Re: [PATCH v2] crypto: aes - Fix undesired override of some optimized AES modes

[Eric Biggers]

On Fri, Sep 25, 2026 at 01:23:53PM -0700, Eric Biggers wrote:
> The new library APIs for AES encryption modes were wired up to the
> traditional crypto API via crypto/aes.c.  However, for now the kernel is
> still in a transitional state where various architectures still have
> architecture-optimized implementations of AES modes in arch/*/crypto/,
> wired up to the traditional crypto API only.  Because of that, the
> crypto/aes.c algorithms were given a cra_priority of only 110 to prevent
> them from overriding arch/*/crypto/ in the traditional crypto API.
> 
> However, because of how the traditional crypto API works, the
> cra_priority trick doesn't work in cases where the relevant algorithm
> isn't directly implemented by arch/*/crypto/ but rather is provided by a
> template instance using other code in arch/*/crypto/.
> 
> For example, x86 doesn't have its own "ccm(aes)" but rather relies on
> the "ccm" template constructing it from the x86-optimized "ctr(aes)".
> The existence of the library-based "ccm(aes)" prevents that, even though
> its priority is lower than what the template would produce.
> 
> Thus, "ccm(aes)" ends up using the slower single-block AES code.
> 
> Therefore, skip wiring up the relevant library-based code to the
> traditional crypto API on architectures where this problem can occur, as
> determined by what exists in arch/*/crypto/ for each architecture.
> 
> This is ugly, but it's also temporary: these conditions will go away as
> architecture-optimized implementations of AES modes are migrated into
> the library.  But until then, we need to prevent performance regressions
> by ensuring that the optimized code continues to be used.
> 
> Note: "xts(aes)" is left alone.  Though the "xts" template can use
> "ecb(aes)" as an inner algorithm, in practice this isn't very efficient
> and a dedicated "xts(aes)" is already provided in all the important
> cases anyway.  (This omission is also consistent with the fact that the
> library isn't planned to provide a similar ECB-to-XTS "adapter".)
> 
> Fixes: 20df21a482aa ("crypto: aes - Add CBC and CBC-CTS support using library")
> Fixes: 8ca62072faa1 ("crypto: aes - Add GCM support using library")
> Fixes: f70ad727d1d6 ("crypto: aes - Add CCM support using library")
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
> 
> This patch is intended to be taken through libcrypto-fixes
> 
> v2: Fixed PowerPC config option, and resent as standalone patch

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

- Eric