Messages in this thread | | | Date | Wed, 28 Jul 2021 13:59:30 +0200 | From | Christian Brauner <> | Subject | Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds |
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[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
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