linux-kernel - Re: [PATCH] set_user: add capability check when rlimit(RLIMIT

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Date:   Fri, 30 Jul 2021 01:23:29 -0700
From:   CGEL <cgel.zte@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     peterz@...radead.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, Ran Xiaokai <ran.xiaokai@....com.cn>,
        James Morris <jamorris@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, NeilBrown <neilb@...e.de>
Subject: Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC)
 exceeds

On Wed, Jul 28, 2021 at 01:59:30PM +0200, Christian Brauner wrote:
> [Ccing a few people that did the PF_NPROC_EXCEEDED changes]
> 
> 
> Hey Cgel,
> Hey Ran,
> 
> The gist seems to me that this code wants to make sure that a program
> can't successfully exec if it has gone through a set*id() transition
> while exceeding its RLIMIT_NPROC.
> 
> But I agree that the semantics here are a bit strange.
> 
> Iicu, a capable but non-INIT_USER caller getting PF_NPROC_EXCEEDED set
> during a set*id() transition wouldn't be able to exec right away if they
> still exceed their RLIMIT_NPROC at the time of exec. So their exec would
> fail in fs/exec.c:
> 
> 	if ((current->flags & PF_NPROC_EXCEEDED) &&
> 	    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
> 		retval = -EAGAIN;
> 		goto out_ret;
> 	}
> 
> However, if the caller were to fork() right after the set*id()
> transition but before the exec while still exceeding their RLIMIT_NPROC
> then they would get PF_NPROC_EXCEEDED cleared (while the child would
> inherit it):
> 
> 	retval = -EAGAIN;
> 	if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(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;
> 
> which means a subsequent exec by the capable caller would now succeed
> even though they could still exceed their RLIMIT_NPROC limit.
> 
> So at first glance, it seems that set_user() should probably get the
> same check as it can be circumvented today unless I misunderstand the
> original motivation.
> 
> Christian

Hi Christian,

I think i didn't give enough information in the commit message.
When switch to a capable but non-INIT_SUER and the RLIMIT_NPROC limit already exceeded,
and calls these funcs:
1. set_xxuid()->exec() 
             ---> fail
2. set_xxuid()->fork()->exec()
             ---> success
Kernel should have the same behavior to uer space.
Also i think non init_user CAN exceed the limit when with proper capability,
so in the patch, set_user() clear PF_NPROC_EXCEEDED flag if capable()
returns true.