squashfs: harden sanity check in squashfs_read_xattr_id_table

While mounting a corrupted filesystem, a signed integer '*xattr_ids' can
become less than zero.  This leads to the incorrect computation of 'len'
and 'indexes' values which can cause null-ptr-deref in copy_bio_to_actor()
or out-of-bounds accesses in the next sanity checks inside
squashfs_read_xattr_id_table().

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Link: https://lkml.kernel.org/r/20230117105226.329303-2-pchelkin@ispras.ru
Fixes: 506220d ("squashfs: add more sanity checks in xattr id lookup")
Reported-by: <syzbot+082fa4af80a5bb1a9843@syzkaller.appspotmail.com>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/squashfs/xattr_id.c

@@ -76,7 +76,7 @@ __le64 *squashfs_read_xattr_id_table(struct super_block *sb, u64 table_start,
 	/* Sanity check values */
 
 	/* there is always at least one xattr id */
-	if (*xattr_ids == 0)
+	if (*xattr_ids <= 0)
 		return ERR_PTR(-EINVAL);
 
 	len = SQUASHFS_XATTR_BLOCK_BYTES(*xattr_ids);