Re: [PATCH] bpf: validate bpf_func when BPF_JIT is enabled

[Yonghong Song <yhs@fb.com>]

On 9/10/19 6:22 PM, Sami Tolvanen wrote:
> On Tue, Sep 10, 2019 at 08:37:19AM +0000, Yonghong Song wrote:
>> You did not mention BPF_BINARY_HEADER_MAGIC and added member
>> of `magic` in bpf_binary_header. Could you add some details
>> on what is the purpose for this `magic` member?
> 
> Sure, I'll add a description to the next version.
> 
> The magic is a random number used to identify bpf_binary_header in
> memory. The purpose of this patch is to limit the possible call
> targets for the function pointer and checking for the magic helps
> ensure we are jumping to a page that contains a jited function,
> instead of allowing calls to arbitrary targets.
> 
> This is particularly useful when combined with the compiler-based
> Control-Flow Integrity (CFI) mitigation, which Google started shipping
> in Pixel kernels last year. The compiler injects checks to all
> indirect calls, but cannot obviously validate jumps to dynamically
> generated code.
> 
>>> +unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
>>> +{
>>> +	const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
>>> +
>>> +	if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
>>> +		return prog->bpf_func(ctx, prog->insnsi);
>>> +
>>> +	if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
>>> +		     !arch_bpf_jit_check_func(prog))) {
>>> +		WARN(1, "attempt to jump to an invalid address");
>>> +		return 0;
>>> +	}
>>> +
>>> +	return prog->bpf_func(ctx, prog->insnsi);
>>> +}
> 
>> The above can be rewritten as
>> 	if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || prog->jited ||
>> 	    hdr->magic != BPF_BINARY_HEADER_MAGIC ||
>> 	    !arch_bpf_jit_check_func(prog))) {
>> 		WARN(1, "attempt to jump to an invalid address");
>> 		return 0;
>> 	}
> 
> That doesn't look quite equivalent, but yes, this can be rewritten as a

Indeed, I made a mistake. Your below change is correct.

> single if statement like this:
> 
> 	if ((IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
> 	     prog->jited) &&
> 	    (hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> 	     !arch_bpf_jit_check_func(prog)))
> 
> I think splitting the interpreter and JIT paths would be more readable,
> but I can certainly change this if you prefer.

How about this:

	if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
		goto out;

	if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
	    !arch_bpf_jit_check_func(prog))) {
		WARN(1, "attempt to jump to an invalid address");
		return 0;
	}
out:
	return prog->bpf_func(ctx, prog->insnsi);

> 
>> BPF_PROG_RUN() will be called during xdp fast path.
>> Have you measured how much slowdown the above change could
>> cost for the performance?
> 
> I have not measured the overhead, but it shouldn't be significant. Is
> there a particular benchmark you'd like me to run?

I am not an expert in XDP testing. Toke, Björn, could you give some
suggestions what to test for XDP performance here?

> 
> Sami
>