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

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Message-ID: <BANLkTimZjwdepwS2mjysRyHytfhd2P7vSA@mail.gmail.com>
Date:	Thu, 26 May 2011 12:02:45 -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 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.

Sorry I wasn't clear!
will
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