[PATCH 1/2] ext2: validate i_nlink before decrement in ext2_unlink()

[Vasiliy Kovalev <kovalev@altlinux.org>]

A crafted ext2 image can provide a directory entry pointing to an inode
with i_links_count == 0 on disk. Calling unlink() on such an entry
triggers WARN_ON inside drop_nlink():

WARNING: CPU: 3 PID: 609 at fs/inode.c:336 drop_nlink+0xad/0xd0 fs/inode.c:336
CPU: 3 UID: 0 PID: 609 Comm: syz-executor Not tainted 6.12.77+ #1
Call Trace:
 <TASK>
 inode_dec_link_count include/linux/fs.h:2518 [inline]
 ext2_unlink+0x26c/0x300 fs/ext2/namei.c:295
 vfs_unlink+0x2fc/0x9b0 fs/namei.c:4477
 do_unlinkat+0x53e/0x730 fs/namei.c:4541
 __do_sys_unlink fs/namei.c:4589 [inline]
 __se_sys_unlink fs/namei.c:4587 [inline]
 __x64_sys_unlink+0xc6/0x110 fs/namei.c:4587
 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>

At the point of the crash, ext2_delete_entry() has already committed
the removal of the directory entry to disk, so returning an error is
not an option. Instead, skip the decrement and report the corruption
via ext2_error(), which marks the superblock as having errors. The
inode will be reclaimed when its last reference is dropped.

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

Cc: stable@vger.kernel.org
Fixes: a513b035eadf ("[PATCH] ext2: switch to inode_inc_count, inode_dec_count")
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
---
 fs/ext2/namei.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index bde617a66cec..ea49e8f2b292 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -293,7 +293,12 @@ static int ext2_unlink(struct inode *dir, struct dentry *dentry)
 		goto out;
 
 	inode_set_ctime_to_ts(inode, inode_get_ctime(dir));
-	inode_dec_link_count(inode);
+	if (!inode->i_nlink)
+		ext2_error(inode->i_sb, __func__,
+			   "inode %lu has zero i_nlink on unlink, fs may be corrupt",
+			   inode->i_ino);
+	else
+		inode_dec_link_count(inode);
 	err = 0;
 out:
 	return err;
-- 
2.50.1