Messages in this thread | | | Date | Fri, 2 Dec 2022 09:30:56 -0500 | Subject | Re: [PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr() | From | Waiman Long <> |
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On 12/2/22 05:18, Will Deacon wrote: > On Thu, Dec 01, 2022 at 12:03:39PM -0500, Waiman Long wrote: >> On 12/1/22 08:44, Will Deacon wrote: >>> On Sun, Nov 27, 2022 at 08:44:41PM -0500, Waiman Long wrote: >>>> Since commit 07ec77a1d4e8 ("sched: Allow task CPU affinity to be >>>> restricted on asymmetric systems"), the setting and clearing of >>>> user_cpus_ptr are done under pi_lock for arm64 architecture. However, >>>> dup_user_cpus_ptr() accesses user_cpus_ptr without any lock >>>> protection. When racing with the clearing of user_cpus_ptr in >>>> __set_cpus_allowed_ptr_locked(), it can lead to user-after-free and >>>> double-free in arm64 kernel. >>>> >>>> Commit 8f9ea86fdf99 ("sched: Always preserve the user requested >>>> cpumask") fixes this problem as user_cpus_ptr, once set, will never >>>> be cleared in a task's lifetime. However, this bug was re-introduced >>>> in commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in >>>> do_set_cpus_allowed()") which allows the clearing of user_cpus_ptr in >>>> do_set_cpus_allowed(). This time, it will affect all arches. >>>> >>>> Fix this bug by always clearing the user_cpus_ptr of the newly >>>> cloned/forked task before the copying process starts and check the >>>> user_cpus_ptr state of the source task under pi_lock. >>>> >>>> Note to stable, this patch won't be applicable to stable releases. >>>> Just copy the new dup_user_cpus_ptr() function over. >>>> >>>> Fixes: 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems") >>>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()") >>>> CC: stable@vger.kernel.org >>>> Reported-by: David Wang 王标 <wangbiao3@xiaomi.com> >>>> Signed-off-by: Waiman Long <longman@redhat.com> >>>> --- >>>> kernel/sched/core.c | 32 ++++++++++++++++++++++++++++---- >>>> 1 file changed, 28 insertions(+), 4 deletions(-) >>> As per my comments on the previous version of this patch: >>> >>> https://lore.kernel.org/lkml/20221201133602.GB28489@willie-the-truck/T/#t >>> >>> I think there are other issues to fix when racing affinity changes with >>> fork() too. >> It is certainly possible that there are other bugs hiding somewhere:-) > Right, but I actually took the time to hit the same race for the other > affinity mask field so it seems a bit narrow-minded for us just to fix the > one issue.
I focused on this particular one because of a double-free bug report from David. What other fields have you found to be subjected to data race?
> >>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>>> index 8df51b08bb38..f2b75faaf71a 100644 >>>> --- a/kernel/sched/core.c >>>> +++ b/kernel/sched/core.c >>>> @@ -2624,19 +2624,43 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask) >>>> int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, >>>> int node) >>>> { >>>> + cpumask_t *user_mask; >>>> unsigned long flags; >>>> + /* >>>> + * Always clear dst->user_cpus_ptr first as their user_cpus_ptr's >>>> + * may differ by now due to racing. >>>> + */ >>>> + dst->user_cpus_ptr = NULL; >>>> + >>>> + /* >>>> + * This check is racy and losing the race is a valid situation. >>>> + * It is not worth the extra overhead of taking the pi_lock on >>>> + * every fork/clone. >>>> + */ >>>> if (!src->user_cpus_ptr) >>>> return 0; >>> data_race() ? >> Race is certainly possible, but the clearing of user_cpus_ptr before will >> mitigate any risk. > Sorry, I meant let's wrap this access in the data_race() macro and add a > comment so that KCSAN won't report the false positive.
Good point. I should have done that.
Thanks, Longman
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