From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
Andrew Morton <akpm@linux-foundation.org>,
Kees Cook <keescook@chromium.org>,
David Howells <dhowells@redhat.com>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Rik van Riel <riel@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm
Date: Thu, 13 Aug 2015 14:55:50 +0200 [thread overview]
Message-ID: <20150813125550.GA13984@redhat.com> (raw)
In-Reply-To: <87614k73mo.fsf@x220.int.ebiederm.org>
Let me first say that CLONE_SIGHAND must die, I think ;) and perhaps
even sighand_struct... I am wondering if we can add something like
if ((clone_flags & (CLONE_THREAD | CLONE_SIGHAND)) == CLONE_SIGHAND)
pr_info("You are crazy, please report this to lkml\n");
into copy_process().
On 08/12, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > On 08/11, Eric W. Biederman wrote:
> >> if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
> >> - /* FIXME: get_task_mm() increments ->mm_users */
> >> - if (atomic_read(¤t->mm->mm_users) > 1)
> >> + if (!thread_group_empty(current))
> >> + return -EINVAL;
> >> + }
> >> + if (unshare_flags & CLONE_VM) {
> >> + if (!current_is_single_threaded())
> >> return -EINVAL;
> >> }
> >
> > OK, but then you can remove "| CLONE_VM" from the previous check...
>
> As an optimization, but I don't think anything cares enough for the
> optimization to be worth the confusion.
current_is_single_threaded() checks task->signal->live at the start,
so there is no optimization. But I won't argue, this doesn't hurt.
> >> /*
> >> + * If unsharing a signal handlers, must also unshare the signal queues.
> >> + */
> >> + if (unshare_flags & CLONE_SIGHAND)
> >> + unshare_flags |= CLONE_THREAD;
> >
> > This looks unnecessary, check_unshare_flags() checks "THREAD | SIGHAND".
> > And to me the comment looks misleading although I won't argue.
>
> I absolutely can not understand this code if we jump 5 steps ahead
> and optimize out the individual dependencies, and try for a flattened
> dependency tree instead. I can validate the individual dependencies
> from first principles.
>
> If we jump several steps ahead I can not validate the individual
> dependencies.
OK,
> > And in fact this doesn't look exactly right, or I am totally confused.
> > Shouldn't we do
> >
> > if (unshare_flags & CLONE_SIGHAND)
> > unshare_flags |= CLONE_VM;
>
> Nope. The backward definitions of the flags in unshare has gotten you.
See below,
> CLONE_SIGHAND means that you want a struct sighand_struct with a count
> of 1.
This is (almost) true,
> Nothing about a sighand_struct with a count of 1 implies or
> requires mm_users == 1. clone can quite happily create those.
See
if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
in copy_process(). So if you have a shared sighand_struct, your ->mm
is also shared, current_is_single_threaded() will notice this.
> > Otherwise suppose that a single threaded process does clone(VM | SIGHAND)
> > and (say) child does sys_unshare(SIGHAND). This will wrongly succeed
> > afaics.
>
> Why would it be wrong to succeed in that case? struct sighand_struct
> has a count of 1.
How that? clone(VM | SIGHAND) will share ->sighand and increment its
count.
> unshare(CLONE_SIGHAND) requests a sighand_struct with
> a count of 1.
Exactly, that is why it is wrong to succeed.
> unshare(SIGHAND) needs to guarantee that when it returns sighand->count == 1.
> So unshare(SIGHAND) needs to test for sighand->count == 1.
Oh, I do not think we should check sighand->count. This can lead to
the same problem we have with the current current->mm->mm_users check.
Most probably today nobody increments sighand->count (I didn't even
try to verify). But this is possible, and I saw the code which did
this to pin ->sighand...
Oleg.
next prev parent reply other threads:[~2015-08-13 12:58 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-28 17:15 [PATCH] user_ns: use correct check for single-threadedness Kees Cook
2015-07-28 18:02 ` Rik van Riel
2015-07-28 18:17 ` Eric W. Biederman
2015-07-28 20:55 ` Ricky Zhou
2015-07-28 21:01 ` Kees Cook
2015-08-05 18:13 ` Eric W. Biederman
2015-08-05 19:40 ` Kees Cook
2015-07-28 21:35 ` Andrew Morton
2015-07-28 21:50 ` Kees Cook
2015-07-28 22:11 ` Kirill A. Shutemov
2015-08-05 11:38 ` Ingo Molnar
2015-08-05 11:53 ` Kirill A. Shutemov
2015-08-05 13:13 ` Ricky Zhou
2015-08-05 17:23 ` Oleg Nesterov
2015-08-05 18:00 ` Eric W. Biederman
2015-08-05 18:52 ` Eric W. Biederman
2015-08-06 13:06 ` Oleg Nesterov
2015-08-06 13:44 ` Oleg Nesterov
2015-08-12 1:17 ` Eric W. Biederman
2015-08-12 14:40 ` Oleg Nesterov
2015-08-12 15:11 ` Eric W. Biederman
2015-08-12 1:22 ` [PATCH 0/2] userns: Creation logic fixes Eric W. Biederman
2015-08-12 1:24 ` [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm Eric W. Biederman
2015-08-12 17:48 ` Oleg Nesterov
2015-08-12 18:39 ` Eric W. Biederman
2015-08-13 12:55 ` Oleg Nesterov [this message]
2015-08-13 15:38 ` Eric W. Biederman
2015-08-13 16:17 ` Oleg Nesterov
2015-08-13 16:27 ` Eric W. Biederman
2015-08-13 16:50 ` Oleg Nesterov
2015-08-14 17:59 ` Oleg Nesterov
2015-08-12 19:59 ` [PATCH v2] " Eric W. Biederman
2015-08-13 12:57 ` Oleg Nesterov
2015-08-13 16:01 ` Eric W. Biederman
2015-08-13 16:30 ` Oleg Nesterov
2015-08-13 16:39 ` Eric W. Biederman
2015-08-12 1:25 ` [PATCH 2/2] userns,pidns: Force thread group sharing, not signal handler sharing Eric W. Biederman
2015-08-12 17:24 ` Oleg Nesterov
2015-08-12 6:29 ` [PATCH 0/2] userns: Creation logic fixes Kees Cook
2015-08-06 14:35 ` [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov
2015-08-06 21:16 ` Eric W. Biederman
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