crypto: af_alg - Add af_alg_restrict sysctl, defaulting to 1

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@gmail.com/).

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@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 10 insertions, 3 deletions

diff --git a/Documentation/crypto/userspace-if.rst b/Documentation/crypto/userspace-if.rst
index ab93300c8e04..d6194346e366 100644
--- a/Documentation/crypto/userspace-if.rst
+++ b/Documentation/crypto/userspace-if.rst
@@ -1,3 +1,5 @@
+.. _crypto_userspace_interface:
+
 User Space Interface
 ====================
 
@@ -12,9 +14,14 @@ AF_ALG is insecure and is deprecated. Originally added to the kernel in 2010,
 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
-disabling ``CONFIG_CRYPTO_USER_API_*``.
+AF_ALG continues to be supported only for backwards compatibility.
+
+Starting in Linux v7.3, the set of algorithms supported by AF_ALG is limited by
+default. See :ref:`/proc/sys/crypto/af_alg_restrict <af_alg_restrict>`.
+
+On systems where no programs using AF_ALG remain, the support for it should be
+disabled entirely by setting ``/proc/sys/crypto/af_alg_restrict`` to 2 or by
+disabling ``CONFIG_CRYPTO_USER_API_*`` in the kernel configuration.
 
 Deprecation
 -----------