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

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Date:   Tue, 3 Aug 2021 03:03:54 -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 Fri, Jul 30, 2021 at 01:23:31AM -0700, CGEL wrote:
> 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.

Hi, Christian

Do you have any further comments on this patch?
is there anything i did not give enough infomation ?