Fix kernfs race condition.

Do not release dirMu between checking whether to create a child and actually
inserting it.

Also fixes a bug in fusefs which was causing it to deadlock under the new
lock ordering. We do not need to call kernfs.Dentry.InsertChild from newEntry
because it will always be called at the kernfs filesystem layer.

Updates #1193.

PiperOrigin-RevId: 334049264

1 files changed, 19 insertions, 5 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/filesystem.go b/pkg/sentry/fsimpl/kernfs/filesystem.go
index 11dc24b07..5cc1c4281 100644
--- a/pkg/sentry/fsimpl/kernfs/filesystem.go
+++ b/pkg/sentry/fsimpl/kernfs/filesystem.go
@@ -207,7 +207,6 @@ func checkCreateLocked(ctx context.Context, rp *vfs.ResolvingPath, parent *Dentr
 	if len(pc) > linux.NAME_MAX {
 		return "", syserror.ENAMETOOLONG
 	}
-	// FIXME(gvisor.dev/issue/1193): Data race due to not holding dirMu.
 	if _, ok := parent.children[pc]; ok {
 		return "", syserror.EEXIST
 	}
@@ -305,6 +304,9 @@ func (fs *Filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 	if err != nil {
 		return err
 	}
+
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
 	pc, err := checkCreateLocked(ctx, rp, parent)
 	if err != nil {
 		return err
@@ -326,7 +328,7 @@ func (fs *Filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 	if err != nil {
 		return err
 	}
-	parent.InsertChild(pc, child)
+	parent.InsertChildLocked(pc, child)
 	return nil
 }
 
@@ -342,6 +344,9 @@ func (fs *Filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 	if err != nil {
 		return err
 	}
+
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
 	pc, err := checkCreateLocked(ctx, rp, parent)
 	if err != nil {
 		return err
@@ -357,7 +362,7 @@ func (fs *Filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		}
 		child = newSyntheticDirectory(rp.Credentials(), opts.Mode)
 	}
-	parent.InsertChild(pc, child)
+	parent.InsertChildLocked(pc, child)
 	return nil
 }
 
@@ -373,6 +378,9 @@ func (fs *Filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 	if err != nil {
 		return err
 	}
+
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
 	pc, err := checkCreateLocked(ctx, rp, parent)
 	if err != nil {
 		return err
@@ -385,7 +393,7 @@ func (fs *Filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 	if err != nil {
 		return err
 	}
-	parent.InsertChild(pc, newD)
+	parent.InsertChildLocked(pc, newD)
 	return nil
 }
 
@@ -483,6 +491,9 @@ afterTrailingSymlink:
 		if err != nil {
 			return nil, err
 		}
+		// FIXME(gvisor.dev/issue/1193): Race between checking existence with
+		// fs.stepExistingLocked and parent.InsertChild. If possible, we should hold
+		// dirMu from one to the other.
 		parent.InsertChild(pc, child)
 		child.inode.IncRef()
 		defer child.inode.DecRef(ctx)
@@ -739,6 +750,9 @@ func (fs *Filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, targ
 	if err != nil {
 		return err
 	}
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
+
 	pc, err := checkCreateLocked(ctx, rp, parent)
 	if err != nil {
 		return err
@@ -751,7 +765,7 @@ func (fs *Filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, targ
 	if err != nil {
 		return err
 	}
-	parent.InsertChild(pc, child)
+	parent.InsertChildLocked(pc, child)
 	return nil
 }