qcow2: Check min_size in qcow2_grow_l1_table()

First, new_l1_size is an int64_t, whereas min_size is a uint64_t.
Therefore, during the loop which adjusts new_l1_size until it equals or
exceeds min_size, new_l1_size might overflow and become negative. The
comparison in the loop condition however will take it as an unsigned
value (because min_size is unsigned) and therefore recognize it as
exceeding min_size. Therefore, the loop is left with a negative
new_l1_size, which is not correct. This could be fixed by making
new_l1_size uint64_t.

On the other hand, however, by doing this, the while loop may take
forever. If min_size is e.g. UINT64_MAX, it will take new_l1_size
probably multiple overflows to reach the exact same value (if it reaches
it at all). Then, right after the loop, new_l1_size will be recognized
as being too big anyway.

Both problems require a ridiculously high min_size value, which is very
unlikely to occur; but both problems are also simply avoided by checking
whether min_size is sane before calculating new_l1_size (which should
still be checked separately, though).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -42,6 +42,13 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
     if (min_size <= s->l1_size)
         return 0;
 
+    /* Do a sanity check on min_size before trying to calculate new_l1_size
+     * (this prevents overflows during the while loop for the calculation of
+     * new_l1_size) */
+    if (min_size > INT_MAX / sizeof(uint64_t)) {
+        return -EFBIG;
+    }
+
     if (exact_size) {
         new_l1_size = min_size;
     } else {