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

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Date:   Tue, 15 Mar 2022 13:32:28 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <hughd@...gle.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <legion@...nel.org>
Subject: Re: [PATCH v2] mm/mlock: fix potential imbalanced rlimit ucounts
 adjustment

Miaohe Lin <linmiaohe@...wei.com> writes:

> On 2022/3/14 23:21, Eric W. Biederman wrote:
>> Miaohe Lin <linmiaohe@...wei.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.

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@...wei.com>
>>> Acked-by: Hugh Dickins <hughd@...gle.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
>> .
>>