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

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Date:   Wed, 5 Apr 2023 20:09:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     jaewon31.kim@...sung.com
Cc:     "jstultz@...gle.com" <jstultz@...gle.com>,
        "tjmercier@...gle.com" <tjmercier@...gle.com>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>
Subject: Re: [PATCH v2] dma-buf/heaps: system_heap: Avoid DoS by limiting
 single allocations to half of all memory

On Thu, 06 Apr 2023 11:17:12 +0900 Jaewon Kim <jaewon31.kim@...sung.com> wrote:

> >> +       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?
> 
> Yes we're considering a buggy caller. I thought even totalram_pages() / 2 in
> the old ion system is also unreasonable. To avoid the /2, I changed it to
> totalram_pages() though.
> 
> Because userspace can request that size repeately, I think WARN_ON() may be
> called to too often, so that it would fill the kernel log buffer.

Oh geeze.  I trust that userspace needs elevated privileges of some form?

If so, then spamming dmesg isn't an issue - root can do much worse than
that.

> Even we think WARN_ON_ONCE rather than WARN_ON, the buggy point is not kernel
> layer. Unlike page fault mechanism, this dma-buf system heap gets the size from 
> userspace, and it is allowing unlimited size. I think we can't fix the buggy
> user space with the kernel warning log. So I think warning is not enough,
> and we need a safeguard in kernel layer.

I really dislike that ram/2 thing - it's so arbitrary, hence is surely
wrong for all cases.  Is there something more thoughtful we can do?

I mean, top priority here is to inform userspace that it's buggy so
that it gets fixed (assuming this requires elevated privileges).  And
userspace which requests (totalram_pages()/2 - 1) bytes is still buggy,
but we did nothing to get the bug fixed.