[BUG]
On-disk corruption setting l_next_free_rec to 0 in an inode's inline
extent list triggers a UBSAN panic on the next write to that file.

[CAUSE]
ocfs2_sum_rightmost_rec() computes
i = le16_to_cpu(el->l_next_free_rec) - 1
and accesses el->l_recs[i] without validating i. When l_next_free_rec
is 0, i becomes -1; when l_next_free_rec exceeds l_count, i falls
past the end of the array. Either case violates the
__counted_by_le(l_count) annotation on l_recs[] and triggers UBSAN.

[FIX]
Validate the inode's inline extent list when the inode is read, in
ocfs2_validate_inode_block(): l_count must be non-zero and no larger
than the inode block can hold, and l_next_free_rec must not exceed
l_count. A corrupt list is rejected at read time, before the b-tree
code can index l_recs[] out of bounds.

Fixes: 2f26f58df041 ("ocfs2: annotate flexible array members with __counted_by_le()")
Reported-by: syzbot+be16e33db01e6644db7a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=be16e33db01e6644db7a
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
Changes in v2:
 - Reject the corrupt inode at read time by validating its inline extent
   list (l_count, l_next_free_rec) in ocfs2_validate_inode_block().

v1: https://lore.kernel.org/all/ah2ljwKRw-Xsi4Ga@dev/

 fs/ocfs2/inode.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index a510a0eb1adc..aff95efd78e7 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1559,6 +1559,38 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 		goto bail;
 	}
 
+	if (ocfs2_dinode_has_extents(di)) {
+		struct ocfs2_extent_list *el = &di->id2.i_list;
+		u16 count = le16_to_cpu(el->l_count);
+		u16 next_free = le16_to_cpu(el->l_next_free_rec);
+
+		if (count == 0) {
+			rc = ocfs2_error(sb,
+					 "Invalid dinode %llu: extent list l_count is zero\n",
+					 (unsigned long long)bh->b_blocknr);
+			goto bail;
+		}
+		/*
+		 * The exact capacity depends on i_xattr_inline_size, another
+		 * unvalidated on-disk field. Inline xattrs only shrink the
+		 * list, so the no-xattr maximum is a safe upper bound that a
+		 * valid l_count never exceeds.
+		 */
+		if (count > ocfs2_extent_recs_per_inode(sb)) {
+			rc = ocfs2_error(sb,
+					 "Invalid dinode %llu: extent list l_count %u exceeds max %u\n",
+					 (unsigned long long)bh->b_blocknr, count,
+					 ocfs2_extent_recs_per_inode(sb));
+			goto bail;
+		}
+		if (next_free > count) {
+			rc = ocfs2_error(sb,
+					 "Invalid dinode %llu: extent list l_next_free_rec %u exceeds l_count %u\n",
+					 (unsigned long long)bh->b_blocknr, next_free, count);
+			goto bail;
+		}
+	}
+
 	rc = 0;
 
 bail:
-- 
2.47.3