RE: [PATCH] seccomp: Improve performance by optimizing memory barrier

[Wanghongzhe (Hongzhe, EulerOS)]

>> On Feb 1, 2021, at 4:06 AM, wanghongzhe <wanghongzhe@xxxxxxxxxx> wrote:
>> 
>> If a thread(A)'s TSYNC flag is set from seccomp(), then it will 
>> synchronize its seccomp filter to other threads(B) in same thread 
>> group. To avoid race condition, seccomp puts rmb() between reading the 
>> mode and filter in seccomp check patch(in B thread).
>> As a result, every syscall's seccomp check is slowed down by the 
>> memory barrier.
>> 
>> However, we can optimize it by calling rmb() only when filter is NULL 
>> and reading it again after the barrier, which means the rmb() is 
>> called only once in thread lifetime.
>> 
>> The 'filter is NULL' conditon means that it is the first time 
>> attaching filter and is by other thread(A) using TSYNC flag.
>> In this case, thread B may read the filter first and mode later in CPU 
>> out-of-order exection. After this time, the thread B's mode is always 
>> be set, and there will no race condition with the filter/bitmap.
>> 
>> In addtion, we should puts a write memory barrier between writing the 
>> filter and mode in smp_mb__before_atomic(), to avoid the race 
>> condition in TSYNC case.
>
> I haven’t fully worked this out, but rmb() is bogus. This should be smp_rmb().

Yes, I think you are right.I will fix it and send another patch.
>> 
>> Signed-off-by: wanghongzhe <wanghongzhe@xxxxxxxxxx>
>> ---
>> kernel/seccomp.c | 31 ++++++++++++++++++++++---------
>> 1 file changed, 22 insertions(+), 9 deletions(-)
>> 
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 
>> 952dc1c90229..b944cb2b6b94 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -397,8 +397,20 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>>            READ_ONCE(current->seccomp.filter);
>> 
>>    /* Ensure unexpected behavior doesn't result in failing open. */
>> -    if (WARN_ON(f == NULL))
>> -        return SECCOMP_RET_KILL_PROCESS;
>> +    if (WARN_ON(f == NULL)) {
>> +        /*
>> +         * Make sure the first filter addtion (from another
>> +         * thread using TSYNC flag) are seen.
>> +         */
>> +        rmb();
>> +        
>> +        /* Read again */
>> +        f = READ_ONCE(current->seccomp.filter);
>> +
>> +        /* Ensure unexpected behavior doesn't result in failing open. */
>> +        if (WARN_ON(f == NULL))
>> +            return SECCOMP_RET_KILL_PROCESS;
>> +    }
>> 
>>    if (seccomp_cache_check_allow(f, sd))
>>        return SECCOMP_RET_ALLOW;
>> @@ -614,9 +626,16 @@ static inline void seccomp_sync_threads(unsigned long flags)
>>         * equivalent (see ptrace_may_access), it is safe to
>>         * allow one thread to transition the other.
>>         */
>> -        if (thread->seccomp.mode == SECCOMP_MODE_DISABLED)
>> +        if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
>> +            /*
>> +             * Make sure mode cannot be set before the filter
>> +             * are set.
>> +             */
>> +            smp_mb__before_atomic();
>> +
>>            seccomp_assign_mode(thread, SECCOMP_MODE_FILTER,
>>                        flags);
>> +        }
>>    }
>> }
>> 
>> @@ -1160,12 +1179,6 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>>    int data;
>>    struct seccomp_data sd_local;
>> 
>> -    /*
>> -     * Make sure that any changes to mode from another thread have
>> -     * been seen after SYSCALL_WORK_SECCOMP was seen.
>> -     */
>> -    rmb();
>> -
>>    if (!sd) {
>>        populate_seccomp_data(&sd_local);
>>        sd = &sd_local;
>> --
>> 2.19.1
>>