Re: [PATCH v2] mm/mlock: fix potential imbalanced rlimit ucounts adjustment

Eric W. Biederman
2022-03-16T14:11:44Z

Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2022/3/16 2:32, Eric W. Biederman wrote:
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>
>>> On 2022/3/14 23:21, Eric W. Biederman wrote:
>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>
>>>>> user_shm_lock forgets to set allowed to 0 when get_ucounts fails. So
>>>>> the later user_shm_unlock might do the extra dec_rlimit_ucounts. Fix
>>>>> this by resetting allowed to 0.
>>>>
>>>> This fix looks correct. But the ability for people to follow and read
>>>> the code seems questionable. I saw in v1 of this patch Hugh originally
>>>> misread the logic.
>>>>
>>>> Could we instead change the code to leave lock_limit at ULONG_MAX aka
>>>> RLIM_INFINITY, leave initialized to 0, and not even need a special case
>>>> of RLIM_INFINITY as nothing can be greater that ULONG_MAX?
>>>>
>>>
>>> Many thanks for your advice. This looks good but it seems this results in different
>>> behavior: When (memlock == LONG_MAX) && !capable(CAP_IPC_LOCK), we would fail now
>>> while it will always success without this change. We should avoid this difference.
>>> Or am I miss something? Maybe the origin patch is more suitable and
>>> simple?
>>
>> Interesting. I think that is an unintended and necessary bug fix.
>>
>> When memlock == LONG_MAX that means inc_rlimit_ucounts failed.
>>
>> It either failed because at another level the limit was exceeded or
>> because the counter wrapped. In either case it is not appropriate to
>> succeed if inc_rlimit_ucounts detects a failure.
>>
>> Which is a long way of saying I think we really want the simplification
>> because it found and fixed another bug as well.
>>
>> Without the simplification I don't think I will be confident the code is
>> correct.
>
> Agree with you. This is a potential bug and you just catch it with the
> code simplification. :)
>
> Am I supposed to do this altogether or will you do this simplification part?
> Many thanks.

If you can that would be great, and you can have the credit.

Otherwise I will make my proposed changes into a proper patch. At this
point we just need to dot the i's and cross the t's and get this fix in.

Eric

>>>> Something like this?
>>>>
>>>> diff --git a/mm/mlock.c b/mm/mlock.c
>>>> index 8f584eddd305..e7eabf5193ab 100644
>>>> --- a/mm/mlock.c
>>>> +++ b/mm/mlock.c
>>>> @@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct ucounts *ucounts)
>>>>
>>>> locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
>>>> lock_limit = rlimit(RLIMIT_MEMLOCK);
>>>> - if (lock_limit == RLIM_INFINITY)
>>>> - allowed = 1;
>>>> - lock_limit >>= PAGE_SHIFT;
>>>> + if (lock_limit != RLIM_INFINITY)
>>>> + lock_limit >>= PAGE_SHIFT;
>>>> spin_lock(&shmlock_user_lock);
>>>> memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
>>>>
>>>> - if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
>>>> + if ((memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
>>>> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
>>>> goto out;
>>>> }
>>>>
>>>>>
>>>>> Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>>> Acked-by: Hugh Dickins <hughd@google.com>
>>>>> ---
>>>>> v1->v2:
>>>>> correct Fixes tag and collect Acked-by tag
>>>>> Thanks Hugh for review!
>>>>> ---
>>>>> mm/mlock.c | 1 +
>>>>> 1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/mm/mlock.c b/mm/mlock.c
>>>>> index 29372c0eebe5..efd2dd2943de 100644
>>>>> --- a/mm/mlock.c
>>>>> +++ b/mm/mlock.c
>>>>> @@ -733,6 +733,7 @@ int user_shm_lock(size_t size, struct ucounts *ucounts)
>>>>> }
>>>>> if (!get_ucounts(ucounts)) {
>>>>> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
>>>>> + allowed = 0;
>>>>> goto out;
>>>>> }
>>>>> allowed = 1;
>>>>
>>>> Eric
>>>> .
>>>>
>> .
>>