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

NeilBrown
2011-07-26T00:47:13Z

On Tue, 26 Jul 2011 03:40:13 +0400 Solar Designer <solar@openwall.com> wrote:

> Vasiliy,
>
> 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... 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.

>
> 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.

>
> Thanks,
>
> Alexander

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

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.

In the case of 'exec' the process will get a SIGKILL as well, which is
probably a good thing.

Thoughts?

NeilBrown