linux-kernel - Re: [PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

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Date:	Thu, 26 May 2011 12:04:43 -0500
From:	Will Drewry <wad@...omium.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Colin Walters <walters@...bum.org>,
	Kees Cook <kees.cook@...onical.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, James Morris <jmorris@...ei.org>
Subject: Re: [PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

On Thu, May 26, 2011 at 12:02 PM, Will Drewry <wad@...omium.org> wrote:
> On Thu, May 26, 2011 at 11:46 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, May 26, 2011 at 9:33 AM, Will Drewry <wad@...omium.org> wrote:
>>>
>>> FWIW, none of the patches deal with privilege escalation via setuid
>>> files or file capabilities.
>>
>> That is NOT AT ALL what I'm talking about.
>>
>> I'm talking about the "setuid()" system call (and all its cousins:
>> setgit/setreuid etc). And the whole thread has been about filtering
>> system calls, no?
>>
>> Do a google code search for setuid.
>>
>> In good code, it will look something like
>>
>>  uid = getuid();
>>
>>  if (setuid(uid)) {
>>    fprintf(stderr, "Unable to drop provileges\n");
>>    exit(1);
>>  }
>>
>> but I guarantee you that there are cases where people just blindly
>> drop privileges. google code search found me at least the "heirloom"
>> source code doing exactly that.
>>
>> And if you filter system calls, it's entirely possible that you can
>> attack suid executables through such a vector. Your "limit system
>> calls for security" security suddenly turned into "avoid the system
>> call that made things secure"!
>>
>> See what I'm saying?
>
> Absolutely - that was what I meant :/  The patches do not currently
> check creds at creation or again at use, which would lead to
> unprivileged filters being used in a privileged context.  Right now,
> though, if setuid() is not allowed by the seccomp-filter, the process
> will be immediately killed with do_exit(SIGKILL) on call -- thus
> avoiding a silent failure. I mentioned file capabilities because they
> can have setuid-like side effects, too.  As long as system call
> rejection results in a process death, I *think* it helps with some of
> this complexity, but I haven't fully vetted the patches for these
> scenarios to be 100% confident.

Bah - by "setuid-like side effects", I meant suid executable-like side
effects.  And I blocking even outside of those scenarios, I think
immediate process-death helps resolves coding mistakes leading to
filtering setuid() calls prior to use.

cheers,
will
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