Messages in this thread | | | Date | Thu, 1 Dec 2022 12:03:39 -0500 | Subject | Re: [PATCH-tip] sched: Fix use-after-free bug in dup_user_cpus_ptr() | From | Waiman Long <> |
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On 12/1/22 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:-) > >> 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. > >> >> - dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node); >> - if (!dst->user_cpus_ptr) >> + user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node); >> + if (!user_mask) >> return -ENOMEM; >> >> - /* Use pi_lock to protect content of user_cpus_ptr */ >> + /* >> + * Use pi_lock to protect content of user_cpus_ptr >> + * >> + * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent >> + * do_set_cpus_allowed(). >> + */ >> raw_spin_lock_irqsave(&src->pi_lock, flags); >> - cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr); >> + if (src->user_cpus_ptr) { >> + swap(dst->user_cpus_ptr, user_mask); > Isn't 'dst->user_cpus_ptr' always NULL here? Why do we need the swap() > instead of just assigning the thing directly?
True. We still need to clear user_mask. So I used swap() instead of 2 assignment statements. I am fine to go with either way.
Cheers, Longman
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