Messages in this thread | | | Date | Wed, 22 Jul 2020 12:06:37 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH -next] fork: silence a false postive warning in __mmdrop |
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On Thu, Jun 04, 2020 at 11:03:44AM -0400, Qian Cai wrote: > The linux-next commit bf2c59fce407 ("sched/core: Fix illegal RCU from > offline CPUs") delayed, > > idle->active_mm = &init_mm; > > into finish_cpu() instead of idle_task_exit() which results in a false > positive warning that was originally designed in the commit 3eda69c92d47 > ("kernel/fork.c: detect early free of a live mm"). > > WARNING: CPU: 127 PID: 72976 at kernel/fork.c:697 > __mmdrop+0x230/0x2c0 > do_exit+0x424/0xfa0 > Call Trace: > do_exit+0x424/0xfa0 > do_group_exit+0x64/0xd0 > sys_exit_group+0x24/0x30 > system_call_exception+0x108/0x1d0 > system_call_common+0xf0/0x278
Please explain; because afaict this is a use-after-free.
The thing is __mmdrop() is going to actually free the mm, so then what is finish_cpu()'s mmdrop() going to do?
->active_mm() should have a refcount on the mm.
> Fixes: bf2c59fce407 ("sched/core: Fix illegal RCU from offline CPUs") > Signed-off-by: Qian Cai <cai@lca.pw> > --- > kernel/fork.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index 142b23645d82..5334efd2a680 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -694,7 +694,6 @@ void __mmdrop(struct mm_struct *mm) > { > BUG_ON(mm == &init_mm); > WARN_ON_ONCE(mm == current->mm); > - WARN_ON_ONCE(mm == current->active_mm); > mm_free_pgd(mm); > destroy_context(mm); > mmu_notifier_subscriptions_destroy(mm); > -- > 2.21.0 (Apple Git-122.2) >
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