Re: [PATCH 2/2] osfs2: Fix kernel BUG in ocfs2_write_cluster

From: heming.zhao@xxxxxxxx
Date: Thu Sep 19 2024 - 03:44:52 EST




On 9/19/24 01:20, Mohammed Anees wrote:
syzbot has found a kernel BUG in ocfs2_write_cluster_by_desc [1].

The issue arises because ocfs2_insert_extent receives start_blk
as 0, which incorrectly maps to a physical address of 0. This
occurs when block is 0 after the call to ocfs2_clusters_to_blocks
which is invoked inside the ocfs2_add_clusters_in_btree. The block
value is then passed to ocfs2_insert_extent, leading to the problem.

[1] https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6

Reported-and-tested-by: syzbot+18a87160c7d64ba2e2f6@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
Signed-off-by: Mohammed Anees <pvmohammedanees2003@xxxxxxxxx>
---
fs/ocfs2/alloc.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/alloc.c b/fs/ocfs2/alloc.c
index 395e23920..926ffeed8 100644
--- a/fs/ocfs2/alloc.c
+++ b/fs/ocfs2/alloc.c
@@ -4843,6 +4843,13 @@ int ocfs2_add_clusters_in_btree(handle_t *handle,
}
block = ocfs2_clusters_to_blocks(osb->sb, bit_off);> + if (block == 0) {
+ mlog(ML_ERROR, "Conversion resulted in zero block number");
+ status = -EIO;
+ need_free = 1;
+ goto bail;
+ }
+

If you check this function, there is no IO operation, so -EIO is not suitable.
In the the ocfs2_clusters_to_blocks() code, there are two possible cases where
the result is zero:
bit_off is 0 or bit_off is out of range for a u64 after a bit shift.
It seems that the root cause is that __ocfs2_claim_clusters allocates an
incorrect bit_off.

-Heming

trace_ocfs2_add_clusters_in_btree(
(unsigned long long)ocfs2_metadata_cache_owner(et->et_ci),
bit_off, num_bits);