On Wed, Mar 20, 2024 at 5:28 AM Tao Lyu <tao.lyu@xxxxxxx> wrote:
>
> Signed-off-by: Tao Lyu <tao.lyu@xxxxxxx>
> ---
> kernel/bpf/verifier.c | 1 +
> 1 file changed, 1 insertion(+)
>
> Hi,
>
> I am sorry to bother you due to my confusion on constraints about stack writes.
>
> 1. When an instruction stores 64-bit values onto the stack with fixed offset and BPF_CAP only,
> the verifier marks the stack slot type as STACK_SPILL, no matter whether they are scalar or pointers.
>
> 2. Then, a store instruction with a **32-bit scalar value** on the same stack slot leads to a verification rejection.
> As it says, this might corrupt the stack pointer by asserting if the stack slot type is STACK_SPILL.
> However, even if the stack slot type is STACK_SPILL, it might store a 64-bit scalar and not a stack pointer.
> IMHO, this "issue" might originate from the incomplete conditions in the check_stack_write_fixed_off() function below.
> It only checks if the stack slot is a spilled register but forgets to check if the spilled register type is a pointer.
>
> 4479 static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> ...
> 4494 if (!env->allow_ptr_leaks &&
> 4495 is_spilled_reg(&state->stack[spi]) &&
> 4496 size != BPF_REG_SIZE) {
> 4497 verbose(env, "attempt to corrupt spilled pointer on stack\n");
> 4498 return -EACCES;
> 4499 }
> ...
> 4600 }
>
> Below is an example bpf program, which stores a 64-bit and 32-bit immediate value on the same stack slot.
> But the second one gets rejected due to the above.
>
> 0: R1=ctx() R10=fp0
> ; asm volatile ( @ repro.bpf.c:679
> 0: (7a) *(u64 *)(r10 -8) = 1 ; R10=fp0 fp-8_w=1
> 1: (62) *(u32 *)(r10 -8) = 1
> attempt to corrupt spilled pointer on stack
> processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0.
>
> If my understanding is correct, then we can apply the attached patch.
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index de7813947981..65f7eb315e9c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4493,6 +4493,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> */
> if (!env->allow_ptr_leaks &&
> is_spilled_reg(&state->stack[spi]) &&
> + state->stack[spi].spilled_ptr.type != SCALAR_VALUE &&
I think it's possible to easily convert PTR_TO_MEM kind of register to
SCALAR_VALUE through arithmetic operations, and so allowing to spill
SCALAR_VALUE to stack is basically just as dangerous as spilling
PTR_TO_MEM directly.
So it feels a bit dangerous to do this.
pw-bot: cr
> size != BPF_REG_SIZE) {
> verbose(env, "attempt to corrupt spilled pointer on stack\n");
> return -EACCES;
> --
> 2.25.1
>
