Re: [kernel-hardening] [PATCH] move RLIMIT_NPROC check from set_user() to do_execve_common()

Solar Designer
2011-07-26T01:16:29Z

On Tue, Jul 26, 2011 at 10:47:13AM +1000, NeilBrown wrote:
> On Tue, 26 Jul 2011 03:40:13 +0400 Solar Designer <solar@openwall.com> wrote:
> > On Mon, Jul 25, 2011 at 09:14:23PM +0400, Vasiliy Kulikov wrote:
> > > @@ -1433,6 +1433,19 @@ static int do_execve_common(const char *filename,
> > > struct files_struct *displaced;
> > > bool clear_in_exec;
> > > int retval;
> > > + const struct cred *cred = current_cred();
> > > +
> > > + /*
> > > + * We move the actual failure in case of RLIMIT_NPROC excess from
> > > + * set*uid() to execve() because too many poorly written programs
> > > + * don't check setuid() return code. Here we additionally recheck
> > > + * whether NPROC limit is still exceeded.
> > > + */
> > > + if ((current->flags & PF_NPROC_EXCEEDED) &&
> > > + atomic_read(&cred->user->processes) > rlimit(RLIMIT_NPROC)) {
> > > + retval = -EAGAIN;
> > > + goto out_ret;
> > > + }
> >
> > Do you possibly need:
> >
> > current->flags &= ~PF_NPROC_EXCEEDED;
> >
> > somewhere after this point?
> >
> > I think it's weird to have past set_user() failure affect other than the
> > very next execve().
>
> So we are hoping that no program uses execvp() or similar...

Why? No, we don't, unless I am missing something.

> Maybe that is
> reasonable but "in for a penny, in for a pound" - I'd fail them all.
>
> I think the flag should only be cleared once we notice that the limit is no
> longer exceeded. So clearing the flag can appear *after* the code you quote
> above, but not in the middle of it.

Definitely. In case execve() fails because of the limit, the flag
remains set, so a second execve() by the process will fail too.

> > Perhaps also reset the flag on fork() because we have an RLIMIT_NPROC
> > check on fork() anyway.
>
> I agree it should be cleared here too.

Great. Just to clarify my own words: on fork(), clear the flag in the
child process only.

> But there is still the issue of 'zygot' like services....

Here's my take on it:

1. It is not known (from the discussion so far) whether Android/Zygote
even cares about RLIMIT_NPROC specifically or not. The code is very
generic, usable for any rlimits, and the rationale behind it might have
been to be able to apply certain other limits. I don't know whether or
not there exists a system that actually sets RLIMIT_NPROC via that
mechanism and expects it working.

2. If desired, Android/Zygote will be able to check the
PF_NPROC_EXCEEDED flag, via procfs or via a prctl() interface that we
might introduce. Or it may simply pass an extra fork().

> Let me try another suggestion. Instead of catching the error in
> do_execve_common, how about we catch it in do_mmap_pgoff.
> i.e. if the flag is set and an attempt it made to create an executable
> mapping, we check the user->processes against the limit then - either failing
> or clearing the flag and succeeding.
>
> This will stop an execve, and an attempt to load a shared library and call it.

This sounds too hackish to me, although if others are (unexpectedly) OK
with it, I don't mind.

Thanks,

Alexander