linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Date:   Fri, 8 Apr 2022 10:04:54 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Will Deacon <will@...nel.org>,
        Marc Zyngier <maz@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN

On Fri, Apr 08, 2022 at 11:25:29AM +0800, Herbert Xu wrote:
> On Thu, Apr 07, 2022 at 05:28:10PM +0100, Catalin Marinas wrote:
> > I can see in many cases that the kmalloc() caller aligns the requested
> > size to something like crypto_tfm_ctx_alignment(). So this would
> > guarantee a kmalloc() object aligned to CRYPTO_MINALIGN.
> 
> crypto_tfm_ctx_alignment is basically the same as CRYPTO_MINALIGN.
> We assume any kmalloced pointers to be aligned to that.
> 
> Specific algorithms may ask for an alignment greater than that
> and we will use the knowledge that kmalloc is aligned to
> CRYPTO_MINALIGN to derive the extra memory we need to get.
> 
> So if kmalloc no longer returns memory aligned to MINALIGN then
> we'll get memory overruns.

My point is that if the crypto code kmallocs a size aligned to
crypto_tfm_ctx_alignment() (and CRYPTO_MINALIGN), the slab allocator
will return memory aligned to CRYPTO_MINALIGN even if
ARCH_KMALLOC_MINALIGN is smaller.

Would the crypto code, say, do a kmalloc(64) and expect a 128 byte
alignment (when CRYPTO_MINALIGN == 128)? Or does it align the size to
CRYPTO_MINALIGN and do a kmalloc(128) directly? If it's the latter, I
don't think there's a problem.

-- 
Catalin