linux-kernel - Re: *alloc API changes

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Date:   Mon, 7 May 2018 14:15:12 -0700
From:   Kees Cook <keescook@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     John Johansen <john.johansen@...onical.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: *alloc API changes

On Mon, May 7, 2018 at 1:49 PM, Matthew Wilcox <willy@...radead.org> wrote:
> On Mon, May 07, 2018 at 01:27:38PM -0700, Kees Cook wrote:
>> On Mon, May 7, 2018 at 1:19 PM, Matthew Wilcox <willy@...radead.org> wrote:
>> > Yes.  And today with kvmalloc.  However, I proposed to Linus that
>> > kvmalloc() shouldn't allow it -- we should have kvmalloc_large() which
>> > would, but kvmalloc wouldn't.  He liked that idea, so I'm going with it.
>>
>> How would we handle size calculations for _large?
>
> I'm not sure we should, at least initially.  The very few places which
> need a large kvmalloc really are special and can do their own careful
> checking.  Because, as Linus pointed out, we shouldn't be letting the
> user ask us to allocate a terabyte of RAM.  We should just fail that.
>
> let's see how those users pan out, and then see what we can offer in
> terms of safety.
>
>> > There are very, very few places which should need kvmalloc_large.
>> > That's one million 8-byte pointers.  If you need more than that inside
>> > the kernel, you're doing something really damn weird and should do
>> > something that looks obviously different.
>>
>> I'm CCing John since I remember long ago running into problems loading
>> the AppArmor DFA with kmalloc and switching it to kvmalloc. John, how
>> large can the DFAs for AppArmor get? Would an 8MB limit be a problem?
>
> Great!  Opinions from people who'll use this interface are exceptionally
> useful.
>
>> And do we have any large IO or network buffers >8MB?
>
> Not that get allocated with kvmalloc ... because you can't DMA map vmalloc
> (without doing some unusual contortions).

Er, yes, right. I meant for _all_ allocators, though. If 8MB is going
to be the new "saturated" value? Maybe I misunderstood? What are you
proposing for the code of array_size()?

>> > but I thought of another problem with array_size.  We already have
>> > ARRAY_SIZE and it means "the number of elements in the array".
>> >
>> > so ... struct_bytes(), array_bytes(), array3_bytes()?
>>
>> Maybe "calc"? struct_calc(), array_calc(), array3_calc()? This has the
>> benefit of actually saying more about what it is doing, rather than
>> its return value... In the end, I don't care. :)
>
> I don't have a strong feeling on this either.

I lean ever so slightly towards *_size(). It'll be hard to mix up
ARRAY_SIZE() and array_size(), given the parameters.

-Kees

-- 
Kees Cook
Pixel Security