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

[Miaohe Lin <linmiaohe@huawei.com>]

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?

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@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
> .
>