From: Solar Designer <solar@openwall.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: CGEL <cgel.zte@gmail.com>,
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: Tue, 7 Sep 2021 23:30:42 +0200 [thread overview]
Message-ID: <20210907213042.GA22626@openwall.com> (raw)
In-Reply-To: <20210803140702.f3rdnka3e2x6vj4r@wittgenstein>
Hi all,
Brad Spengler brought this to my attention on Twitter, and Christian
Brauner agreed I should follow up. So here goes, below the quote:
On Tue, Aug 03, 2021 at 04:07:02PM +0200, Christian Brauner wrote:
> On Tue, Aug 03, 2021 at 03:03:54AM -0700, CGEL wrote:
> > 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 ?
>
> Yeah, this is fine and how I understood it too. I don't see anything
> obviously wrong with it and the weird detour workaround via fork() seems
> inconsistent. So if I don't here anyone come up with a good reason the
> current behavior makes sense I'll pick this up.
>
> Christian
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.
Yes, I can see how having a detour via fork() was inconsistent, but
since the privileged process can be assumed non-malicious it was no big
deal. suexec just doesn't have fork() in there.
Historically, the resetting on fork() appears to have been due to my
suggestion here:
https://www.openwall.com/lists/kernel-hardening/2011/07/25/4
"Perhaps also reset the flag on fork() because we have an RLIMIT_NPROC
check on fork() anyway."
Looks like I didn't consider the inconsistency for capable() processes
(or maybe that exception wasn't yet in there?)
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;
Alexander
next prev parent reply other threads:[~2021-09-07 21:37 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-28 7:26 [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds cgel.zte
2021-07-28 11:59 ` Christian Brauner
2021-07-30 8:23 ` CGEL
2021-08-03 10:03 ` CGEL
2021-08-03 14:07 ` Christian Brauner
2021-09-07 21:30 ` Solar Designer [this message]
2021-09-08 10:24 ` Solar Designer
2021-09-13 10:01 ` Christian Brauner
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