Fix reference counting on kcov mappings.

Reported-by: syzbot+078580ce5dd6d607fcd8@syzkaller.appspotmail.com
Reported-by: syzbot+2096681f6891e7bf8aed@syzkaller.appspotmail.com
PiperOrigin-RevId: 337973519

pkg/sentry/fsimpl/sys/kcov.go

@@ -102,7 +102,7 @@ func (fd *kcovFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) erro
 func (fd *kcovFD) Release(ctx context.Context) {
 	// kcov instances have reference counts in Linux, but this seems sufficient
 	// for our purposes.
-	fd.kcov.Clear()
+	fd.kcov.Clear(ctx)
 }
 
 // SetStat implements vfs.FileDescriptionImpl.SetStat.

pkg/sentry/kernel/kcov.go

@@ -199,23 +199,25 @@ func (kcov *Kcov) DisableTrace(ctx context.Context) error {
 	}
 	kcov.mode = linux.KCOV_MODE_INIT
 	kcov.owningTask = nil
-	kcov.mappable = nil
+	if kcov.mappable != nil {
+		kcov.mappable.DecRef(ctx)
+		kcov.mappable = nil
+	}
 	return nil
 }
 
 // Clear resets the mode and clears the owning task and memory mapping for kcov.
 // It is called when the fd corresponding to kcov is closed. Note that the mode
 // needs to be set so that the next call to kcov.TaskWork() will exit early.
-func (kcov *Kcov) Clear() {
+func (kcov *Kcov) Clear(ctx context.Context) {
 	kcov.mu.Lock()
-	kcov.clearLocked()
-	kcov.mu.Unlock()
-}
-
-func (kcov *Kcov) clearLocked() {
 	kcov.mode = linux.KCOV_MODE_INIT
 	kcov.owningTask = nil
-	kcov.mappable = nil
+	if kcov.mappable != nil {
+		kcov.mappable.DecRef(ctx)
+		kcov.mappable = nil
+	}
+	kcov.mu.Unlock()
 }
 
 // OnTaskExit is called when the owning task exits. It is similar to
@@ -254,6 +256,7 @@ func (kcov *Kcov) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) erro
 		// will look different under /proc/[pid]/maps than they do on Linux.
 		kcov.mappable = mm.NewSpecialMappable(fmt.Sprintf("[kcov:%d]", t.ThreadID()), kcov.mfp, fr)
 	}
+	kcov.mappable.IncRef()
 	opts.Mappable = kcov.mappable
 	opts.MappingIdentity = kcov.mappable
 	return nil

pkg/sentry/kernel/kcov_unsafe.go

@@ -20,9 +20,9 @@ import (
 	"gvisor.dev/gvisor/pkg/safemem"
 )
 
-// countBlock provides a safemem.BlockSeq for k.count.
+// countBlock provides a safemem.BlockSeq for kcov.count.
 //
 // Like k.count, the block returned is protected by k.mu.
-func (k *Kcov) countBlock() safemem.BlockSeq {
-	return safemem.BlockSeqOf(safemem.BlockFromSafePointer(unsafe.Pointer(&k.count), int(unsafe.Sizeof(k.count))))
+func (kcov *Kcov) countBlock() safemem.BlockSeq {
+	return safemem.BlockSeqOf(safemem.BlockFromSafePointer(unsafe.Pointer(&kcov.count), int(unsafe.Sizeof(kcov.count))))
 }