Messages in this thread | | | Date | Mon, 27 Feb 2012 13:54:00 -0600 | Subject | Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF | From | Will Drewry <> |
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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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