kvm: prefault a floating point state before restoring it

If physical pages of a memory region are not mapped yet, the kernel will
trigger KVM_EXIT_MMIO and we will map physical pages in bluepillHandler().

An instruction that triggered a fault will not be re-executed, it
will be emulated in the kernel, but it can't  emulate complex
instructions like xsave, xrstor. We can touch the memory with
simple instructions to workaround this problem.

5 files changed, 34 insertions, 11 deletions

diff --git a/pkg/sentry/platform/kvm/bluepill_amd64.go b/pkg/sentry/platform/kvm/bluepill_amd64.go
index f4b9a5321..308696efe 100644
--- a/pkg/sentry/platform/kvm/bluepill_amd64.go
+++ b/pkg/sentry/platform/kvm/bluepill_amd64.go
@@ -73,7 +73,7 @@ func (c *vCPU) KernelSyscall() {
 	// We only trigger a bluepill entry in the bluepill function, and can
 	// therefore be guaranteed that there is no floating point state to be
 	// loaded on resuming from halt. We only worry about saving on exit.
-	ring0.SaveFloatingPoint((*byte)(c.floatingPointState)) // escapes: no.
+	ring0.SaveFloatingPoint(&c.floatingPointState[0]) // escapes: no.
 	ring0.Halt()
 	ring0.WriteFS(uintptr(regs.Fs_base)) // escapes: no, reload host segment.
 }
@@ -92,7 +92,7 @@ func (c *vCPU) KernelException(vector ring0.Vector) {
 		regs.Rip = 0
 	}
 	// See above.
-	ring0.SaveFloatingPoint((*byte)(c.floatingPointState)) // escapes: no.
+	ring0.SaveFloatingPoint(&c.floatingPointState[0]) // escapes: no.
 	ring0.Halt()
 	ring0.WriteFS(uintptr(regs.Fs_base)) // escapes: no; reload host segment.
 }
@@ -124,5 +124,5 @@ func bluepillArchExit(c *vCPU, context *arch.SignalContext64) {
 	// Set the context pointer to the saved floating point state. This is
 	// where the guest data has been serialized, the kernel will restore
 	// from this new pointer value.
-	context.Fpstate = uint64(uintptrValue((*byte)(c.floatingPointState)))
+	context.Fpstate = uint64(uintptrValue(&c.floatingPointState[0]))
 }
diff --git a/pkg/sentry/platform/kvm/bluepill_arm64.go b/pkg/sentry/platform/kvm/bluepill_arm64.go
index e26b7da8d..c317f1e99 100644
--- a/pkg/sentry/platform/kvm/bluepill_arm64.go
+++ b/pkg/sentry/platform/kvm/bluepill_arm64.go
@@ -92,7 +92,7 @@ func bluepillArchExit(c *vCPU, context *arch.SignalContext64) {
 
 	lazyVfp := c.GetLazyVFP()
 	if lazyVfp != 0 {
-		fpsimd := fpsimdPtr((*byte)(c.floatingPointState))
+		fpsimd := fpsimdPtr(&c.floatingPointState[0])
 		context.Fpsimd64.Fpsr = fpsimd.Fpsr
 		context.Fpsimd64.Fpcr = fpsimd.Fpcr
 		context.Fpsimd64.Vregs = fpsimd.Vregs
@@ -112,12 +112,12 @@ func (c *vCPU) KernelSyscall() {
 
 	fpDisableTrap := ring0.CPACREL1()
 	if fpDisableTrap != 0 {
-		fpsimd := fpsimdPtr((*byte)(c.floatingPointState))
+		fpsimd := fpsimdPtr(&c.floatingPointState[0])
 		fpcr := ring0.GetFPCR()
 		fpsr := ring0.GetFPSR()
 		fpsimd.Fpcr = uint32(fpcr)
 		fpsimd.Fpsr = uint32(fpsr)
-		ring0.SaveVRegs((*byte)(c.floatingPointState))
+		ring0.SaveVRegs(&c.floatingPointState[0])
 	}
 
 	ring0.Halt()
@@ -136,12 +136,12 @@ func (c *vCPU) KernelException(vector ring0.Vector) {
 
