Merge pull request #3617 from laijs:upperhalf

PiperOrigin-RevId: 340484823

5 files changed, 115 insertions, 15 deletions

diff --git a/pkg/sentry/platform/ring0/pagetables/pagetables.go b/pkg/sentry/platform/ring0/pagetables/pagetables.go
index 7f18ac296..bc16a1622 100644
--- a/pkg/sentry/platform/ring0/pagetables/pagetables.go
+++ b/pkg/sentry/platform/ring0/pagetables/pagetables.go
@@ -30,6 +30,10 @@ type PageTables struct {
 	Allocator Allocator
 
 	// root is the pagetable root.
+	//
+	// For same archs such as amd64, the upper of the PTEs is cloned
+	// from and owned by upperSharedPageTables which are shared among
+	// many PageTables if upperSharedPageTables is not nil.
 	root *PTEs
 
 	// rootPhysical is the cached physical address of the root.
@@ -39,15 +43,52 @@ type PageTables struct {
 
 	// archPageTables includes architecture-specific features.
 	archPageTables
+
+	// upperSharedPageTables represents a read-only shared upper
+	// of the Pagetable. When it is not nil, the upper is not
+	// allowed to be modified.
+	upperSharedPageTables *PageTables
+
+	// upperStart is the start address of the upper portion that
+	// are shared from upperSharedPageTables
+	upperStart uintptr
+
+	// readOnlyShared indicates the Pagetables are read-only and
+	// own the ranges that are shared with other Pagetables.
+	readOnlyShared bool
 }
 
-// New returns new PageTables.
-func New(a Allocator) *PageTables {
+// NewWithUpper returns new PageTables.
+//
+// upperSharedPageTables are used for mapping the upper of addresses,
+// starting at upperStart. These pageTables should not be touched (as
+// invalidations may be incorrect) after they are passed as an
+// upperSharedPageTables. Only when all dependent PageTables are gone
+// may they be used. The intenteded use case is for kernel page tables,
+// which are static and fixed.
+//
+// Precondition: upperStart must be between canonical ranges.
+// Precondition: upperStart must be pgdSize aligned.
+// precondition: upperSharedPageTables must be marked read-only shared.
+func NewWithUpper(a Allocator, upperSharedPageTables *PageTables, upperStart uintptr) *PageTables {
 	p := new(PageTables)
 	p.Init(a)
+	if upperSharedPageTables != nil {
+		if !upperSharedPageTables.readOnlyShared {
+			panic("Only read-only shared pagetables can be used as upper")
+		}
+		p.upperSharedPageTables = upperSharedPageTables
+		p.upperStart = upperStart
+		p.cloneUpperShared()
+	}
 	return p
 }
 
+// New returns new PageTables.
+func New(a Allocator) *PageTables {
+	return NewWithUpper(a, nil, 0)
+}
+
 // mapVisitor is used for map.
 type mapVisitor struct {
 	target   uintptr // Input.
@@ -90,6 +131,21 @@ func (*mapVisitor) requiresSplit() bool { return true }
 //
 //go:nosplit
 func (p *PageTables) Map(addr usermem.Addr, length uintptr, opts MapOpts, physical uintptr) bool {
+	if p.readOnlyShared {
+		panic("Should not modify read-only shared pagetables.")
+	}
+	if uintptr(addr)+length < uintptr(addr) {
+		panic("addr & length overflow")
+	}
+	if p.upperSharedPageTables != nil {
+		// ignore change to the read-only upper shared portion.
+		if uintptr(addr) >= p.upperStart {
+			return false
+		}
+		if uintptr(addr)+length > p.upperStart {
+			length = p.upperStart - uintptr(addr)
+		}
+	}
 	if !opts.AccessType.Any() {
 		return p.Unmap(addr, length)
 	}
@@ -128,12 +184,27 @@ func (v *unmapVisitor) visit(start uintptr, pte *PTE, align uintptr) {
 //
 // True is returned iff there was a previous mapping in the range.
 //
-// Precondition: addr & length must be page-aligned.
+// Precondition: addr & length must be page-aligned, their sum must not overflow.
 //
 // +checkescape:hard,stack
 //
 //go:nosplit
 func (p *PageTables) Unmap(addr usermem.Addr, length uintptr) bool {
+	if p.readOnlyShared {
+		panic("Should not modify read-only shared pagetables.")
+	}
+	if uintptr(addr)+length < uintptr(addr) {
+		panic("addr & length overflow")
+	}
+	if p.upperSharedPageTables != nil {
+		// ignore change to the read-only upper shared portion.
+		if uintptr(addr) >= p.upperStart {
+			return false
+		}
+		if uintptr(addr)+length > p.upperStart {
+			length = p.upperStart - uintptr(addr)
+		}
+	}
 	w := unmapWalker{
 		pageTables: p,
 		visitor: unmapVisitor{
@@ -218,3 +289,10 @@ func (p *PageTables) Lookup(addr usermem.Addr) (physical uintptr, opts MapOpts)
 	w.iterateRange(uintptr(addr), uintptr(addr)+1)
 	return w.visitor.physical + offset, w.visitor.opts
 }
+
+// MarkReadOnlyShared marks the pagetables read-only and can be shared.
+//
+// It is usually used on the pagetables that are used as the upper
+func (p *PageTables) MarkReadOnlyShared() {
+	p.readOnlyShared = true
+}
diff --git a/pkg/sentry/platform/ring0/pagetables/pagetables_aarch64.go b/pkg/sentry/platform/ring0/pagetables/pagetables_aarch64.go
index 520161755..a4e416af7 100644
--- a/pkg/sentry/platform/ring0/pagetables/pagetables_aarch64.go
+++ b/pkg/sentry/platform/ring0/pagetables/pagetables_aarch64.go
@@ -24,14 +24,6 @@ import (
 
