Re: [PATCH 1/4] seccomp: Provide matching filter for introspection

[Tyler Hicks <tyhicks@canonical.com>]

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On 08/07/2017 08:03 PM, Tyler Hicks wrote:
> On 08/02/2017 10:19 PM, Kees Cook wrote:
>> Both the upcoming logging improvements and changes to RET_KILL will need
>> to know which filter a given seccomp return value originated from. In
>> order to delay logic processing of result until after the seccomp loop,
>> this adds a single pointer assignment on matches. This will allow both
>> log and RET_KILL logic to work off the filter rather than doing more
>> expensive tests inside the time-critical run_filters loop.
>>
>> Running tight cycles of getpid() with filters attached shows no measurable
>> difference in speed.
>>
>> Suggested-by: Tyler Hicks <tyhicks@canonical.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  kernel/seccomp.c | 11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index 98b59b5db90b..8bdcf01379e4 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -171,10 +171,12 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
>>  /**
>>   * seccomp_run_filters - evaluates all seccomp filters against @sd
>>   * @sd: optional seccomp data to be passed to filters
>> + * @match: stores struct seccomp_filter that resulted in the return value

Thinking just a bit more about this patch, can you document that @match
may be NULL upon return?

Tyler

>>   *
>>   * Returns valid seccomp BPF response codes.
>>   */
>> -static u32 seccomp_run_filters(const struct seccomp_data *sd)
>> +static u32 seccomp_run_filters(const struct seccomp_data *sd,
>> +			       struct seccomp_filter **match)
>>  {
>>  	struct seccomp_data sd_local;
>>  	u32 ret = SECCOMP_RET_ALLOW;
> 
> My version of this patch initialized *match to f here. The reason I did
> that is because if BPF_PROG_RUN() returns RET_ALLOW for all
> filters, I didn't want *match to remain NULL when seccomp_run_filters()
> returns. FILTER_FLAG_LOG nor FILTER_FLAG_KILL_PROCESS would be affected
> by this because they don't care about RET_ALLOW actions but there could
> conceivably be a filter flag in the future that cares about RET_ALLOW
> and not initializing *match to the first filter could result in a latent
> bug for that filter flag.
> 
> I'm fine with not adding the initialization since this is a hot path and
> it doesn't help any of the currently existing/planned filter flags but I
> wanted to at least mention it.
> 
> Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
> 
> Tyler
> 
>> @@ -198,8 +200,10 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd)
>>  	for (; f; f = f->prev) {
>>  		u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
>>  
>> -		if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
>> +		if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) {
>>  			ret = cur_ret;
>> +			*match = f;
>> +		}
>>  	}
>>  	return ret;
>>  }
>> @@ -566,6 +570,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>>  			    const bool recheck_after_trace)
>>  {
>>  	u32 filter_ret, action;
>> +	struct seccomp_filter *match = NULL;
>>  	int data;
>>  
>>  	/*
>> @@ -574,7 +579,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>>  	 */
>>  	rmb();
>>  
>> -	filter_ret = seccomp_run_filters(sd);
>> +	filter_ret = seccomp_run_filters(sd, &match);
>>  	data = filter_ret & SECCOMP_RET_DATA;
>>  	action = filter_ret & SECCOMP_RET_ACTION;
>>  
>>
> 
> 



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