Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

[Will Drewry <wad@chromium.org>]

On Mon, Feb 27, 2012 at 11:09 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> Hello Will.
>
> I missed the previous discussions, and I don't think I can read
> all these emails now. So I apologize in advance if this was already
> discussed.

No worries - any review is appreciated :)

> On 02/24, Will Drewry wrote:
>>
>>  struct seccomp {
>>       int mode;
>> +     struct seccomp_filter *filter;
>>  };
>
> Minor nit, it seems that the new member can be "ifdef CONFIG_SECCOMP_FILTER"

Good call - I'll add that.

>> +static long seccomp_attach_filter(struct sock_fprog *fprog)
>> +{
>> +     struct seccomp_filter *filter;
>> +     unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
>> +     long ret;
>> +
>> +     if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
>> +             return -EINVAL;
>
> OK, this limits the memory PR_SET_SECCOMP can use.
>
> But,
>
>> +     /*
>> +      * If there is an existing filter, make it the prev and don't drop its
>> +      * task reference.
>> +      */
>> +     filter->prev = current->seccomp.filter;
>> +     current->seccomp.filter = filter;
>> +     return 0;
>
> this doesn't limit the number of filters, looks like a DoS.
>
> What if the application simply does prctl(PR_SET_SECCOMP, dummy_filter)
> in an endless loop?

It consumes a massive amount of kernel memory and, maybe, the OOM
killer gives it a boot :)

I wasn't sure what the normal convention was for avoiding memory
consumption by user processes. Should I just add a sysctl and a
per-task counter for the max number of filters?

I'm fine doing whatever makes sense here.

>
>
>> +static struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig)
>> +{
>> +     if (!orig)
>> +             return NULL;
>> +     /* Reference count is bounded by the number of total processes. */
>> +     atomic_inc(&orig->usage);
>> +     return orig;
>> +}
>> ...
>> +void copy_seccomp(struct seccomp *child, const struct seccomp *parent)
>> +{
>> +     /* Other fields are handled by dup_task_struct. */
>> +     child->filter = get_seccomp_filter(parent->filter);
>> +}
>
> This is purely cosmetic, but imho looks a bit confusing.
>
> We do not copy seccomp->mode and this is correct, it was already copied
> implicitely. So why do we copy ->filter? This is not "symmetrical", afaics
> you can simply do
>
>        void copy_seccomp(struct seccomp *child)
>        {
>                if (child->filter)
>                        atomic_inc(child->filter->usage);
>
> But once again, this is cosmetic, feel free to ignore.

Right now get_seccomp_filter does the NULL check, so really this could
be reduced to adding an external get_seccomp_filter(p->seccomp.filter)
in place of copy_seccomp().

As to removing the extra arg, that should be fine since the parent
can't drop its refcount when copy_seccomp is called.  At the very
least, I can make that change so it reads more cleanly.

thanks!
will