Re: [PATCH v2] dma-buf/heaps: system_heap: Avoid DoS by limiting single allocations to half of all memory

[T.J. Mercier]

On Thu, Apr 6, 2023 at 7:24 PM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote:
>
> >On Thu, Apr 6, 2023 at 4:38?PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> On Thu, 6 Apr 2023 16:27:28 -0700 "T.J. Mercier" <tjmercier@xxxxxxxxxx> wrote:
> >>
> >> > > When you say "decide what's the largest reasonable size", I think it
> >> > > is difficult as with the variety of RAM sizes and buffer sizes I don't
> >> > > think there's a fixed limit. Systems with more ram will use larger
> >> > > buffers for image/video capture buffers.  And yes, you're right that
> >> > > ram/2-1 in a single allocation is just as broken, but I'm not sure how
> >> > > to establish a better guard rail.
> >> > >
> >> > > thanks
> >> > > -john
> >> >
> >> > I like ENOMEM with the len / PAGE_SIZE > totalram_pages() check and
> >> > WARN_ON. We know for sure that's an invalid request, and it's pretty
> >> > cheap to check as opposed to trying a bunch of reclaim before failing.
> >>
> >> Well, if some buggy caller has gone and requested eleventy bigabytes of
> >:)
> >> memory, doing a lot of reclaiming before failing isn't really a problem
> >> - we don't want to optimize for this case!
> >>
> >The issue I see is that it could delay other non-buggy callers, or
> >cause reclaim that wouldn't have happened if we just outright rejected
> >a known-bad allocation request from the beginning.
> >
> >> > For buffers smaller than that I agree with John in that I'm not sure
> >> > there's a definitive threshold.
> >>
> >> Well...  why do we want to do _anything_ here?  Why cater for buggy
> >> callers?  I think it's because "dma-buf behaves really badly with very
> >> large allocation requests".  Again, can we fix that instead?
> >>
> >There are a variety of different allocation strategies used by
> >different exporters so I don't think there's one dma-buf thing we
> >could fix for slow, large allocations in general. For the system_heap
> >in this patch it's really just alloc_pages. I'm saying I don't think
> >the kernel should ever ask alloc_pages for more memory than exists on
> >the system, which seems like a pretty reasonable sanity check to me.
> >Given that, I don't think we should do anything for buffers smaller
> >than totalram_pages() (except maybe to prevent OOM panics via
> >__GFP_RETRY_MAYFAIL when we attempt to exhaust system memory on any
> >request - valid or otherwise).
>
> I think T. J. also agree with me on what I shared.
>   if (len / PAGE_SIZE > totalram_pages()) return ERR_PTR(-ENOMEM);
>   #define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO | __GFP_COMP | __GFP_RETRY_MAYFAIL)
>
Oh yeah, sorry if that wasn't clear. I was referring to your updated
check for just totalram_pages() above, not totalram_pages() / 2.

> Regarding the dma-buf behavior, I also would like to say that the dma-buf
> system heap seems to be designed to allocate that large memory. In mobile
> devices, we need that large memory for camera buffers or grahpics
> rendendering buffers. So that large memory should be allowed but the invalid
> huge size over ram should be avoided.
>
> I agree on that mm should reclaim even for the large size. But that reclaim
> process may affect system performance or user convenience. In that perspective
> I thought ram / 2 was reasonable, but yes not a golden value. I hope to use
> just ram size as sanity check.
>
> Additionally if all agree, we may be able to apply __GFP_RETRY_MAYFAIL too.
>
> BR