[PATCH v2] ocfs2: Invalidate inode if i_mode is zero after block read

[Ahmet Eray Karadag]

A panic occurs in ocfs2_unlink due to WARN_ON(inode->i_nlink == 0) when
handling a corrupted inode with i_mode=0 and i_nlink=0 in memory.

This "zombie" inode is created because ocfs2_read_locked_inode proceeds
even after ocfs2_validate_inode_block successfully validates a block
that structurally looks okay (passes checksum, signature etc.) but
contains semantically invalid data (specifically i_mode=0). The current
validation function doesn't check for i_mode being zero.

This results in an in-memory inode with i_mode=0 being added to the VFS
cache, which later triggers the panic during unlink.

Prevent this by adding an explicit check for i_mode == 0 within
ocfs2_validate_inode_block. If i_mode is zero, return -EFSCORRUPTED to signal
corruption. This causes the caller (ocfs2_read_locked_inode) to invoke
make_bad_inode(), correctly preventing the zombie inode from entering
the cache.

---
[RFC]:
The current fix handles i_mode=0 corruption detected during inode read
by returning -EFSCORRUPTED from ocfs2_validate_inode_block, which leads to
make_bad_inode() being called, preventing the corrupted inode from
entering the cache. This approach avoids immediately forcing the entire
filesystem read-only, assuming the corruption might be localized to
this inode.

Is this less aggressive error handling strategy appropriate for i_mode=0
corruption? Or is this condition considered severe enough that we *should*
explicitly call ocfs2_error() within the validation function to guarantee
the filesystem is marked read-only immediately upon detection?
Feedback and testing on the correct severity assessment and error
handling for this type of corruption would be appreciated.
---
v2:
 - Reviewed how ext4 handling same situation and we come up with this
   solution
---
Reported-by: syzbot+55c40ae8a0e5f3659f2b@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@xxxxxxxxx>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@xxxxxxxxx>
Signed-off-by: Ahmet Eray Karadag <eraykrdg1@xxxxxxxxx>
---
 fs/ocfs2/inode.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 14bf440ea4df..6c936f62b169 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1455,7 +1455,15 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 		     (unsigned long long)bh->b_blocknr);
 		goto bail;
 	}
-
+	if (di->i_links_count == 0) {
+		if (le16_to_cpu(di->i_mode) == 0 ||
+			!(le32_to_cpu(di->i_flags) & OCFS2_ORPHANED_FL)) {
+			mlog(ML_ERROR, "Invalid dinode #%llu: i_mode is zero!\n",
+			           (unsigned long long)bh->b_blocknr);
+			rc = -EFSCORRUPTED;
+			goto bail;
+		}
+	}
 	/*
 	 * Errors after here are fatal.
 	 */
-- 
2.43.0