Re: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD_W instruction

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2013-04-26 at 10:50 -0400, Xi Wang wrote:
> On Fri, Apr 26, 2013 at 10:18 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Fri, 2013-04-26 at 03:51 -0400, Xi Wang wrote:
> >
> >> +#ifdef CONFIG_SECCOMP_FILTER
> >> +                     case BPF_S_ANC_SECCOMP_LD_W:
> >> +                             if (K == offsetof(struct seccomp_data, arch)) {
> >> +                                     int arch = syscall_get_arch(current, NULL);
> >> +
> >> +                                     EMIT1_off32(0xb8, arch); /* mov arch,%eax */
> >> +                                     break;
> >> +                             }
> >> +                             func = (u8 *)seccomp_bpf_load;
> >> +                             t_offset = func - (image + addrs[i]);
> >> +                             EMIT1_off32(0xbf, K); /* mov imm32,%edi */
> >> +                             EMIT1_off32(0xe8, t_offset); /* call seccomp_bpf_load */
> >> +                             break;
> >> +#endif
> >
> > This seems seriously wrong to me.
> 
> Can you elaborate?
> 
> > This cannot have been tested at all.
> 
> Thanks to QEMU for hiding bugs then. :)



1) 'current' at the time the code is jitted (compiled) is not the
'current' at the time the filter will be evaluated.

On x86_64, if CONFIG_IA32_EMULATION=y, syscall_get_arch() evaluates to :

if (task_thread_info(task)->status & TS_COMPAT)
	return AUDIT_ARCH_I386;
return AUDIT_ARCH_X86_64;

So your code is completely wrong.

2) Calling a function potentially destroys some registers.
   %rdi,%r8,%r9 for instance, so we are going to crash very easily.

I dont know, I feel a bit uncomfortable having to explain this to
someone sending security related patches...