[PATCH] ocfs2: Add check for total number of chains in chain list

[Prithvi Tambewagh <activprithvi@gmail.com>]

The functions ocfs2_reserve_suballoc_bits(), ocfs2_block_group_alloc(),
ocfs2_block_group_alloc_contig() and ocfs2_find_smallest_chain() trust
the on-disk values related to the allocation chain. However, KASAN bug
was triggered in these functions, and the kernel panicked when accessing
redzoned memory. This occurred due to the corrupted value of `cl_count`
field of `struct ocfs2_chain_list`. Upon analysis, the value of `cl_count`
was observed to be overwhemingly large, due to which the code accessed
redzoned memory.

The fix introduces an if statement which validates value of `cl_count`
(both lower and upper bounds). Lower bound check ensures the value of
`cl_count` is not zero and upper bound check ensures that the value of
`cl_count` is in the range such that it has a value less than the total
size of struct ocfs2_chain_list and maximum number of chains that can be
present, so as to fill one block.

Reported-by: syzbot+af14efe17dfa46173239@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=af14efe17dfa46173239
Tested-by: syzbot+af14efe17dfa46173239@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
---
 fs/ocfs2/suballoc.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
index f7b483f0de2a..7ea63e9cc4f8 100644
--- a/fs/ocfs2/suballoc.c
+++ b/fs/ocfs2/suballoc.c
@@ -671,6 +671,21 @@ static int ocfs2_block_group_alloc(struct ocfs2_super *osb,
 	BUG_ON(ocfs2_is_cluster_bitmap(alloc_inode));
 
 	cl = &fe->id2.i_chain;
+	unsigned int block_size = osb->sb->s_blocksize;
+	unsigned int max_cl_count =
+	(block_size - offsetof(struct ocfs2_chain_list, cl_recs)) /
+	sizeof(struct ocfs2_chain_rec);
+
+	if (!le16_to_cpu(cl->cl_count) ||
+	    le16_to_cpu(cl->cl_count) > max_cl_count) {
+		ocfs2_error(osb->sb,
+			    "Invalid chain list: cl_count %u "
+			    "exceeds max %u",
+			    le16_to_cpu(cl->cl_count), max_cl_count);
+		status = -EIO;
+		goto bail;
+	}
+
 	status = ocfs2_reserve_clusters_with_limit(osb,
 						   le16_to_cpu(cl->cl_cpg),
 						   max_block, flags, &ac);

base-commit: 36c254515dc6592c44db77b84908358979dd6b50
-- 
2.34.1