Move page tables lock into the address space.

This is necessary to prevent races with invalidation. It is currently
possible that page tables are garbage collected while paging caches
refer to them. We must ensure that pages are held until caches can be
invalidated. This is not achieved by this goal alone, but moving locking
to outside the page tables themselves is a requisite.

PiperOrigin-RevId: 198920784
Change-Id: I66fffecd49cb14aa2e676a84a68cabfc0c8b3e9a

pkg/sentry/platform/kvm/address_space.go

@@ -16,6 +16,7 @@ package kvm
 
 import (
 	"reflect"
+	"sync"
 
 	"gvisor.googlesource.com/gvisor/pkg/sentry/platform"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/filemem"
@@ -27,6 +28,11 @@ import (
 type addressSpace struct {
 	platform.NoAddressSpaceIO
 
+	// mu is the lock for modifications to the address space.
+	//
+	// Note that the page tables themselves are not locked.
+	mu sync.Mutex
+
 	// filemem is the memory instance.
 	filemem *filemem.FileMem
 
@@ -49,8 +55,8 @@ type addressSpace struct {
 	files hostMap
 }
 
-// Invalidate interrupts all dirty contexts.
-func (as *addressSpace) Invalidate() {
+// invalidate is the implementation for Invalidate.
+func (as *addressSpace) invalidate() {
 	for i := 0; i < as.dirtySet.size(); i++ {
 		if c := as.dirtySet.swap(i, nil); c != nil && c.active.get() == as {
 			c.BounceToKernel() // Force a kernel transition.
@@ -58,6 +64,13 @@ func (as *addressSpace) Invalidate() {
 	}
 }
 
+// Invalidate interrupts all dirty contexts.
+func (as *addressSpace) Invalidate() {
+	as.mu.Lock()
+	defer as.mu.Unlock()
+	as.invalidate()
+}
+
 // Touch adds the given vCPU to the dirty list.
 //
 // The return value indicates whether a flush is required.
@@ -120,7 +133,7 @@ func (as *addressSpace) mapHostFile(addr usermem.Addr, fd int, fr platform.FileR
 		addr += usermem.Addr(m.length)
 	}
 	if inv {
-		as.Invalidate()
+		as.invalidate()
 	}
 
 	return nil
@@ -169,7 +182,7 @@ func (as *addressSpace) mapFilemem(addr usermem.Addr, fr platform.FileRange, at
 		addr += usermem.Addr(len(s))
 	}
 	if inv {
-		as.Invalidate()
+		as.invalidate()
 		as.files.DeleteMapping(orig)
 	}
 
@@ -178,6 +191,9 @@ func (as *addressSpace) mapFilemem(addr usermem.Addr, fr platform.FileRange, at
 
 // MapFile implements platform.AddressSpace.MapFile.
 func (as *addressSpace) MapFile(addr usermem.Addr, fd int, fr platform.FileRange, at usermem.AccessType, precommit bool) error {
+	as.mu.Lock()
+	defer as.mu.Unlock()
+
 	// Create an appropriate mapping. If this is filemem, we don't create
 	// custom mappings for each in-application mapping. For files however,
 	// we create distinct mappings for each address space. Unfortunately,
@@ -195,8 +211,11 @@ func (as *addressSpace) MapFile(addr usermem.Addr, fd int, fr platform.FileRange
 
 // Unmap unmaps the given range by calling pagetables.PageTables.Unmap.
 func (as *addressSpace) Unmap(addr usermem.Addr, length uint64) {
+	as.mu.Lock()
+	defer as.mu.Unlock()
+
 	if prev := as.pageTables.Unmap(addr, uintptr(length)); prev {
-		as.Invalidate()
+		as.invalidate()
 		as.files.DeleteMapping(usermem.AddrRange{
 			Start: addr,
 			End:   addr + usermem.Addr(length),

pkg/sentry/platform/ring0/pagetables/pagetables.go

@@ -16,8 +16,6 @@
 package pagetables
 
 import (
-	"sync"
-
 	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
 )
 
@@ -39,8 +37,6 @@ type Node struct {
 
 // PageTables is a set of page tables.
 type PageTables struct {
-	mu sync.Mutex
-
 	// root is the pagetable root.
 	root *Node
 
@@ -122,7 +118,6 @@ func (p *PageTables) Map(addr usermem.Addr, length uintptr, opts MapOpts, physic
 		return p.Unmap(addr, length)
 	}
 	prev := false
-	p.mu.Lock()
 	end, ok := addr.AddLength(uint64(length))
 	if !ok {
 		panic("pagetables.Map: overflow")
@@ -139,7 +134,6 @@ func (p *PageTables) Map(addr usermem.Addr, length uintptr, opts MapOpts, physic
 		}
 		pte.Set(p, opts)
 	})
-	p.mu.Unlock()
 	return prev
 }
 
@@ -147,13 +141,11 @@ func (p *PageTables) Map(addr usermem.Addr, length uintptr, opts MapOpts, physic
 //
 // True is returned iff there was a previous mapping in the range.
 func (p *PageTables) Unmap(addr usermem.Addr, length uintptr) bool {
-	p.mu.Lock()
 	count := 0
 	p.iterateRange(uintptr(addr), uintptr(addr)+length, false, func(s, e uintptr, pte *PTE, align uintptr) {
 		pte.Clear()
 		count++
 	})
-	p.mu.Unlock()
 	return count > 0
 }