 	fpDisableTrap := ring0.CPACREL1()
 	if fpDisableTrap != 0 {
-		fpsimd := fpsimdPtr((*byte)(c.floatingPointState))
+		fpsimd := fpsimdPtr(&c.floatingPointState[0])
 		fpcr := ring0.GetFPCR()
 		fpsr := ring0.GetFPSR()
 		fpsimd.Fpcr = uint32(fpcr)
 		fpsimd.Fpsr = uint32(fpsr)
-		ring0.SaveVRegs((*byte)(c.floatingPointState))
+		ring0.SaveVRegs(&c.floatingPointState[0])
 	}
 
 	ring0.Halt()
diff --git a/pkg/sentry/platform/kvm/context.go b/pkg/sentry/platform/kvm/context.go
index aeae01dbd..706fa53dc 100644
--- a/pkg/sentry/platform/kvm/context.go
+++ b/pkg/sentry/platform/kvm/context.go
@@ -65,7 +65,7 @@ func (c *context) Switch(ctx pkgcontext.Context, mm platform.MemoryManager, ac a
 	// Prepare switch options.
 	switchOpts := ring0.SwitchOpts{
 		Registers:          &ac.StateData().Regs,
-		FloatingPointState: (*byte)(ac.FloatingPointData()),
+		FloatingPointState: ac.FloatingPointData(),
 		PageTables:         localAS.pageTables,
 		Flush:              localAS.Touch(cpu),
 		FullRestore:        ac.FullRestore(),
diff --git a/pkg/sentry/platform/kvm/machine_amd64.go b/pkg/sentry/platform/kvm/machine_amd64.go
index 6e583baa3..916903881 100644
--- a/pkg/sentry/platform/kvm/machine_amd64.go
+++ b/pkg/sentry/platform/kvm/machine_amd64.go
@@ -70,7 +70,7 @@ type vCPUArchState struct {
 
 	// floatingPointState is the floating point state buffer used in guest
 	// to host transitions. See usage in bluepill_amd64.go.
-	floatingPointState *arch.FloatingPointData
+	floatingPointState arch.FloatingPointData
 }
 
 const (
@@ -293,6 +293,28 @@ func (c *vCPU) fault(signal int32, info *arch.SignalInfo) (usermem.AccessType, e
 	return accessType, platform.ErrContextSignal
 }
 
+//go:nosplit
+//go:noinline
+func loadByte(ptr *byte) byte {
+	return *ptr
+}
+
+// prefaultFloatingPointState touches each page of the floating point state to
+// be sure that its physical pages are mapped.
+//
+// Otherwise the kernel can trigger KVM_EXIT_MMIO and an instruction that
+// triggered a fault will be emulated by the kvm kernel code, but it can't
+// emulate instructions like xsave and xrstor.
+//
+//go:nosplit
+func prefaultFloatingPointState(data arch.FloatingPointData) {
+	size := len(data)
+	for i := 0; i < size; i += usermem.PageSize {
+		loadByte(&(data)[i])
+	}
+	loadByte(&(data)[size-1])
+}
+
 // SwitchToUser unpacks architectural-details.
 func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo) (usermem.AccessType, error) {
 	// Check for canonical addresses.
@@ -323,6 +345,7 @@ func (c *vCPU) SwitchToUser(switchOpts ring0.SwitchOpts, info *arch.SignalInfo)
 	// allocations occur.
 	entersyscall()
 	bluepill(c)
+	prefaultFloatingPointState(switchOpts.FloatingPointState)
 	vector = c.CPU.SwitchToUser(switchOpts)
 	exitsyscall()
 
diff --git a/pkg/sentry/platform/kvm/machine_arm64.go b/pkg/sentry/platform/kvm/machine_arm64.go
index 7d7857067..3d715e570 100644
--- a/pkg/sentry/platform/kvm/machine_arm64.go
+++ b/pkg/sentry/platform/kvm/machine_arm64.go
@@ -32,7 +32,7 @@ type vCPUArchState struct {
 
 	// floatingPointState is the floating point state buffer used in guest
 	// to host transitions. See usage in bluepill_arm64.go.
-	floatingPointState *arch.FloatingPointData
+	floatingPointState arch.FloatingPointData
 }
 
 const (