Messages in this thread | | | Date | Tue, 10 Sep 2019 10:22:53 -0700 | From | Sami Tolvanen <> | Subject | Re: [PATCH] bpf: validate bpf_func when BPF_JIT is enabled |
| |
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
| |