bpf: Reject non-negative offsets in stack_slot_obj_get_spi()

bpf_get_spi() computes (-off - 1) / BPF_REG_SIZE using C division,
which truncates toward zero. For off == 0, this produces spi 0, the
same index used by the valid stack slot at fp-8.

stack_slot_obj_get_spi() currently checks alignment and the resulting
spi bounds, but does not reject the non-negative offset itself. It can
therefore validate a PTR_TO_STACK register holding fp+0 against an
iterator stored at fp-8 even though the runtime receives the actual fp+0
pointer. An effectful iterator kfunc can then interpret memory outside
the BPF stack as iterator state.

Reject non-negative offsets before converting the offset to an spi. All
valid stack objects begin at a negative offset from the frame pointer.

Fixes: 06accc8779 ("bpf: add support for open-coded iterator loops")
Signed-off-by: Xu Yunxiang <xyx2021@mail.ustc.edu.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://lore.kernel.org/bpf/20260920210423.345636-2-xyx2021@mail.ustc.edu.cn

kernel/bpf/verifier.c

@@ -567,7 +567,7 @@ static int stack_slot_obj_get_spi(struct bpf_verifier_env *env, struct bpf_reg_s
 	}
 
 	off = reg->var_off.value;
-	if (off % BPF_REG_SIZE) {
+	if (off >= 0 || off % BPF_REG_SIZE) {
 		verbose(env, "cannot pass in %s at an offset=%d\n", obj_kind, off);
 		return -EINVAL;
 	}