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

From: Sami Tolvanen <[email-masked]>
Date: Tue, 10 Sep 2019 10:22:53 -0700

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
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.

> 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?

Sami