linux-kernel - [PATCH v3] ocfs2: validate cl_bpc in allocator inodes to prevent divide-by-zero

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Message-ID: <20251028042919.18704-1-kartikey406@gmail.com>
Date: Tue, 28 Oct 2025 09:59:19 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: mark@...heh.com,
	jlbec@...lplan.org,
	joseph.qi@...ux.alibaba.com,
	heming.zhao@...e.com
Cc: ocfs2-devel@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Deepanshu Kartikey <kartikey406@...il.com>,
	syzbot+fd8af97c7227fe605d95@...kaller.appspotmail.com
Subject: [PATCH v3] ocfs2: validate cl_bpc in allocator inodes to prevent divide-by-zero

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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fd8af97c7227fe605d95
Tested-by: syzbot+fd8af97c7227fe605d95@...kaller.appspotmail.com
Suggested-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
Changes in v3:
- Removed unnecessary local osb variable as suggested by Joseph Qi
- Added blank line before rc = 0 for better readability

Changes in v2:
- Moved validation from ocfs2_block_group_alloc() to ocfs2_validate_inode_block()
  as suggested by Joseph Qi to benefit all code paths
- Added OCFS2_CHAIN_FL check to only validate chain allocator inodes
- Updated commit message to reflect the new location
---
 fs/ocfs2/inode.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index fcc89856ab95..dbbfe72b1dc7 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1502,6 +1502,21 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 				 le16_to_cpu(di->i_suballoc_slot));
 		goto bail;
 	}
+	/* Validate cl_bpc for chain allocator inodes */
+	if (le32_to_cpu(di->i_flags) & OCFS2_CHAIN_FL) {
+		struct ocfs2_chain_list *cl = &di->id2.i_chain;
+		u16 cl_bpc = le16_to_cpu(cl->cl_bpc);
+		u16 expected_bpc = 1 << (OCFS2_SB(sb)->s_clustersize_bits -
+					 sb->s_blocksize_bits);
+
+		if (cl_bpc != expected_bpc) {
+			rc = ocfs2_error(sb,
+				"Inode %llu has corrupted cl_bpc: ondisk=%u expected=%u\n",
+				(unsigned long long)bh->b_blocknr,
+				cl_bpc, expected_bpc);
+			goto bail;
+		}
+	}
 
 	rc = 0;
 
-- 
2.43.0