[ Upstream commit 543bdb2d825fe2400d6e951f1786d92139a16931 ]
Make mmu_notifier_register() safer by issuing a memory barrier before registering a new notifier. This fixes a theoretical bug on weakly ordered CPUs. For example, take this simplified use of notifiers by a driver:
my_struct->mn.ops = &my_ops; /* (1) */ mmu_notifier_register(&my_struct->mn, mm) ... hlist_add_head(&mn->hlist, &mm->mmu_notifiers); /* (2) */ ...
Once mmu_notifier_register() releases the mm locks, another thread can invalidate a range:
mmu_notifier_invalidate_range() ... hlist_for_each_entry_rcu(mn, &mm->mmu_notifiers, hlist) { if (mn->ops->invalidate_range)
The read side relies on the data dependency between mn and ops to ensure that the pointer is properly initialized. But the write side doesn't have any dependency between (1) and (2), so they could be reordered and the readers could dereference an invalid mn->ops. mmu_notifier_register() does take all the mm locks before adding to the hlist, but those have acquire semantics which isn't sufficient.
By calling hlist_add_head_rcu() instead of hlist_add_head() we update the hlist using a store-release, ensuring that readers see prior initialization of my_struct. This situation is better illustated by litmus test MP+onceassign+derefonce.
Link: http://lkml.kernel.org/r/20190502133532.24981-1-jean-philippe.brucker@arm.co... Fixes: cddb8a5c14aa ("mmu-notifiers: core") Signed-off-by: Jean-Philippe Brucker jean-philippe.brucker@arm.com Cc: Jérôme Glisse jglisse@redhat.com Cc: Michal Hocko mhocko@suse.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- mm/mmu_notifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c index f4259e496f83..7a66e37efb4d 100644 --- a/mm/mmu_notifier.c +++ b/mm/mmu_notifier.c @@ -286,7 +286,7 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn, * thanks to mm_take_all_locks(). */ spin_lock(&mm->mmu_notifier_mm->lock); - hlist_add_head(&mn->hlist, &mm->mmu_notifier_mm->list); + hlist_add_head_rcu(&mn->hlist, &mm->mmu_notifier_mm->list); spin_unlock(&mm->mmu_notifier_mm->lock);
mm_drop_all_locks(mm);