Subject: Re: [PATCH 2/3] AF_ALG: Drop support for off-CPU cryptography

From: Harald Freudenberger <[email-masked]>
Date: Wed, 3 Jun 2026 15:33:12 +0200

> AF_ALG is deprecated and exposed to unprivileged userspace.  Only
> use the least buggy algorithm implementations: the pure software ones.
>

I thought AF_ALG is marked as deprecated but still usable. This patch
now actively disables groups of crypto implementations. Also it just
assumes that all algorithms which are asynchronously implemented or
do not have a fallback are to be disabled via AF_ALG.

There are may reasons for not having a synchronous implementation. For
example if you need to fetch (asynch) some information from a HSM before
doing the job of the algorithm. Also all secure key operations can't
by definition run directly on the CPU but need to be fed into some
hardware. Same is true with just acceleration - and acceleration via
special hardware (crypto hw, or AI hardware for example) is very common
on platforms priced by CPU cycles.

I also can't find any arguments for the statement 'Hardware accelerator
drivers are frequently buggy.' Does this mean that the linux kernel
from now on will not accept any hardware accelerator drivers any more?
Statements about code quality should be addressed to the driver
maintainer but not lead to tagging of groups of drivers.

I can understand that the AF_ALG shall be deprecated and fade away.
But this patch out of the sudden disables the long standing AF_ALG
interface at least for testing purpose and causes some failures in
the s390 crypto test area without any chance to react at all.

