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

On Thu, 7 Jan 2021 at 20:02, Eric Biggers [off-list ref] wrote:

On Thu, Jan 07, 2021 at 01:41:28PM +0100, Ard Biesheuvel wrote:

quoted

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@kernel.org>
---
 include/crypto/hash.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/crypto/hash.h b/include/crypto/hash.h
index af2ff31ff619..13f8a6a54ca8 100644
--- a/include/crypto/hash.h
+++ b/include/crypto/hash.h

@@ -149,7 +149,7 @@ struct ahash_alg {

 struct shash_desc {
      struct crypto_shash *tfm;
-     void *__ctx[] CRYPTO_MINALIGN_ATTR;
+     void *__ctx[] __aligned(ARCH_SLAB_MINALIGN);
 };

 #define HASH_MAX_DIGESTSIZE   64

@@ -162,9 +162,9 @@ struct shash_desc {

 #define HASH_MAX_STATESIZE   512

-#define SHASH_DESC_ON_STACK(shash, ctx)                                \
-     char __##shash##_desc[sizeof(struct shash_desc) +         \
-             HASH_MAX_DESCSIZE] CRYPTO_MINALIGN_ATTR; \
+#define SHASH_DESC_ON_STACK(shash, ctx)                                           \
+     char __##shash##_desc[sizeof(struct shash_desc) + HASH_MAX_DESCSIZE] \
+             __aligned(__alignof__(struct shash_desc));                   \
      struct shash_desc *shash = (struct shash_desc *)__##shash##_desc

Looks good to me, but it would be helpful if the comment above the definition of
CRYPTO_MINALIGN in include/linux/crypto.h was updated.

I'd be inclined to update CRYPTO_MINALIGN altogether, given that there
should be very few cases where this actually matter (we've had to fix
some non-coherent DMA issues in the past, but in the general case, all
buffers that are passed to devices for DMA should be described via
scatterlists, and I don't think we permit pointing the scatterlist
into request structures)