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

From: Christian Brauner <christian.brauner@ubuntu.com>
To: cgel.zte@gmail.com
Cc: peterz@infradead.org, tglx@linutronix.de,
	linux-kernel@vger.kernel.org,
	Ran Xiaokai <ran.xiaokai@zte.com.cn>,
	James Morris <jamorris@linux.microsoft.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>, NeilBrown <neilb@suse.de>
Subject: Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds
Date: Wed, 28 Jul 2021 13:59:30 +0200	[thread overview]
Message-ID: <20210728115930.2lzs57h4hvnqipue@wittgenstein> (raw)
In-Reply-To: <20210728072629.530435-1-ran.xiaokai@zte.com.cn>

[Ccing a few people that did the PF_NPROC_EXCEEDED changes]

On Wed, Jul 28, 2021 at 12:26:29AM -0700, cgel.zte@gmail.com wrote:
> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> 
> 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.
> 
> Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> ---
>  kernel/sys.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index ef1a78f5d71c..72c7639e3c98 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -480,7 +480,8 @@ static int set_user(struct cred *new)
>  	 * failure to the execve() stage.
>  	 */
>  	if (is_ucounts_overlimit(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
> -			new_user != INIT_USER)
> +			new_user != INIT_USER &&
> +			!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  		current->flags |= PF_NPROC_EXCEEDED;
>  	else
>  		current->flags &= ~PF_NPROC_EXCEEDED;

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