[PATCH] ocfs2: validate global bitmap cl_bpc before resize

[BUG]
A corrupted global bitmap inode can make online group extension scan past
the end of a group descriptor bitmap:

BUG: KASAN: use-after-free in _find_next_bit+0xef/0x120 lib/find_bit.c:157
Read of size 8 at addr ffff888021b52000 by task syz.0.34/409
Call Trace:
 <TASK>
 ...
 _find_next_bit+0xef/0x120 lib/find_bit.c:157
 find_next_bit include/linux/find.h:73 [inline]
 find_next_bit_le include/linux/find.h:518 [inline]
 ocfs2_find_max_contig_free_bits+0x53/0xb0 fs/ocfs2/suballoc.c:1292
 ocfs2_update_last_group_and_inode fs/ocfs2/resize.c:127 [inline]
 ocfs2_group_extend+0x83e/0x1ae0 fs/ocfs2/resize.c:350
 ocfs2_ioctl+0x175/0x6e0 fs/ocfs2/ioctl.c:869
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x197/0x1e0 fs/ioctl.c:583
 ...

[CAUSE]
ocfs2_group_extend() trusts the global bitmap dinode's cl_bpc value.
If its high byte corrupted from zero to 0xc9 makes cl_bpc 51457.
Extending by seven clusters narrows their product to a u16 value of
32519 and raises a 2048-bit group to 34567 bits, beyond its 32256-bit
bitmap. The subsequent maximum-free-run scan then reads into the next
page.

[FIX]
Reject a global bitmap whose cl_bpc is not one before using it in any
resize arithmetic. The global allocator has exactly one bitmap bit per
cluster, so this validates the invariant at the cold online-resize
boundary and reports metadata corruption instead of enlarging bg_bits
past the descriptor.

Fixes: d659072f7368 ("[PATCH 1/2] ocfs2: Add group extend for online resize")
Signed-off-by: ZhengYuan Huang <[email protected]>
---
 fs/ocfs2/resize.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/resize.c b/fs/ocfs2/resize.c
index 6375d5035972..556aaa319621 100644
--- a/fs/ocfs2/resize.c
+++ b/fs/ocfs2/resize.c
@@ -311,6 +311,14 @@ int ocfs2_group_extend(struct inode * inode, int new_clusters)
 		goto out_unlock;
 	}
 
+	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
+	if (cl_bpc != 1) {
+		ret = ocfs2_error(main_bm_inode->i_sb,
+				  "Invalid global bitmap bits per cluster %u\n",
+				  cl_bpc);
+		goto out_unlock;
+	}
+
 	if (le16_to_cpu(fe->id2.i_chain.cl_cpg) !=
 		ocfs2_group_bitmap_size(osb->sb, 0,
 					osb->s_feature_incompat) * 8) {
@@ -332,7 +340,6 @@ int ocfs2_group_extend(struct inode * inode, int new_clusters)
 	}
 	group = (struct ocfs2_group_desc *)group_bh->b_data;
 
-	cl_bpc = le16_to_cpu(fe->id2.i_chain.cl_bpc);
 	if (le16_to_cpu(group->bg_bits) / cl_bpc + new_clusters >
 		le16_to_cpu(fe->id2.i_chain.cl_cpg)) {
 		ret = -EINVAL;
-- 
2.43.0