[PATCH 2/2] ext2: guard against zero i_nlink on new_inode in ext2_rename()

[Vasiliy Kovalev <kovalev@altlinux.org>]

A crafted ext2 image can provide a target inode with i_links_count == 0
on disk. When rename() resolves to an existing target, ext2_rename()
calls drop_nlink(new_inode) for the directory case and
inode_dec_link_count(new_inode) unconditionally. Both reach
drop_nlink(), which triggers WARN_ON:

WARNING: CPU: 0 PID: 646 at fs/inode.c:336 drop_nlink+0xad/0xd0 fs/inode.c:336
CPU: 0 UID: 0 PID: 646 Comm: syz.0.17 Not tainted 6.12.77+ #1
Call Trace:
 <TASK>
 inode_dec_link_count include/linux/fs.h:2518 [inline]
 ext2_rename+0x35e/0x850 fs/ext2/namei.c:374
 vfs_rename+0xf2f/0x2060 fs/namei.c:5021
 do_renameat2+0xbe2/0xd50 fs/namei.c:5178
 __do_sys_rename fs/namei.c:5225 [inline]
 __se_sys_rename fs/namei.c:5223 [inline]
 __x64_sys_rename+0x7e/0xa0 fs/namei.c:5223
 do_syscall_x64 arch/x86/entry/common.c:47 [inline]
 do_syscall_64+0xf5/0x220 arch/x86/entry/common.c:78
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

No disk state has been modified at this point in the function, so
return -EFSCORRUPTED after reporting the corruption via ext2_error().

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Cc: stable@vger.kernel.org
Fixes: 9a53c3a783c2 ("[PATCH] r/o bind mounts: unlink: monitor i_nlink")
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
---
 fs/ext2/namei.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index ea49e8f2b292..419e844f2e54 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -334,6 +334,13 @@ static int ext2_rename (struct mnt_idmap * idmap,
 	bool old_is_dir = S_ISDIR(old_inode->i_mode);
 	int err;
 
+	if (new_inode && new_inode->i_nlink == 0) {
+		ext2_error(old_dir->i_sb, __func__,
+			   "target inode %lu has zero i_nlink, filesystem may be corrupt",
+			   new_inode->i_ino);
+		return -EFSCORRUPTED;
+	}
+
 	if (flags & ~RENAME_NOREPLACE)
 		return -EINVAL;
 
-- 
2.50.1