ovl: take a snapshot of the name for copy up

The copy up of a directory reads a freed dentry name when a lookup
moves the directory at the same time.

A pointer to the name of a dentry is only valid while something stops
a move of the dentry. ovl_do_copy_up() keeps a plain copy of
c->dentry->d_name in c->destname, and ovl_copy_up_start() only keeps
ovl_rename() away. When a lookup finds the same directory under another
name after a change to the lower layer, d_splice_alias() moves the
dentry and frees its external name.

With a delay in ovl_do_copy_up(), a chmod of a lower-only directory with
a 60 byte name raced with a rename of that directory in the lower layer
and a stat of the new name, and KASAN reported a use-after-free in
lookup_noperm_common().

Fix this by using take_dentry_name_snapshot() for c->destname.

Fixes: 829c28be9bb9 ("ovl: use d_splice_alias() in place of d_add() in lookup")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 92b3d08a0234..12000e45fefc 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -920,6 +920,7 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 	struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
 	struct dentry *origin = c->lowerpath.dentry;
 	struct ovl_fh *fh = NULL;
+	struct name_snapshot name;
 	bool to_index = false;
 
 	/*
@@ -957,12 +958,15 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 		goto out_free_fh;
 	} else {
 		/*
-		 * c->dentry->d_name is stabilzed by ovl_copy_up_start(),
-		 * because if we got here, it means that c->dentry has no upper
-		 * alias and changing ->d_name means going through ovl_rename()
-		 * that will call ovl_copy_up() on source and target dentry.
+		 * ovl_copy_up_start() keeps ovl_rename() away from c->dentry,
+		 * because ovl_rename() calls ovl_copy_up() on source and target
+		 * dentry.  It does not stop d_splice_alias() from moving a
+		 * directory that a lookup found under another name after a
+		 * change to the lower layer.  That move frees an external name,
+		 * so take a snapshot.
 		 */
-		c->destname = c->dentry->d_name;
+		take_dentry_name_snapshot(&name, c->dentry);
+		c->destname = name.name;
 		/*
 		 * Mark parent "impure" because it may now contain non-pure
 		 * upper
@@ -971,7 +975,7 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 		err = ovl_set_impure(c->parent, c->destdir);
 		ovl_end_write(c->dentry);
 		if (err)
-			goto out_free_fh;
+			goto out;
 	}
 
 	/* Should we copyup with O_TMPFILE or with workdir? */
@@ -1005,6 +1009,8 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 out:
 	if (to_index)
 		kfree(c->destname.name);
+	else
+		release_dentry_name_snapshot(&name);
 out_free_fh:
 	kfree(fh);
 	return err;