On Tue, 2024-02-20 at 14:14 +0100, Toke Høiland-Jørgensen wrote: > Paolo Abeni <pabeni@xxxxxxxxxx> writes: > > > On Tue, 2024-02-20 at 10:06 +0100, Daniel Borkmann wrote: > > > On 2/15/24 2:26 PM, Toke Høiland-Jørgensen wrote: > > > > The BPF_TEST_RUN code in XDP live frame mode creates a new page pool > > > > each time it is called and uses that to allocate the frames used for the > > > > XDP run. This works well if the syscall is used with a high repetitions > > > > number, as it allows for efficient page recycling. However, if used with > > > > a small number of repetitions, the overhead of creating and tearing down > > > > the page pool is significant, and can even lead to system stalls if the > > > > syscall is called in a tight loop. > > > > > > > > Now that we have a persistent system page pool instance, it becomes > > > > pretty straight forward to change the test_run code to use it. The only > > > > wrinkle is that we can no longer rely on a custom page init callback > > > > from page_pool itself; instead, we change the test_run code to write a > > > > random cookie value to the beginning of the page as an indicator that > > > > the page has been initialised and can be re-used without copying the > > > > initial data again. > > > > > > > > Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > > > > > > [...] > > > > - > > > > /* We create a 'fake' RXQ referencing the original dev, but with an > > > > * xdp_mem_info pointing to our page_pool > > > > */ > > > > xdp_rxq_info_reg(&xdp->rxq, orig_ctx->rxq->dev, 0, 0); > > > > - xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL; > > > > - xdp->rxq.mem.id = pp->xdp_mem_id; > > > > + xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL; /* mem id is set per-frame below */ > > > > xdp->dev = orig_ctx->rxq->dev; > > > > xdp->orig_ctx = orig_ctx; > > > > > > > > + /* We need a random cookie for each run as pages can stick around > > > > + * between runs in the system page pool > > > > + */ > > > > + get_random_bytes(&xdp->cookie, sizeof(xdp->cookie)); > > > > + > > > > > > So the assumption is that there is only a tiny chance of collisions with > > > users outside of xdp test_run. If they do collide however, you'd leak data. > > > > Good point. @Toke: what is the worst-case thing that could happen in > > case a page is recycled from another pool's user? > > > > could we possibly end-up matching the cookie for a page containing > > 'random' orig_ctx/ctx, so that bpf program later tries to access > > equally random ptrs? > > Well, yes, if there's a collision in the cookie value we'll end up > basically dereferencing garbage pointer values, with all the badness > that ensues (most likely just a crash, but system compromise is probably > also possible in such a case). > > A 64-bit value is probably too small to be resistant against random > collisions in a "protect global data across the internet" type scenario > (for instance, a 64-bit cryptographic key is considered weak). However, > in this case the collision domain is only for the lifetime of the > running system, and each cookie value only stays valid for the duration > of a single syscall (seconds, at most), so I figured it was acceptable. > > We could exclude all-zeros as a valid cookie value (and also anything > that looks as a valid pointer), but that only removes a few of the > possible random collision values, so if we're really worried about > random collisions of 64-bit numbers, I think a better approach would be > to just make the cookie a 128-bit value instead. I can respin with that > if you prefer? :) I must admit that merging a code that will allow trashing the kernel - even with a very low probability - is quite scaring to me. How much relevant is the recycle case optimization? Could removing completely that optimization be considered? Thanks! Paolo
