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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/3/16 22:11, Eric W. Biederman wrote:
> 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.

I will try to do this. Many thanks!

> 
> 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
>>>>> .
>>>>>
>>> .
>>>
> .
>