bcachefs: slab-use-after-free Read in bch2_sb_errors_from_cpu

Acquire fsck_error_counts_lock before accessing the critical section
protected by this lock.

syzbot has tested the proposed patch and the reproducer did not trigger
any issue.

Reported-by: syzbot+a2bc0e838efd7663f4d9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a2bc0e838efd7663f4d9
Signed-off-by: Pei Li <peili.dev@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: peilii

fs/bcachefs/sb-errors.c

@@ -110,19 +110,25 @@ void bch2_sb_error_count(struct bch_fs *c, enum bch_sb_error_id err)
 void bch2_sb_errors_from_cpu(struct bch_fs *c)
 {
 	bch_sb_errors_cpu *src = &c->fsck_error_counts;
-	struct bch_sb_field_errors *dst =
-		bch2_sb_field_resize(&c->disk_sb, errors,
-				     bch2_sb_field_errors_u64s(src->nr));
+	struct bch_sb_field_errors *dst;
 	unsigned i;
 
+	mutex_lock(&c->fsck_error_counts_lock);
+
+	dst = bch2_sb_field_resize(&c->disk_sb, errors,
+				   bch2_sb_field_errors_u64s(src->nr));
+
 	if (!dst)
-		return;
+		goto err;
 
 	for (i = 0; i < src->nr; i++) {
 		SET_BCH_SB_ERROR_ENTRY_ID(&dst->entries[i], src->data[i].id);
 		SET_BCH_SB_ERROR_ENTRY_NR(&dst->entries[i], src->data[i].nr);
 		dst->entries[i].last_error_time = cpu_to_le64(src->data[i].last_error_time);
 	}
+
+err:
+	mutex_unlock(&c->fsck_error_counts_lock);
 }
 
 static int bch2_sb_errors_to_cpu(struct bch_fs *c)