 // archPageTables is architecture-specific data.
 type archPageTables struct {
-	// root is the pagetable root for kernel space.
-	root *PTEs
-
-	// rootPhysical is the cached physical address of the root.
-	//
-	// This is saved only to prevent constant translation.
-	rootPhysical uintptr
-
 	asid uint16
 }
 
@@ -46,7 +38,7 @@ func (p *PageTables) TTBR0_EL1(noFlush bool, asid uint16) uint64 {
 //
 //go:nosplit
 func (p *PageTables) TTBR1_EL1(noFlush bool, asid uint16) uint64 {
-	return uint64(p.archPageTables.rootPhysical) | (uint64(asid)&ttbrASIDMask)<<ttbrASIDOffset
+	return uint64(p.upperSharedPageTables.rootPhysical) | (uint64(asid)&ttbrASIDMask)<<ttbrASIDOffset
 }
 
 // Bits in page table entries.
diff --git a/pkg/sentry/platform/ring0/pagetables/pagetables_amd64.go b/pkg/sentry/platform/ring0/pagetables/pagetables_amd64.go
index 0c153cf8c..e7ab887e5 100644
--- a/pkg/sentry/platform/ring0/pagetables/pagetables_amd64.go
+++ b/pkg/sentry/platform/ring0/pagetables/pagetables_amd64.go
@@ -50,5 +50,26 @@ func (p *PageTables) Init(allocator Allocator) {
 	p.rootPhysical = p.Allocator.PhysicalFor(p.root)
 }
 
+func pgdIndex(upperStart uintptr) uintptr {
+	if upperStart&(pgdSize-1) != 0 {
+		panic("upperStart should be pgd size aligned")
+	}
+	if upperStart >= upperBottom {
+		return entriesPerPage/2 + (upperStart-upperBottom)/pgdSize
+	}
+	if upperStart < lowerTop {
+		return upperStart / pgdSize
+	}
+	panic("upperStart should be in canonical range")
+}
+
+// cloneUpperShared clone the upper from the upper shared page tables.
+//
+//go:nosplit
+func (p *PageTables) cloneUpperShared() {
+	start := pgdIndex(p.upperStart)
+	copy(p.root[start:entriesPerPage], p.upperSharedPageTables.root[start:entriesPerPage])
+}
+
 // PTEs is a collection of entries.
 type PTEs [entriesPerPage]PTE
diff --git a/pkg/sentry/platform/ring0/pagetables/pagetables_arm64.go b/pkg/sentry/platform/ring0/pagetables/pagetables_arm64.go
index 5ddd10256..5392bf27a 100644
--- a/pkg/sentry/platform/ring0/pagetables/pagetables_arm64.go
+++ b/pkg/sentry/platform/ring0/pagetables/pagetables_arm64.go
@@ -49,8 +49,17 @@ func (p *PageTables) Init(allocator Allocator) {
 	p.Allocator = allocator
 	p.root = p.Allocator.NewPTEs()
 	p.rootPhysical = p.Allocator.PhysicalFor(p.root)
-	p.archPageTables.root = p.Allocator.NewPTEs()
-	p.archPageTables.rootPhysical = p.Allocator.PhysicalFor(p.archPageTables.root)
+}
+
+// cloneUpperShared clone the upper from the upper shared page tables.
+//
+//go:nosplit
+func (p *PageTables) cloneUpperShared() {
+	if p.upperStart != upperBottom {
+		panic("upperStart should be the same as upperBottom")
+	}
+
+	// nothing to do for arm.
 }
 
 // PTEs is a collection of entries.
diff --git a/pkg/sentry/platform/ring0/pagetables/walker_arm64.go b/pkg/sentry/platform/ring0/pagetables/walker_arm64.go
index c261d393a..157c9a7cc 100644
--- a/pkg/sentry/platform/ring0/pagetables/walker_arm64.go
+++ b/pkg/sentry/platform/ring0/pagetables/walker_arm64.go
@@ -116,7 +116,7 @@ func next(start uintptr, size uintptr) uintptr {
 func (w *Walker) iterateRangeCanonical(start, end uintptr) {
 	pgdEntryIndex := w.pageTables.root
 	if start >= upperBottom {
-		pgdEntryIndex = w.pageTables.archPageTables.root
+		pgdEntryIndex = w.pageTables.upperSharedPageTables.root
 	}
 
 	for pgdIndex := (uint16((start & pgdMask) >> pgdShift)); start < end && pgdIndex < entriesPerPage; pgdIndex++ {