bpf: account for current allocated stack depth in widen_imprecise_scalars()

The usage pattern for widen_imprecise_scalars() looks as follows:

    prev_st = find_prev_entry(env, ...);
    queued_st = push_stack(...);
    widen_imprecise_scalars(env, prev_st, queued_st);

Where prev_st is an ancestor of the queued_st in the explored states
tree. This ancestor is not guaranteed to have same allocated stack
depth as queued_st. E.g. in the following case:

    def main():
      for i in 1..2:
        foo(i)        // same callsite, differnt param

    def foo(i):
      if i == 1:
        use 128 bytes of stack
      iterator based loop

Here, for a second 'foo' call prev_st->allocated_stack is 128,
while queued_st->allocated_stack is much smaller.
widen_imprecise_scalars() needs to take this into account and avoid
accessing bpf_verifier_state->frame[*]->stack out of bounds.

Fixes: 2793a8b ("bpf: exact states comparison for iterator convergence checks")
Reported-by: Emil Tsalapatis <[email protected]>
Signed-off-by: Eduard Zingerman <[email protected]>

Author: eddyz87

kernel/bpf/verifier.c

@@ -8866,7 +8866,7 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
 				   struct bpf_verifier_state *cur)
 {
 	struct bpf_func_state *fold, *fcur;
-	int i, fr;
+	int i, fr, num_slots;
 
 	reset_idmap_scratch(env);
 	for (fr = old->curframe; fr >= 0; fr--) {
@@ -8879,7 +8879,9 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
 					&fcur->regs[i],
 					&env->idmap_scratch);
 
-		for (i = 0; i < fold->allocated_stack / BPF_REG_SIZE; i++) {
+		num_slots = min(fold->allocated_stack / BPF_REG_SIZE,
+				fcur->allocated_stack / BPF_REG_SIZE);
+		for (i = 0; i < num_slots; i++) {
 			if (!is_spilled_reg(&fold->stack[i]) ||
 			    !is_spilled_reg(&fcur->stack[i]))
 				continue;