Re: [PATCH] crypto: af_alg - Add af_alg_restrict sysctl, defaulting to 1

[Herbert Xu]

On Mon, Jun 22, 2026 at 04:48:03PM -0700, Eric Biggers wrote:
> AF_ALG is a frequent source of vulnerabilities and a maintenance
> nightmare.  It exposes far more functionality to userspace than ever
> should have been exposed, especially to unprivileged processes.  Recent
> exploits have targeted kernel internal implementation details like
> "authencesn" that have zero use case for userspace access.
> 
> Fortunately, AF_ALG is rarely used in practice, as userspace crypto
> libraries exist.  And when it is used, only some functionality is known
> to be used, and many users are known to hold capabilities already.
> iwd for example requires CAP_NET_ADMIN and has a known algorithm list
> (https://lore.kernel.org/linux-crypto/bcbbef00-5881-421b-8892-7be6c04b832d@xxxxxxxxx/).
> 
> Thus, let's restrict the set of allowed algorithms by default, depending
> on the capabilities held.
> 
> Add a sysctl /proc/sys/crypto/af_alg_restrict with meaning:
> 
>     0: unrestricted
>     1: limited functionality
>     2: completely disabled
> 
> Set the default value to 1, which enables an algorithm allowlist for
> unprivileged processes and a slightly longer allowlist for privileged
> processes.
> 
> Note that the list may be tweaked in the future.  However, the common
> use cases such as iwd and bluez are taken into account already.  I've
> tested that iwd still works with the default value of 1.
> 
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/sysctl/crypto.rst | 36 +++++++++++
>  Documentation/crypto/userspace-if.rst       | 13 +++-
>  crypto/af_alg.c                             | 72 +++++++++++++++++++--
>  crypto/algif_aead.c                         | 11 ++++
>  crypto/algif_hash.c                         | 24 +++++++
>  crypto/algif_rng.c                          |  9 +++
>  crypto/algif_skcipher.c                     | 20 ++++++
>  include/crypto/if_alg.h                     |  8 +++
>  8 files changed, 184 insertions(+), 9 deletions(-)

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