Messages in this thread | | | Date | Tue, 7 Sep 2021 23:30:42 +0200 | From | Solar Designer <> | Subject | Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds |
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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
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