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

Miaohe Lin
2022-03-15T12:18:05Z

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
> .
>