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

On 02/27, Will Drewry wrote:

On Mon, Feb 27, 2012 at 11:09 AM, Oleg Nesterov [off-list ref] wrote:

quoted

quoted

+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,

quoted

+     /*
+      * 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 :)

may be ;) but most probably oom-killer kills another innocent task,
this memory is not accounted.

I wasn't sure what the normal convention was for avoiding memory
consumption by user processes. Should I just add a sysctl

Perhaps we can add a sysctl later, but personally I think that we
can start with some "arbitrary" #define BPF_MAXFILTERS.

and a
per-task counter for the max number of filters?

Do we really need the counter? attach_filter is not the fast path,
perhaps seccomp_attach_filter() could simply iterate the chain and
count the number?

In any case, if this hurts perfomance-wise then seccomp_run_filters()
has even more problems.

I'm fine doing whatever makes sense here.

I am fine either way too.

Oleg.