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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 15 Apr 2022 13:38:15 +0200
Message-ID: <CAMj1kXH5O32H1nnm6y7=3KiH7R-_oakxzBpZ20wK+8kaD46aKw@mail.gmail.com>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
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On Fri, 15 Apr 2022 at 12:45, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Fri, Apr 15, 2022 at 12:22:27PM +0200, Ard Biesheuvel wrote:
> >
> > Subsequent objects are owned by the driver, and it is the
> > responsibility of the driver not to modify the fields while it is also
> > mapped for DMA (and we have had issues in the past where drivers
> > violated this rule). So as long as ARCH_KMALLOC_ALIGN guarantees
> > actual DMA minimum alignment for both the start and the end, we
> > shouldn't need any explicit padding at the end.
>
> I don't understand why this is guaranteed.  The driver context
> size is arbitrary so it could end in the middle of a cacheline.
> The slab allocator could well lay it out so that the next kmalloc
> object starts right after the end of the context, in which case
> they would share a cache-line.
>

If this is the case, things are already broken today. We never take
ARCH_DMA_MINALIGN into account when adding the driver ctx size to the
overall allocation size.

> The next kmalloc object could be (and in fact is likely to be)
> of the same type.
>
> Previously this wasn't possible because kmalloc guaranteed
> alignment.
>

Either it does or it doesn't. If kmalloc() guarantees the actual DMA
alignment at both ends, the situation you describe cannot occur, given
that the driver's slice of the request/TFM structure would be padded
up to actual DMA alignment, in spite of whether or not
ARCH_DMA_MINALIGN exceeds that. So it would never share a cacheline in
practice, even though they might live in the same 128 byte aligned
region on a system that has a minimum DMA alignment that is lower than
that.

If kmalloc() does not guarantee that the end of the buffer is aligned
to actual DMA alignment, things are already broken today.