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

[Miaohe Lin]

On 2022/3/16 2:32, Eric W. Biederman wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> 
>> On 2022/3/14 23:21, Eric W. Biederman wrote:
>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> 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.

> 
> Eric
> 
> 
>> Thanks.
>>
>>> 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@xxxxxxxxxx>
>>>> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
>>>> ---
>>>> 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
>>> .
>>>
> .
>