[PATCH v2] ocfs2: fix kernel BUG in ocfs2_find_victim_chain

[Prithvi Tambewagh <activprithvi@gmail.com>]

syzbot reported a kernel BUG in ocfs2_find_victim_chain() because the
`cl_next_free_rec` field of the allocation chain list is 0, triggring the
BUG_ON(!cl->cl_next_free_rec) condition and panicking the kernel.

To fix this, `cl_next_free_rec` is checked inside the caller of
ocfs2_find_victim_chain() i.e. ocfs2_claim_suballoc_bits() and if it is
equal to 0, ocfs2_error() is called, to log the corruption and force the
filesystem into read-only mode, to prevent further damage.

Reported-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
Tested-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
---
v1->v2:
 - Remove extra line before the if statement in patch
 - Add upper limit check for cl->cl_next_free_rec in the if condition

 fs/ocfs2/suballoc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
index 6ac4dcd54588..1257c39c2c11 100644
--- a/fs/ocfs2/suballoc.c
+++ b/fs/ocfs2/suballoc.c
@@ -1992,6 +1992,13 @@ static int ocfs2_claim_suballoc_bits(struct ocfs2_alloc_context *ac,
 	}
 
 	cl = (struct ocfs2_chain_list *) &fe->id2.i_chain;
+	if (!le16_to_cpu(cl->cl_next_free_rec) ||
+	    le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)) {
+		status = ocfs2_error(ac->ac_inode->i_sb,
+					"Chain allocator dinode %llu has 0 chains\n",
+					(unsigned long long)le64_to_cpu(fe->i_blkno));
+		goto bail;
+	}
 
 	victim = ocfs2_find_victim_chain(cl);
 	ac->ac_chain = victim;

base-commit: 939f15e640f193616691d3bcde0089760e75b0d3
-- 
2.34.1