Forwarded: [PATCH] nsfs: reject file handles with invalid inode number

syzbot
2025-09-21T03:19:09Z

Subject: [PATCH] nsfs: reject file handles with invalid inode number
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

Reject nsfs file handles that claim to have inode number 0, as no
legitimate namespace can have inode 0. This prevents a warning in
nsfs_fh_to_dentry() when open_by_handle_at() is called with malformed
file handles.

The issue occurs when userspace provides a file handle with valid
namespace type and ID but claims the namespace has inode number 0.
The namespace lookup succeeds but triggers VFS_WARN_ON_ONCE() when
comparing the real inode number against the impossible claim of 0.

Since inode 0 is reserved in all filesystems and no namespace can
legitimately have inode 0, we can safely reject such handles early
to prevent reaching the consistency check that triggers the warning.

Reported-by: syzbot+9eefe09bedd093f156c2@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
fs/nsfs.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 32cb8c835a2b..42672cec293c 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -469,7 +469,8 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,

if (fh_len < NSFS_FID_SIZE_U32_VER0)
return NULL;
-
+ if (fid->ns_inum == 0)
+ return NULL;
/* Check that any trailing bytes are zero. */
if ((fh_len > NSFS_FID_SIZE_U32_LATEST) &&
memchr_inv((void *)fid + NSFS_FID_SIZE_U32_LATEST, 0,
--
2.43.0