[PATCH v3] crypto - shash: reduce minimum alignment of shash_desc structure

[Herbert Xu]

On Wed, Jan 13, 2021 at 10:11:35AM +0100, Ard Biesheuvel wrote:
> Unlike many other structure types defined in the crypto API, the
> 'shash_desc' structure is permitted to live on the stack, which
> implies its contents may not be accessed by DMA masters. (This is
> due to the fact that the stack may be located in the vmalloc area,
> which requires a different virtual-to-physical translation than the
> one implemented by the DMA subsystem)
> 
> Our definition of CRYPTO_MINALIGN_ATTR is based on ARCH_KMALLOC_MINALIGN,
> which may take DMA constraints into account on architectures that support
> non-cache coherent DMA such as ARM and arm64. In this case, the value is
> chosen to reflect the largest cacheline size in the system, in order to
> ensure that explicit cache maintenance as required by non-coherent DMA
> masters does not affect adjacent, unrelated slab allocations. On arm64,
> this value is currently set at 128 bytes.
> 
> This means that applying CRYPTO_MINALIGN_ATTR to struct shash_desc is both
> unnecessary (as it is never used for DMA), and undesirable, given that it
> wastes stack space (on arm64, performing the alignment costs 112 bytes in
> the worst case, and the hole between the 'tfm' and '__ctx' members takes
> up another 120 bytes, resulting in an increased stack footprint of up to
> 232 bytes.) So instead, let's switch to the minimum SLAB alignment, which
> does not take DMA constraints into account.
> 
> Note that this is a no-op for x86.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
> v3: - drop skcipher_request change again - this needs more careful thought
> 
> v2: - reduce alignment for SYNC_SKCIPHER_REQUEST_ON_STACK as well
>     - update CRYPTO_MINALIGN_ATTR comment with DMA requirements.
> 
>  include/crypto/hash.h  | 8 ++++----
>  include/linux/crypto.h | 9 ++++++---
>  2 files changed, 10 insertions(+), 7 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert at gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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