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

Will Drewry
2011-05-26T17:02:47Z

On Thu, May 26, 2011 at 11:46 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, May 26, 2011 at 9:33 AM, Will Drewry <wad@chromium.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