Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

From: Waiman Long <longman@redhat.com>
To: Will Deacon <will@kernel.org>
Cc: "Ingo Molnar" <mingo@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Juri Lelli" <juri.lelli@redhat.com>,
	"Vincent Guittot" <vincent.guittot@linaro.org>,
	"Dietmar Eggemann" <dietmar.eggemann@arm.com>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Ben Segall" <bsegall@google.com>, "Mel Gorman" <mgorman@suse.de>,
	"Daniel Bristot de Oliveira" <bristot@redhat.com>,
	"Phil Auld" <pauld@redhat.com>,
	"Wenjie Li" <wenjieli@qti.qualcomm.com>,
	"David Wang 王标" <wangbiao3@xiaomi.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()
Date: Mon, 28 Nov 2022 10:11:52 -0500	[thread overview]
Message-ID: <d49a78bb-dce1-92b1-0f67-d71259609263@redhat.com> (raw)
In-Reply-To: <20221128120008.GA25090@willie-the-truck>

On 11/28/22 07:00, Will Deacon wrote:
> On Sun, Nov 27, 2022 at 08:43:27PM -0500, Waiman Long wrote:
>> On 11/24/22 21:39, Waiman Long wrote:
>>> In general, a non-null user_cpus_ptr will remain set until the task dies.
>>> A possible exception to this is the fact that do_set_cpus_allowed()
>>> will clear a non-null user_cpus_ptr. To allow this possible racing
>>> condition, we need to check for NULL user_cpus_ptr under the pi_lock
>>> before duping the user mask.
>>>
>>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>> This is actually a pre-existing use-after-free bug since commit 07ec77a1d4e8
>> ("sched: Allow task CPU affinity to be restricted on asymmetric systems").
>> So it needs to be fixed in the stable release as well. Will resend the patch
>> with an additional fixes tag and updated commit log.
> Please can you elaborate on the use-after-free here? Looking at
> 07ec77a1d4e8, the mask is only freed in free_task() when the usage refcount
> has dropped to zero and I can't see how that can race with fork().
>
> What am I missing?

I missed that at first. The current task cloning process copies the 
content of the task structure over to the newly cloned/forked task. IOW, 
if user_cpus_ptr had been set up previously, it will be copied over to 
the cloned task. Now if user_cpus_ptr of the source task is cleared 
right after that and before dup_user_cpus_ptr() is called. The obsolete 
user_cpus_ptr value in the cloned task will remain and get used even if 
it has been freed. That is what I call as use-after-free and double-free.

Cheers,
Longman