bpf: fix check for attempt to corrupt spilled pointer

When register is spilled onto a stack as a 1/2/4-byte register, we set
slot_type[BPF_REG_SIZE - 1] (plus potentially few more below it,
depending on actual spill size). So to check if some stack slot has
spilled register we need to consult slot_type[7], not slot_type[0].

To avoid the need to remember and double-check this in the future, just
use is_spilled_reg() helper.

Fixes: 27113c5 ("bpf: Check the other end of slot_type for STACK_SPILL")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -4431,7 +4431,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 	 * so it's aligned access and [off, off + size) are within stack limits
 	 */
 	if (!env->allow_ptr_leaks &&
-	    state->stack[spi].slot_type[0] == STACK_SPILL &&
+	    is_spilled_reg(&state->stack[spi]) &&
 	    size != BPF_REG_SIZE) {
 		verbose(env, "attempt to corrupt spilled pointer on stack\n");
 		return -EACCES;