[PATCH v2] ocfs2: update xattr count before moving bucket entries

[Su Yue <glass.su@suse.com>]

Since commit 2f26f58df041 ("ocfs2: annotate flexible array members
with __counted_by_le()"), the xh_entries array is annotated with
__counted_by_le(xh_count), so FORTIFY uses xh_count to determine its
bounds. When inserting an entry into a bucket, ocfs2_xa_bucket_add_entry()
shifts existing entries before incrementing xh_count.
The destination therefore extends one entry past the bounds described
by the old count.

With CONFIG_CC_HAS_COUNTED_BY and CONFIG_FORTIFY_SOURCE enabled,
ocfs2-test: single_run-WIP.sh -t reflink triggers the following failure
while adding an extended attribute:

[  150.156484] memmove: detected buffer overflow: 352 byte write of buffer size 336
[  150.156487] WARNING: lib/string_helpers.c:1036 at __fortify_report+0x3d/0x50, CPU#15: reflink_test/2336
[  150.160496] RIP: 0010:__fortify_report+0x40/0x50
[  150.164031] Call Trace:
[  150.164141]  <TASK>
[  150.164232]  __fortify_panic+0x9/0xb
[  150.164383]  ocfs2_xa_bucket_add_entry.cold+0x17/0x28 [ocfs2]
[  150.164661]  ocfs2_xa_set+0x8fb/0xf40 [ocfs2]

Increment xh_count before memmove() so the destination bounds include
the new entry. Keep the local count unchanged to calculate the insertion
position and move length from the original number of entries. The caller
has already checked that there is enough space for the new entry.

Signed-off-by: Su Yue <glass.su@suse.com>
---
Changelog:
v2:
   Remove fixes tag, mention the commit id in message.
---
 fs/ocfs2/xattr.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/xattr.c b/fs/ocfs2/xattr.c
index 35bcbb0ff607b..49d82ea5a1271 100644
--- a/fs/ocfs2/xattr.c
+++ b/fs/ocfs2/xattr.c
@@ -2076,12 +2076,17 @@ static void ocfs2_xa_bucket_add_entry(struct ocfs2_xa_loc *loc, u32 name_hash)
 		}
 	}
 
+	/*
+	 * Increment xh_count before memmove() so __counted_by_le(xh_count)
+	 * includes the new entry in the destination bounds.
+	 */
+	le16_add_cpu(&xh->xh_count, 1);
+
 	if (low != count)
 		memmove(&xh->xh_entries[low + 1],
 			&xh->xh_entries[low],
 			((count - low) * sizeof(struct ocfs2_xattr_entry)));
 
-	le16_add_cpu(&xh->xh_count, 1);
 	loc->xl_entry = &xh->xh_entries[low];
 	memset(loc->xl_entry, 0, sizeof(struct ocfs2_xattr_entry));
 }
-- 
2.55.0