeqCPUAddr is exclusive to the task goroutine.
+	oldRSeqCPUAddr usermem.Addr
+
+	// rseqAddr is a pointer to the userspace linux.RSeq structure.
+	//
+	// rseqAddr is exclusive to the task goroutine.
+	rseqAddr usermem.Addr
+
+	// rseqSignature is the signature that the rseq abort IP must be signed
+	// with.
+	//
+	// rseqSignature is exclusive to the task goroutine.
+	rseqSignature uint32
 
 	// copyScratchBuffer is a buffer available to CopyIn/CopyOut
 	// implementations that require an intermediate buffer to copy data
diff --git a/pkg/sentry/kernel/task_clone.go b/pkg/sentry/kernel/task_clone.go
index 5f3589493..247bd4aba 100644
--- a/pkg/sentry/kernel/task_clone.go
+++ b/pkg/sentry/kernel/task_clone.go
@@ -236,7 +236,10 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 	} else if opts.NewPIDNamespace {
 		pidns = pidns.NewChild(userns)
 	}
+
 	tg := t.tg
+	rseqAddr := usermem.Addr(0)
+	rseqSignature := uint32(0)
 	if opts.NewThreadGroup {
 		tg.mounts.IncRef()
 		sh := t.tg.signalHandlers
@@ -244,6 +247,8 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 			sh = sh.Fork()
 		}
 		tg = t.k.NewThreadGroup(tg.mounts, pidns, sh, opts.TerminationSignal, tg.limits.GetCopy())
+		rseqAddr = t.rseqAddr
+		rseqSignature = t.rseqSignature
 	}
 
 	cfg := &TaskConfig{
@@ -260,6 +265,8 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 		UTSNamespace:            utsns,
 		IPCNamespace:            ipcns,
 		AbstractSocketNamespace: t.abstractSockets,
+		RSeqAddr:                rseqAddr,
+		RSeqSignature:           rseqSignature,
 		ContainerID:             t.ContainerID(),
 	}
 	if opts.NewThreadGroup {
diff --git a/pkg/sentry/kernel/task_exec.go b/pkg/sentry/kernel/task_exec.go
index 90a6190f1..fa6528386 100644
--- a/pkg/sentry/kernel/task_exec.go
+++ b/pkg/sentry/kernel/task_exec.go
@@ -190,9 +190,11 @@ func (r *runSyscallAfterExecStop) execute(t *Task) taskRunState {
 	t.updateRSSLocked()
 	// Restartable sequence state is discarded.
 	t.rseqPreempted = false
-	t.rseqCPUAddr = 0
 	t.rseqCPU = -1
-	t.tg.rscr.Store(&RSEQCriticalRegion{})
+	t.rseqAddr = 0
+	t.rseqSignature = 0
+	t.oldRSeqCPUAddr = 0
+	t.tg.oldRSeqCritical.Store(&OldRSeqCriticalRegion{})
 	t.tg.pidns.owner.mu.Unlock()
 
 	// Remove FDs with the CloseOnExec flag set.
diff --git a/pkg/sentry/kernel/task_run.go b/pkg/sentry/kernel/task_run.go
index d97f8c189..6357273d3 100644
--- a/pkg/sentry/kernel/task_run.go
+++ b/pkg/sentry/kernel/task_run.go
@@ -169,12 +169,22 @@ func (*runApp) execute(t *Task) taskRunState {
 	// Apply restartable sequences.
 	if t.rseqPreempted {
 		t.rseqPreempted = false
-		if t.rseqCPUAddr != 0 {
+		if t.rseqAddr != 0 || t.oldRSeqCPUAddr != 0 {
+			// Linux writes the CPU on every preemption. We only do
+			// so if it changed. Thus we may delay delivery of
+			// SIGSEGV if rseqAddr/oldRSeqCPUAddr is invalid.
 			cpu := int32(hostcpu.GetCPU())
 			if t.rseqCPU != cpu {
 				t.rseqCPU = cpu
 				if err := t.rseqCopyOutCPU(); err != nil {
-					t.Warningf("Failed to copy CPU to %#x for RSEQ: %v", t.rseqCPUAddr, err)
+					t.Debugf("Failed to copy CPU to %#x for rseq: %v", t.rseqAddr, err)
+					t.forceSignal(linux.SIGSEGV, false)
+					t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
+					// Re-enter the task run loop for signal delivery.
+					return (*runApp)(nil)
+				}
+				if err := t.oldRSeqCopyOutCPU(); err != nil {
+					t.Debugf("Failed to copy CPU to %#x for old rseq: %v", t.oldRSeqCPUAddr, err)
 					t.forceSignal(linux.SIGSEGV, false)
 					t.SendSignal(SignalInfoPriv(linux.SIGSEGV))
 					// Re-enter the task run loop for signal delivery.
@@ -320,7 +330,7 @@ func (*runApp) execute(t *Task) taskRunState {
 		return (*runApp)(nil)
 
