[lvc-project] [PATCH v2] ocfs2: adjust ocfs2_xa_remove_entry() to match UBSAN boundary checks

[Dmitry Antipov]

After introducing 2f26f58df041 ("ocfs2: annotate flexible array members
with __counted_by_le()"), syzbot has reported the following issue:

UBSAN: array-index-out-of-bounds in fs/ocfs2/xattr.c:1955:3
index 2 is out of range for type 'struct ocfs2_xattr_entry[]
__counted_by(xh_count)' (aka 'struct ocfs2_xattr_entry[]')
...
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 ubsan_epilogue+0xa/0x40 lib/ubsan.c:233
 __ubsan_handle_out_of_bounds+0xe9/0xf0 lib/ubsan.c:455
 ocfs2_xa_remove_entry+0x36d/0x3e0 fs/ocfs2/xattr.c:1955
 ...

To address this issue, 'xh_entries[]' member removal should be performed
before actually changing 'xh_count', thus making sure that all array
accesses matches the boundary checks performed by UBSAN.

Reported-by: syzbot+cf96bc82a588a27346a8 at syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=cf96bc82a588a27346a8
Signed-off-by: Dmitry Antipov <dmantipov at yandex.ru>
---
v2: adjust commit metadata per Heming's notices
---
 fs/ocfs2/xattr.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ocfs2/xattr.c b/fs/ocfs2/xattr.c
index dc1761e84814..de99276d12f0 100644
--- a/fs/ocfs2/xattr.c
+++ b/fs/ocfs2/xattr.c
@@ -1941,8 +1941,7 @@ static void ocfs2_xa_remove_entry(struct ocfs2_xa_loc *loc)
 	ocfs2_xa_wipe_namevalue(loc);
 	loc->xl_entry = NULL;
 
-	le16_add_cpu(&xh->xh_count, -1);
-	count = le16_to_cpu(xh->xh_count);
+	count = le16_to_cpu(xh->xh_count) - 1;
 
 	/*
 	 * Only zero out the entry if there are more remaining.  This is
@@ -1957,6 +1956,8 @@ static void ocfs2_xa_remove_entry(struct ocfs2_xa_loc *loc)
 		memset(&xh->xh_entries[count], 0,
 		       sizeof(struct ocfs2_xattr_entry));
 	}
+
+	xh->xh_count = cpu_to_le16(count);
 }
 
 /*
-- 
2.52.0