> This removes one of the main advantages of AF_ALG, which is the
> ability to use it with off-CPU accelerators.  However, using off-CPU
> accelerators has huge overheads, both in performance and attack surface.
> I have yet to see real-world, performance-critical workloads where using
> an accelerator via AF_ALG is actually a win over doing cryptography in
> userspace.
>
> If using an off-CPU accelerator really does turn out to be a win, a new
> API should be developed that is actually a good fit for it.
>
> Signed-off-by: Demi Marie Obenour <[email-masked]>
> ---
>  Documentation/crypto/userspace-if.rst |  7 ++++++-
>  crypto/af_alg.c                       |  2 +-
>  crypto/algif_aead.c                   |  4 ++--
>  crypto/algif_hash.c                   |  4 ++--
>  crypto/algif_rng.c                    |  4 ++--
>  crypto/algif_skcipher.c               |  4 ++--
>  include/crypto/if_alg.h               | 14 +++++++++++++-
>  7 files changed, 28 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/crypto/userspace-if.rst b/Documentation/crypto/userspace-if.rst
> index ea1b1b3f4049fd4673528dc2a6234f6376a3489f..b31117d4415dda6ad6ca36275e615bec7df9552e 100644
> --- a/Documentation/crypto/userspace-if.rst
> +++ b/Documentation/crypto/userspace-if.rst
> @@ -9,7 +9,8 @@ symmetric cipher, AEAD, and RNG algorithms that are implemented in kernel-mode
>  code.
>
>  AF_ALG is insecure and is deprecated. Originally added to the kernel in 2010,
> -most kernel developers now consider it to be a mistake.
> +most kernel developers now consider it to be a mistake. Support for hardware
> +accelerators, which was the original purpose of AF_ALG, has been removed.
>
>  AF_ALG continues to be supported only for backwards compatibility. On systems
>  where no programs using AF_ALG remain, the support for it should be disabled by
> @@ -59,6 +60,10 @@ Some of the examples include:
>  - CVE-2013-7421
>  - CVE-2011-4081
>
> +Hardware accelerator drivers are frequently buggy. To reduce attack surface,
> +AF_ALG now only provides access to algorithms implemented in software. This
> +means that AF_ALG no longer fulfills its original purpose.
> +
>  It is recommended that, whenever possible, userspace programs be migrated to
>  userspace crypto code (which again, is what is normally used anyway) and
>  ``CONFIG_CRYPTO_USER_API_*`` be disabled.  On systems that use SELinux, SELinux
> diff --git a/crypto/af_alg.c b/crypto/af_alg.c
> index 8ccf7a737cd6ca9a5d5bf47050c9afea0dfd61bf..cce000e8590e469927b5a5a0ceccfdf0ef54633d 100644
> --- a/crypto/af_alg.c
> +++ b/crypto/af_alg.c
> @@ -181,7 +181,7 @@ static int alg_bind(struct socket *sock, struct sockaddr_unsized *uaddr, int add
>	if (IS_ERR(type))
>		return PTR_ERR(type);
>
> -	private = type->bind(sa->salg_name, sa->salg_feat, sa->salg_mask);
> +	private = type->bind(sa->salg_name);
>	if (IS_ERR(private)) {
>		module_put(type->owner);
>		return PTR_ERR(private);
> diff --git a/crypto/algif_aead.c b/crypto/algif_aead.c
> index 60f06597cb0b13036bc975641a0b02ea8a41ad03..787aac8aeb24eed128f08345ba730478113919b3 100644
> --- a/crypto/algif_aead.c
> +++ b/crypto/algif_aead.c
> @@ -342,9 +342,9 @@ static struct proto_ops algif_aead_ops_nokey = {
>	.poll		=	af_alg_poll,
>  };
>
> -static void *aead_bind(const char *name, u32 type, u32 mask)
> +static void *aead_bind(const char *name)
>  {
> -	return crypto_alloc_aead(name, type, mask);
> +	return crypto_alloc_aead(name, 0, AF_ALG_CRYPTOAPI_MASK);
>  }
>
>  static void aead_release(void *private)
> diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
> index 4d3dfc60a16a6d8b677d903d209df18d67202c98..5452ad6c15069c3cb0ff78fe58868fe7ce4b0fc3 100644
> --- a/crypto/algif_hash.c
> +++ b/crypto/algif_hash.c
> @@ -380,9 +380,9 @@ static struct proto_ops algif_hash_ops_nokey = {
>	.accept		=	hash_accept_nokey,
>  };
>
> -static void *hash_bind(const char *name, u32 type, u32 mask)
> +static void *hash_bind(const char *name)
>  {
> -	return crypto_alloc_ahash(name, type, mask);
> +	return crypto_alloc_ahash(name, 0, AF_ALG_CRYPTOAPI_MASK);
>  }
>
>  static void hash_release(void *private)
> diff --git a/crypto/algif_rng.c b/crypto/algif_rng.c
> index a9fb492e929a70c94476f296f5f5e7c42f0313b7..4dfe7899f8fa4ce82d5f2236297230fb44bc35d6 100644
> --- a/crypto/algif_rng.c
> +++ b/crypto/algif_rng.c
> @@ -197,7 +197,7 @@ static struct proto_ops __maybe_unused algif_rng_test_ops = {
>	.sendmsg	=	rng_test_sendmsg,
>  };
>
> -static void *rng_bind(const char *name, u32 type, u32 mask)
> +static void *rng_bind(const char *name)
>  {
>	struct rng_parent_ctx *pctx;
>	struct crypto_rng *rng;
> @@ -206,7 +206,7 @@ static void *rng_bind(const char *name, u32 type, u32 mask)
>	if (!pctx)
>		return ERR_PTR(-ENOMEM);
>
> -	rng = crypto_alloc_rng(name, type, mask);
> +	rng = crypto_alloc_rng(name, 0, AF_ALG_CRYPTOAPI_MASK);
>	if (IS_ERR(rng)) {
>		kfree(pctx);
>		return ERR_CAST(rng);
> diff --git a/crypto/algif_skcipher.c b/crypto/algif_skcipher.c
> index 9dbccabd87b13920c27aff5a450a235cc6a27d59..df20bdfe1f1f4e453782dee3b743dd1939ab4c6c 100644
> --- a/crypto/algif_skcipher.c
> +++ b/crypto/algif_skcipher.c
> @@ -307,9 +307,9 @@ static struct proto_ops algif_skcipher_ops_nokey = {
>	.poll		=	af_alg_poll,
>  };
>
> -static void *skcipher_bind(const char *name, u32 type, u32 mask)
> +static void *skcipher_bind(const char *name)
>  {
> -	return crypto_alloc_skcipher(name, type, mask);
> +	return crypto_alloc_skcipher(name, 0, AF_ALG_CRYPTOAPI_MASK);
>  }
>
>  static void skcipher_release(void *private)
> diff --git a/include/crypto/if_alg.h b/include/crypto/if_alg.h
> index 62867daca47d76c9ea1a7ed233188788c5f6c3c0..7643ba954125aba0c06aaf19de087985325885ad 100644
> --- a/include/crypto/if_alg.h
> +++ b/include/crypto/if_alg.h
> @@ -41,7 +41,7 @@ struct af_alg_control {
>  };
>
>  struct af_alg_type {
> -	void *(*bind)(const char *name, u32 type, u32 mask);
> +	void *(*bind)(const char *name);
>	void (*release)(void *private);
>	int (*setkey)(void *private, const u8 *key, unsigned int keylen);
>	int (*setentropy)(void *private, sockptr_t entropy, unsigned int len);
> @@ -243,4 +243,16 @@ int af_alg_get_rsgl(struct sock *sk, struct msghdr *msg, int flags,
>		    struct af_alg_async_req *areq, size_t maxsize,
>		    size_t *outlen);
>
> +/*
> + * Mask used to disable unsupported algorithm implementations.
> + *
> + * This is the same as FSCRYPT_CRYPTOAPI_MASK in fs/crypto/fscrypt_private.h.
> + * In additions to the motivations there, this API is exposed to userspace
> + * that might not be fully trusted.
> + */
> +#define AF_ALG_CRYPTOAPI_MASK                             \
> +	(CRYPTO_ALG_ASYNC | CRYPTO_ALG_ALLOCATES_MEMORY | \
> +	 CRYPTO_ALG_KERN_DRIVER_ONLY)
> +
> +
>  #endif	/* _CRYPTO_IF_ALG_H */
>
> --
> 2.54.0
>

Harald Freudenberger