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

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Date:   Mon, 14 Mar 2022 10:49:04 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     <akpm@...ux-foundation.org>, <Herbert.van.den.Bergh@...cle.com>,
        <chris.mason@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/mlock: fix potential imbalanced rlimit ucounts
 adjustment

On 2022/3/14 10:40, Hugh Dickins wrote:
> On Thu, 10 Mar 2022, Miaohe Lin wrote:
> 
>> 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.
>>
>> Fixes: 5ed44a401ddf ("do not limit locked memory when RLIMIT_MEMLOCK is RLIM_INFINITY")
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> 
> NAK.  user_shm_lock() remembers to declare "int allowed = 0" on entry.
> 

If lock_limit is RLIM_INFINITY, "allowed" will be set to 1. And if get_ucounts fails
in some corner cases, "allowed" will remain to be 1 while the user_shm_lock ops indeed
fails. Or am I miss something?

Many thanks for comment.

>> ---
>>  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;
>> -- 
>> 2.23.0
>>
>>
>>
> .
>