[PATCH] ovl: fix UAF in ovl_do_mkdir() debug print

[PATCH] ovl: fix UAF in ovl_do_mkdir() debug print

[Amir Goldstein]

ovl_do_mkdir() prints the input dentry with %pd after vfs_mkdir().
Since commit fe497f0759e0 ("VFS: change vfs_mkdir() to unlock on
failure."), vfs_mkdir() calls end_creating() on the input dentry on
failure and may replace it on success, so the post-call %pd can
use-after-free the dentry when CONFIG_OVERLAY_FS_DEBUG is enabled.

Print the dentry before the call and only the result afterward.

Reported-by: syzbot+ced26b784bf977d223dd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ced26b784bf977d223dd
Fixes: fe497f0759e0 ("VFS: change vfs_mkdir() to unlock on failure.")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Christian,

One more fallout from directory locking prep series reported by syzbot.
I have asked LLM to audit the kernel for other similar uses of %pd
post end_creating() and it did not find any.

Could you take this via vfs.fixes?

Thanks,
Amir.

 fs/overlayfs/overlayfs.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index e0d8c6152e9fc..7f3558372c599 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -254,8 +254,10 @@ static inline struct dentry *ovl_do_mkdir(struct ovl_fs *ofs,
 {
 	struct dentry *ret;
 
+	/* vfs_mkdir() drops @dentry on failure and may replace it on success */
+	pr_debug("mkdir(%pd2, 0%o)\n", dentry, mode);
 	ret = vfs_mkdir(ovl_upper_mnt_idmap(ofs), dir, dentry, mode, NULL);
-	pr_debug("mkdir(%pd2, 0%o) = %i\n", dentry, mode, PTR_ERR_OR_ZERO(ret));
+	pr_debug("...mkdir = %i\n", PTR_ERR_OR_ZERO(ret));
 	return ret;
 }
 
-- 
2.55.0

Re: [PATCH] ovl: fix UAF in ovl_do_mkdir() debug print

[Christian Brauner]

On Mon, 21 Sep 2026 12:40:13 +0200, Amir Goldstein wrote:
> ovl_do_mkdir() prints the input dentry with %pd after vfs_mkdir().
> Since commit fe497f0759e0 ("VFS: change vfs_mkdir() to unlock on
> failure."), vfs_mkdir() calls end_creating() on the input dentry on
> failure and may replace it on success, so the post-call %pd can
> use-after-free the dentry when CONFIG_OVERLAY_FS_DEBUG is enabled.
> 
> Print the dentry before the call and only the result afterward.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] ovl: fix UAF in ovl_do_mkdir() debug print
      https://git.kernel.org/vfs/vfs/c/ae146bc1abde