 	case platform.ErrContextCPUPreempted:
-		// Ensure that RSEQ critical sections are interrupted and per-thread
+		// Ensure that rseq critical sections are interrupted and per-thread
 		// CPU values are updated before the next platform.Context.Switch().
 		t.rseqPreempted = true
 		return (*runApp)(nil)
diff --git a/pkg/sentry/kernel/task_start.go b/pkg/sentry/kernel/task_start.go
index 3522a4ae5..58af16ee2 100644
--- a/pkg/sentry/kernel/task_start.go
+++ b/pkg/sentry/kernel/task_start.go
@@ -21,6 +21,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel/futex"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/sched"
 	"gvisor.dev/gvisor/pkg/sentry/usage"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
@@ -79,6 +80,13 @@ type TaskConfig struct {
 	// AbstractSocketNamespace is the AbstractSocketNamespace of the new task.
 	AbstractSocketNamespace *AbstractSocketNamespace
 
+	// RSeqAddr is a pointer to the the userspace linux.RSeq structure.
+	RSeqAddr usermem.Addr
+
+	// RSeqSignature is the signature that the rseq abort IP must be signed
+	// with.
+	RSeqSignature uint32
+
 	// ContainerID is the container the new task belongs to.
 	ContainerID string
 }
@@ -126,6 +134,8 @@ func (ts *TaskSet) newTask(cfg *TaskConfig) (*Task, error) {
 		ipcns:           cfg.IPCNamespace,
 		abstractSockets: cfg.AbstractSocketNamespace,
 		rseqCPU:         -1,
+		rseqAddr:        cfg.RSeqAddr,
+		rseqSignature:   cfg.RSeqSignature,
 		futexWaiter:     futex.NewWaiter(),
 		containerID:     cfg.ContainerID,
 	}
diff --git a/pkg/sentry/kernel/thread_group.go b/pkg/sentry/kernel/thread_group.go
index 0cded73f6..c0197a563 100644
--- a/pkg/sentry/kernel/thread_group.go
+++ b/pkg/sentry/kernel/thread_group.go
@@ -238,8 +238,8 @@ type ThreadGroup struct {
 	// execed is protected by the TaskSet mutex.
 	execed bool
 
-	// rscr is the thread group's RSEQ critical region.
-	rscr atomic.Value `state:".(*RSEQCriticalRegion)"`
+	// oldRSeqCritical is the thread group's old rseq critical region.
+	oldRSeqCritical atomic.Value `state:".(*OldRSeqCriticalRegion)"`
 
 	// mounts is the thread group's mount namespace. This does not really
 	// correspond to a "mount namespace" in Linux, but is more like a
@@ -273,18 +273,18 @@ func (k *Kernel) NewThreadGroup(mntns *fs.MountNamespace, pidns *PIDNamespace, s
 	}
 	tg.itimerRealTimer = ktime.NewTimer(k.monotonicClock, &itimerRealListener{tg: tg})
 	tg.timers = make(map[linux.TimerID]*IntervalTimer)
-	tg.rscr.Store(&RSEQCriticalRegion{})
+	tg.oldRSeqCritical.Store(&OldRSeqCriticalRegion{})
 	return tg
 }
 
-// saveRscr is invoked by stateify.
-func (tg *ThreadGroup) saveRscr() *RSEQCriticalRegion {
-	return tg.rscr.Load().(*RSEQCriticalRegion)
+// saveOldRSeqCritical is invoked by stateify.
+func (tg *ThreadGroup) saveOldRSeqCritical() *OldRSeqCriticalRegion {
+	return tg.oldRSeqCritical.Load().(*OldRSeqCriticalRegion)
 }
 
