[lvc-project] [PATCH 2/2] ocfs2: validate cl_bpc in allocator inodes to prevent divide-by-zero

[Dmitry Antipov]

From: Deepanshu Kartikey <kartikey406 at gmail.com>

The chain allocator field cl_bpc (blocks per cluster) is read from disk
and used in division operations without validation. A corrupted filesystem
image with cl_bpc=0 causes a divide-by-zero crash in the kernel:

  divide error: 0000 [#1] PREEMPT SMP KASAN
  RIP: 0010:ocfs2_bg_discontig_add_extent fs/ocfs2/suballoc.c:335 [inline]
  RIP: 0010:ocfs2_block_group_fill+0x5bd/0xa70 fs/ocfs2/suballoc.c:386
  Call Trace:
   ocfs2_block_group_alloc+0x7e9/0x1330 fs/ocfs2/suballoc.c:703
   ocfs2_reserve_suballoc_bits+0x20a6/0x4640 fs/ocfs2/suballoc.c:834
   ocfs2_reserve_new_inode+0x4f4/0xcc0 fs/ocfs2/suballoc.c:1074
   ocfs2_mknod+0x83c/0x2050 fs/ocfs2/namei.c:306

This patch adds validation in ocfs2_validate_inode_block() to ensure cl_bpc
matches the expected value calculated from the superblock's cluster size
and block size for chain allocator inodes (identified by OCFS2_CHAIN_FL).

Moving the validation to inode validation time (rather than allocation time)
has several benefits:
- Validates once when the inode is read, rather than on every allocation
- Protects all code paths that use cl_bpc (allocation, resize, etc.)
- Follows the existing pattern of inode validation in OCFS2
- Centralizes validation logic

The validation catches both:
- Zero values that cause divide-by-zero crashes
- Non-zero but incorrect values indicating filesystem corruption or
  mismatched filesystem geometry

With this fix, mounting a corrupted filesystem produces:

  OCFS2: ERROR (device loop0): ocfs2_validate_inode_block: Inode 74
         has corrupted cl_bpc: ondisk=0 expected=16

instead of a kernel crash.

Link: https://lore.kernel.org/ocfs2-devel/20251026132625.12348-1-kartikey406@gmail.com/T/#u [v1]
Link: https://lore.kernel.org/all/20251027124131.10002-1-kartikey406@gmail.com/T/ [v2]
Reported-by: syzbot+fd8af97c7227fe605d95 at syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fd8af97c7227fe605d95
Tested-by: syzbot+fd8af97c7227fe605d95 at syzkaller.appspotmail.com
Suggested-by: Joseph Qi <joseph.qi at linux.alibaba.com>
Signed-off-by: Deepanshu Kartikey <kartikey406 at gmail.com>
[dmantipov: combine into the series and tweak
 the message to fit the commonly used style]
Signed-off-by: Dmitry Antipov <dmantipov at yandex.ru>
---
 fs/ocfs2/inode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 1b6bdd9d7755..efb930da0920 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1505,6 +1505,8 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 
 	if (le32_to_cpu(di->i_flags) & OCFS2_CHAIN_FL) {
 		struct ocfs2_chain_list *cl = &di->id2.i_chain;
+		u16 bpc = 1 << (OCFS2_SB(sb)->s_clustersize_bits -
+				sb->s_blocksize_bits);
 
 		if (le16_to_cpu(cl->cl_count) != ocfs2_chain_recs_per_inode(sb)) {
 			rc = ocfs2_error(sb, "Invalid dinode %llu: chain list count %u\n",
@@ -1518,6 +1520,12 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 					 le16_to_cpu(cl->cl_next_free_rec));
 			goto bail;
 		}
+		if (le16_to_cpu(cl->cl_bpc) != bpc) {
+			rc = ocfs2_error(sb, "Invalid dinode %llu: bits per cluster %u\n",
+					 (unsigned long long)bh->b_blocknr,
+					 le16_to_cpu(cl->cl_bpc));
+			goto bail;
+		}
 	}
 
 	rc = 0;
-- 
2.51.0