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

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"

> +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?



> +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.

Oleg.



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