-// loadRscr is invoked by stateify.
-func (tg *ThreadGroup) loadRscr(rscr *RSEQCriticalRegion) {
-	tg.rscr.Store(rscr)
+// loadOldRSeqCritical is invoked by stateify.
+func (tg *ThreadGroup) loadOldRSeqCritical(r *OldRSeqCriticalRegion) {
+	tg.oldRSeqCritical.Store(r)
 }
 
 // SignalHandlers returns the signal handlers used by tg.
diff --git a/pkg/sentry/syscalls/linux/BUILD b/pkg/sentry/syscalls/linux/BUILD
index 6766ba587..a76975cee 100644
--- a/pkg/sentry/syscalls/linux/BUILD
+++ b/pkg/sentry/syscalls/linux/BUILD
@@ -30,6 +30,7 @@ go_library(
         "sys_random.go",
         "sys_read.go",
         "sys_rlimit.go",
+        "sys_rseq.go",
         "sys_rusage.go",
         "sys_sched.go",
         "sys_seccomp.go",
diff --git a/pkg/sentry/syscalls/linux/linux64_amd64.go b/pkg/sentry/syscalls/linux/linux64_amd64.go
index 272ae9991..479c5f6ff 100644
--- a/pkg/sentry/syscalls/linux/linux64_amd64.go
+++ b/pkg/sentry/syscalls/linux/linux64_amd64.go
@@ -377,7 +377,7 @@ var AMD64 = &kernel.SyscallTable{
 		331: syscalls.ErrorWithEvent("pkey_free", syserror.ENOSYS, "", nil),
 		332: syscalls.Supported("statx", Statx),
 		333: syscalls.ErrorWithEvent("io_pgetevents", syserror.ENOSYS, "", nil),
-		334: syscalls.ErrorWithEvent("rseq", syserror.ENOSYS, "", nil),
+		334: syscalls.PartiallySupported("rseq", RSeq, "Not supported on all platforms.", nil),
 
 		// Linux skips ahead to syscall 424 to sync numbers between arches.
 		424: syscalls.ErrorWithEvent("pidfd_send_signal", syserror.ENOSYS, "", nil),
diff --git a/pkg/sentry/syscalls/linux/linux64_arm64.go b/pkg/sentry/syscalls/linux/linux64_arm64.go
index 3b584eed9..d3f61f5e8 100644
--- a/pkg/sentry/syscalls/linux/linux64_arm64.go
+++ b/pkg/sentry/syscalls/linux/linux64_arm64.go
@@ -307,7 +307,7 @@ var ARM64 = &kernel.SyscallTable{
 		290: syscalls.ErrorWithEvent("pkey_free", syserror.ENOSYS, "", nil),
 		291: syscalls.Supported("statx", Statx),
 		292: syscalls.ErrorWithEvent("io_pgetevents", syserror.ENOSYS, "", nil),
-		293: syscalls.ErrorWithEvent("rseq", syserror.ENOSYS, "", nil),
+		293: syscalls.PartiallySupported("rseq", RSeq, "Not supported on all platforms.", nil),
 
 		// Linux skips ahead to syscall 424 to sync numbers between arches.
 		424: syscalls.ErrorWithEvent("pidfd_send_signal", syserror.ENOSYS, "", nil),
diff --git a/pkg/sentry/syscalls/linux/sys_rseq.go b/pkg/sentry/syscalls/linux/sys_rseq.go
new file mode 100644
index 000000000..90db10ea6
--- /dev/null
+++ b/pkg/sentry/syscalls/linux/sys_rseq.go
@@ -0,0 +1,48 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package linux
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// RSeq implements syscall rseq(2).
+func RSeq(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	addr := args[0].Pointer()
+	length := args[1].Uint()
+	flags := args[2].Int()
+	signature := args[3].Uint()
+
+	if !t.RSeqAvailable() {
+		// Event for applications that want rseq on a configuration
+		// that doesn't support them.
+		t.Kernel().EmitUnimplementedEvent(t)
+		return 0, nil, syserror.ENOSYS
+	}
+
+	switch flags {
+	case 0:
+		// Register.
+		return 0, nil, t.SetRSeq(addr, length, signature)
+	case linux.RSEQ_FLAG_UNREGISTER:
+		return 0, nil, t.ClearRSeq(addr, length, signature)
+	default:
+		// Unknown flag.
+		return 0, nil, syserror.EINVAL
+	}
+}