Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Sep 08, 2021 at 12:24:00PM +0200, Solar Designer wrote:
> Here's a further observation:
> 
> On Tue, Sep 07, 2021 at 11:30:42PM +0200, Solar Designer wrote:
> > As I understand, the resulting commit:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2863643fb8b92291a7e97ba46e342f1163595fa8
> > 
> > broke RLIMIT_NPROC support for Apache httpd suexec and likely similar.
> 
> The commit above tries to make things consistent by duplicating the
> check from copy_process() also in set_user().  However, the check isn't
> actually the same because set_user(new) is called _before_
> security_task_fix_setuid(new, ...), whereas in the described detour via
> fork() its check would be reached already as the new user.  So those
> capable() calls just look the same, but are actually very different, and
> that's the problem.  My current understanding is the commit actually
> increases inconsistency.
> 
> The commit message starts with:
> 
> "in copy_process(): non root users but with capability CAP_SYS_RESOURCE
> or CAP_SYS_ADMIN will clean PF_NPROC_EXCEEDED flag even
> rlimit(RLIMIT_NPROC) exceeds. Add the same capability check logic here."
> 
> It talks about the obscure case of "non root users but with capability".
> However, the capable() calls added by the commit actually also apply to
> root, such as in suexec.
> 
> > Anyway, now I suggest that 2863643fb8b92291a7e97ba46e342f1163595fa8 be
> > reverted, and if there's any reason to make any change (what reason?
> > mere consistency or any real issue?) then I suggest that the flag
> > resetting on fork() be made conditional.  Something like this:
> > 
> > 	if (atomic_read(&p->real_cred->user->processes) >=
> > 			task_rlimit(p, RLIMIT_NPROC)) {
> > 		if (p->real_cred->user != INIT_USER &&
> > 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
> > 			goto bad_fork_free;
> > -	}
> > -	current->flags &= ~PF_NPROC_EXCEEDED;
> > +	} else
> > +		current->flags &= ~PF_NPROC_EXCEEDED;
> 
> Alternatively, we could postpone the set_user() calls until we're
> running with the new user's capabilities, but that's an invasive change
> that's likely to create its own issues.  So my suggestion above holds.

Thanks for taking a look at this. We can surely revert this.  Fwiw,
given how non-obvious this whole thing turned out to be a few comments
in the code would've been helpful. I'll try to send a revert by the end
of this week with your explanations added in the revert message.

Christian