Re: [PATCH] arm64: bpf: fix JIT stack setup

["Shi, Yang" <yang.shi@linaro.org>]

On 11/8/2015 2:29 PM, Z Lim wrote:
> On Sat, Nov 7, 2015 at 6:27 PM, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>> On Fri, Nov 06, 2015 at 09:36:17PM -0800, Yang Shi wrote:
>>> ARM64 JIT used FP (x29) as eBPF fp register, but FP is subjected to
>>> change during function call so it may cause the BPF prog stack base address
>>> change too. Whenever, it pointed to the bottom of BPF prog stack instead of
>>> the top.
>>>
>>> So, when copying data via bpf_probe_read, it will be copied to (SP - offset),
>>> then it may overwrite the saved FP/LR.
>>>
>>> Use x25 to replace FP as BPF stack base register (fp). Since x25 is callee
>>> saved register, so it will keep intact during function call.
>>> It is initialized in BPF prog prologue when BPF prog is started to run
>>> everytime. When BPF prog exits, it could be just tossed.
>>>
>>> Other than this the BPf prog stack base need to be setup before function
>>> call stack.
>>>
>>> So, the BPF stack layout looks like:
>>>
>>>                                   high
>>>           original A64_SP =>   0:+-----+ BPF prologue
>>>                                  |     | FP/LR and callee saved registers
>>>           BPF fp register => +64:+-----+
>>>                                  |     |
>>>                                  | ... | BPF prog stack
>>>                                  |     |
>>>                                  |     |
>>>           current A64_SP =>      +-----+
>>>                                  |     |
>>>                                  | ... | Function call stack
>>>                                  |     |
>>>                                  +-----+
>>>                                    low
>>>
>>> Signed-off-by: Yang Shi <yang.shi@linaro.org>
>>> CC: Zi Shen Lim <zlim.lnx@gmail.com>
>>> CC: Xi Wang <xi.wang@gmail.com>
>>
>> Thanks for tracking it down.
>> That looks like fundamental bug in arm64 jit. I'm surprised function calls worked at all.
>> Zi please review.
>>
>
> For function calls (BPF_JMP | BPF_CALL), we are compliant with AAPCS64
> [1]. That part is okay.
>
>
> bpf_probe_read accesses the BPF program stack, which is based on BPF_REG_FP.
>
> This exposes an issue with how BPF_REG_FP was setup, as Yang pointed out.
> Instead of having BPF_REG_FP point to top of stack, we erroneously
> point it to the bottom of stack. When there are function calls, we run
> the risk of clobbering of BPF stack. Bad idea.

Yes, exactly.

>
> Otherwise, since BPF_REG_FP is read-only, and is setup exactly once in
> prologue, it remains consistent throughout lifetime of the BPF
> program.
>
>
> Yang, can you please try the following?

It should work without the below change:

+       emit(A64_MOV(1, A64_FP, A64_SP), ctx);

I added it to stay align with ARMv8 AAPCS to maintain the correct FP 
during function call. It makes us get correct stack backtrace.

I think we'd better to keep compliant with ARMv8 AAPCS in BPF JIT 
prologue too.

If nobody thinks it is necessary, we definitely could remove that change.

Thanks,
Yang

>
> 8<-----
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -161,12 +161,12 @@ static void build_prologue(struct jit_ctx *ctx)
>          if (ctx->tmp_used)
>                  emit(A64_PUSH(tmp1, tmp2, A64_SP), ctx);
>
> -       /* Set up BPF stack */
> -       emit(A64_SUB_I(1, A64_SP, A64_SP, stack_size), ctx);
> -
>          /* Set up frame pointer */
>          emit(A64_MOV(1, fp, A64_SP), ctx);
>
> +       /* Set up BPF stack */
> +       emit(A64_SUB_I(1, A64_SP, A64_SP, stack_size), ctx);
> +
>          /* Clear registers A and X */
>          emit_a64_mov_i64(ra, 0, ctx);
>          emit_a64_mov_i64(rx, 0, ctx);
> ----->8
>
> [1] http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf
>