On Wed, Jul 22, 2020 at 12:06:37PM +0200, pet...@infradead.org wrote: > 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.
Well, the refcount issue you mentioned then happens all before bf2c59fce407 was introduced as well, but then it looks harmless because mmdrop() in finish_cpu() will do, if (unlikely(atomic_dec_and_test(&mm->mm_count))) __mmdrop(mm); where that atomic_dec_and_test() see the negative refcount and will not involve __mmdrop() again. It is not clear to me that once the CPU is offline if it needs to care about its idle thread mm_count at all. Even if this refcount issue is finally addressed, it could hit this warning in finish_cpu() without this patch. On the other hand, if you look at the commit 3eda69c92d47, it is clearly that the assumption of, WARN_ON_ONCE(mm == current->active_mm); is totally gone due to bf2c59fce407. Thus, the patch is to fix that discrepancy first and then I'll look at that the imbalance mmdrop()/mmgrab() elsewhere. > > > Fixes: bf2c59fce407 ("sched/core: Fix illegal RCU from offline CPUs") > > Signed-off-by: Qian Cai <c...@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) > >