Jason Gunthorpe <jgg@xxxxxxxx> writes:
> On Thu, Apr 14, 2022 at 01:18:10PM +1000, Alistair Popple wrote:
>> In some cases it is possible for mmu_interval_notifier_remove() to race
>> with mn_tree_inv_end() allowing it to return while the notifier data
>> structure is still in use. Consider the following sequence:
>>
>> CPU0 - mn_tree_inv_end() CPU1 - mmu_interval_notifier_remove()
>> spin_lock(subscriptions->lock);
>> seq = subscriptions->invalidate_seq;
>> spin_lock(subscriptions->lock); spin_unlock(subscriptions->lock);
>> subscriptions->invalidate_seq++;
>> wait_event(invalidate_seq != seq);
>> return;
>> interval_tree_remove(interval_sub); kfree(interval_sub);
>> spin_unlock(subscriptions->lock);
>> wake_up_all();
>>
>> As the wait_event() condition is true it will return immediately. This
>> can lead to use-after-free type errors if the caller frees the data
>> structure containing the interval notifier subscription while it is
>> still on a deferred list. Fix this by changing invalidate_seq to an
>> atomic type as it is read outside of the lock and moving the increment
>> until after deferred lists have been updated.
>
> Oh, yes, that is a mistake.
>
> I would not solve it with more unlocked atomics though, this is just a
> simple case of a missing lock - can you look at this and if you like
> it post it as a patch please?
Yep, that looks good and is easier to understand. For some reason I had assumed
the lack of locking was intentional. Will post the below fix as v2.
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 459d195d2ff64b..f45ff1b7626a62 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -1036,6 +1036,18 @@ int mmu_interval_notifier_insert_locked(
> }
> EXPORT_SYMBOL_GPL(mmu_interval_notifier_insert_locked);
>
> +static bool
> +mmu_interval_seq_released(struct mmu_notifier_subscriptions *subscriptions,
> + unsigned long seq)
> +{
> + bool ret;
> +
> + spin_lock(&subscriptions->lock);
> + ret = subscriptions->invalidate_seq != seq;
> + spin_unlock(&subscriptions->lock);
> + return ret;
> +}
> +
> /**
> * mmu_interval_notifier_remove - Remove a interval notifier
> * @interval_sub: Interval subscription to unregister
> @@ -1083,7 +1095,7 @@ void mmu_interval_notifier_remove(struct mmu_interval_notifier *interval_sub)
> lock_map_release(&__mmu_notifier_invalidate_range_start_map);
> if (seq)
> wait_event(subscriptions->wq,
> - READ_ONCE(subscriptions->invalidate_seq) != seq);
> + mmu_interval_seq_released(subscriptions, seq));
>
> /* pairs with mmgrab in mmu_interval_notifier_insert() */
> mmdrop(mm);
>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
