[PATCH v6] ocfs2: Invalidate inode if i_mode is zero after block read

[Ahmet Eray Karadag <eraykrdg1@gmail.com>]

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, i_nlink == 0, non-orphan)
within ocfs2_validate_inode_block. If the check is true, 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.

Reported-by: syzbot+55c40ae8a0e5f3659f2b@syzkaller.appspotmail.com
Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
Previous link: https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/
---
v2:
 - Only checking either i_links_count == 0 or i_mode == 0
 - Not performing le16_to_cpu() anymore
 - Tested with ocfs2-test
---
v3:
 - Add checking both high and low bits of i_links_count
---
v4:
 - Reading i_links_count hi and low bits without helper function
  to save few cpu cycles
---
v5:
 - Clear i_links_count check
 - Log the actual i_nlink/i_mode
---
v6:
 - Use `ocfs2_read_links_count` to get nlink
 - Convert di->imode to cpu endian
---
 fs/ocfs2/inode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 14bf440ea4df..f83b8e4f4d8b 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1456,6 +1456,14 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 		goto bail;
 	}
 
+	if ((!di->i_links_count && !di->i_links_count_hi) || !di->i_mode) {
+		mlog(ML_ERROR, "Invalid dinode #%llu: "
+			"Corrupt state (nlink = %u or mode = %u) detected!\n",
+		        (unsigned long long)bh->b_blocknr,
+			ocfs2_read_links_count(di), le16_to_cpu(di->i_mode));
+		rc = -EFSCORRUPTED;
+		goto bail;
+	}
 	/*
 	 * Errors after here are fatal.
 	 */
-- 
2.43.0