linux-kernel - Re: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD

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Date:	Fri, 26 Apr 2013 08:11:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Xi Wang <xi.wang@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Russell King <linux@....linux.org.uk>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Will Drewry <wad@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support
 BPF_S_ANC_SECCOMP_LD_W instruction

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@...il.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...




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