kernel-hardening - Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

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Message-ID: <CAGXu5j+ZsP1q97=-DKxbyteNtX6zcw1LLWnKbTSKW90LN-_FHQ@mail.gmail.com>
Date: Mon, 27 Feb 2012 23:52:28 -0800
From: Kees Cook <keescook@...omium.org>
To: Indan Zupancic <indan@....nu>
Cc: Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, 
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org, x86@...nel.org, 
	arnd@...db.de, davem@...emloft.net, hpa@...or.com, mingo@...hat.com, 
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net, 
	mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu, eparis@...hat.com, 
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com, 
	pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net, 
	eric.dumazet@...il.com, markus@...omium.org, coreyb@...ux.vnet.ibm.com
Subject: Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

On Mon, Feb 27, 2012 at 10:51 PM, Indan Zupancic <indan@....nu> wrote:
> On Sat, February 25, 2012 04:21, Will Drewry wrote:
>> @@ -169,6 +170,7 @@ void free_task(struct task_struct *tsk)
>>       free_thread_info(tsk->stack);
>>       rt_mutex_debug_task_free(tsk);
>>       ftrace_graph_exit_task(tsk);
>> +     put_seccomp_filter(tsk->seccomp.filter);
>>       free_task_struct(tsk);
>> }
>> EXPORT_SYMBOL(free_task);
>> @@ -1113,6 +1115,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>               goto fork_out;
>>
>>       ftrace_graph_init_task(p);
>> +     copy_seccomp(&p->seccomp, &current->seccomp);
>
> I agree it's more symmetrical when get_seccomp_filter() is used here
> directly instead of copy_seccomp(). That should put Kees at ease.

Yeah, that does feel more symmetrical.

>> +static void seccomp_filter_log_failure(int syscall)
>> +{
>> +     int compat = 0;
>> +#ifdef CONFIG_COMPAT
>> +     compat = is_compat_task();
>> +#endif
>> +     pr_info("%s[%d]: %ssystem call %d blocked at 0x%lx\n",
>> +             current->comm, task_pid_nr(current),
>> +             (compat ? "compat " : ""),
>> +             syscall, KSTK_EIP(current));
>> +}
>
> This should be at least rate limited, but could be dropped altogether,
> as it's mostly useful for debugging filters. There is no kernel message
> when a process is killed because it exceeds a ulimit either. The death
> by SIGSYS is hopefully clear enough for users, and filter writers can
> return different errno values when debugging where it goes wrong.

I've already sent a patch to take care of this. It was redundant with
the later call to audit_seccomp() on the exit path.
https://lkml.org/lkml/2012/2/26/70
https://lkml.org/lkml/2012/2/27/369

>> @@ -48,6 +308,14 @@ void __secure_computing(int this_syscall)
>>                               return;
>>               } while (*++syscall);
>>               break;
>> +#ifdef CONFIG_SECCOMP_FILTER
>> +     case SECCOMP_MODE_FILTER:
>> +             if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
>> +                     return;
>> +             seccomp_filter_log_failure(this_syscall);
>> +             exit_code = SIGSYS;
>
> Wouldn't it make more sense to always kill with SIGSYS, also for mode 1?
> I suppose it's too late for that now.

Right, this should (somewhat unfortunately) stay SIGKILL for mode 1.

-Kees

-- 
Kees Cook
ChromeOS Security

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