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Date:   Wed, 5 Apr 2023 18:56:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     jaewon31.kim@samsung.com
Cc:     "jstultz@google.com" <jstultz@google.com>,
        "tjmercier@google.com" <tjmercier@google.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jaewon31.kim@gmail.com" <jaewon31.kim@gmail.com>
Subject: Re: [PATCH v2] dma-buf/heaps: system_heap: Avoid DoS by limiting
 single allocations to half of all memory
Message-Id: <20230405185650.239f9721f066aa480e83d543@linux-foundation.org>
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        <CGME20230406000841epcas1p3630010a770682be0f1d540a448f3e00e@epcms1p3>
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On Thu, 06 Apr 2023 10:44:19 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote:

> >> ...
> >>
> >> --- a/drivers/dma-buf/heaps/system_heap.c
> >> +++ b/drivers/dma-buf/heaps/system_heap.c
> >> @@ -351,6 +351,9 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
> >>  	struct page *page, *tmp_page;
> >>  	int i, ret = -ENOMEM;
> >>  
> >> +	if (len / PAGE_SIZE > totalram_pages() / 2)
> >> +		return ERR_PTR(-ENOMEM);
> >> +
> >
> >This seems so random.  Why ram/2 rather than ram/3 or 17*ram/35?
> 
> Hello
> 
> Thank you for your comment.
> 
> I just took the change from the old ion driver code, and actually I thought the
> half of all memory is unrealistic. It could be unwanted size like negative,
> or too big size which incurs slowness or OoM panic.
> 
> >
> >Better behavior would be to try to allocate what the caller asked
> >for and if that doesn't work out, fail gracefully after freeing the
> >partial allocations which have been performed thus far.  If dma_buf
> >is changed to do this then that change is useful in many scenarios other
> >than this crazy corner case.
> 
> I think you would like __GFP_RETRY_MAYFAIL. Actually T.J. Mercier recommended
> earlier, here's what we discussed.
> https://lore.kernel.org/linux-mm/20230331005140epcms1p1ac5241f02f645e9dbc29626309a53b24@epcms1p1/
> 
> I just worried about a case in which we need oom kill to get more memory but
> let me change my mind. That case seems to be rare. I think now it's time when
> we need to make a decision and not to allow oom kill for dma-buf system heap
> allocations.
> 
> But I still want to block that huge size over ram. For an unavailabe size,
> I think, we don't have to do memory reclaim or killing processes, and we can
> avoid freezing screen in user perspecitve.
> 
> This is eventually what I want. Can we check totalram_pages and and apply
> __GFP_RETRY_MAYFAIL?
> 
> --- a/drivers/dma-buf/heaps/system_heap.c
> +++ b/drivers/dma-buf/heaps/system_heap.c
> @@ -41,7 +41,7 @@ struct dma_heap_attachment {
>         bool mapped;
>  };
>  
> -#define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO | __GFP_COMP)
> +#define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO | __GFP_COMP | __GFP_RETRY_MAYFAIL)
>  #define MID_ORDER_GFP (LOW_ORDER_GFP | __GFP_NOWARN)
>  #define HIGH_ORDER_GFP  (((GFP_HIGHUSER | __GFP_ZERO | __GFP_NOWARN \
>                                 | __GFP_NORETRY) & ~__GFP_RECLAIM) \
> @@ -351,6 +351,9 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
>         struct page *page, *tmp_page;
>         int i, ret = -ENOMEM;
>  
> +       if (len / PAGE_SIZE > totalram_pages())
> +               return ERR_PTR(-ENOMEM);

We're catering for a buggy caller here, aren't we?  Are such large
requests ever reasonable?

How about we decide what's the largest reasonable size and do a
WARN_ON(larger-than-that), so the buggy caller gets fixed?