crypto: af_alg - Document the deprecation of AF_ALG

AF_ALG is almost completely unnecessary, and it exposes a massive attack
surface that hasn't been standing up to modern vulnerability discovery
tools.  The latest one even has its own website, providing a small
Python script that reliably roots most Linux distros: https://copy.fail/

This isn't sustainable, especially as LLMs have accelerated the rate the
vulnerabilities are coming in.  The effort that is being put into this
thing is vastly disproportional to the few programs that actually use
it, and those programs would be better served by userspace code anyway.

These issues have been noted in many mailing list discussions already.
But until now they haven't been reflected in the documentation or
kconfig menu itself, and the vulnerabilities are still coming in.

Let's go ahead and document the deprecation.

This isn't intended to change anything overnight.  After all, most Linux
distros won't be able to disable the kconfig options quite yet, mainly
because of iwd.  But this should create a bit more impetus for these
userspace programs to be fixed, and the documentation update should also
help prevent more users from appearing.

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

1 files changed, 62 insertions, 20 deletions

diff --git a/Documentation/crypto/userspace-if.rst b/Documentation/crypto/userspace-if.rst
index a3b13ff83cb3..2c4e9a1ac3e5 100644
--- a/Documentation/crypto/userspace-if.rst
+++ b/Documentation/crypto/userspace-if.rst
@@ -4,26 +4,68 @@ User Space Interface
 Introduction
 ------------
 
-The concepts of the kernel crypto API visible to kernel space is fully
-applicable to the user space interface as well. Therefore, the kernel
-crypto API high level discussion for the in-kernel use cases applies
-here as well.
-
-The major difference, however, is that user space can only act as a
-consumer and never as a provider of a transformation or cipher
-algorithm.
-
-The following covers the user space interface exported by the kernel
-crypto API. A working example of this description is libkcapi that can
-be obtained from [1]. That library can be used by user space
-applications that require cryptographic services from the kernel.
-
-Some details of the in-kernel kernel crypto API aspects do not apply to
-user space, however. This includes the difference between synchronous
-and asynchronous invocations. The user space API call is fully
-synchronous.
-
-[1] https://www.chronox.de/libkcapi/index.html
+AF_ALG provides unprivileged userspace programs access to arbitrary hash,
+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.
+
+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_*``.
+
+Deprecation
+-----------
+
+AF_ALG was originally intended to provide userspace programs access to crypto
+accelerators that they wouldn't otherwise have access to.
+
+However, that capability turned out to not be useful on very many systems. More
+significantly, the actual implementation exposes a vastly greater amount of
+functionality than that. It actually provides access to all software algorithms.
+
+This includes arbitrary compositions of different algorithms created via a
+complex template system, as well as algorithms that only make sense as internal
+implementation details of other algorithms. It also includes full zero-copy
+support, which is difficult for the kernel to implement securely.
+
+Ultimately, these algorithms are just math computations. They use the same
+instructions that userspace programs already have access to, just accessed in a
+much more convoluted and less efficient way.
+
+Indeed, userspace code is nearly always what is being used anyway. These same
+algorithms are widely implemented in userspace crypto libraries.
+
+Meanwhile, AF_ALG hasn't been withstanding modern vulnerability discovery tools
+such as syzbot and large language models. It receives a steady stream of CVEs.
+Some of the examples include:
+
+- CVE-2026-31677
+- CVE-2026-31431 (https://copy.fail)
+- CVE-2025-38079
+- CVE-2025-37808
+- CVE-2024-26824
+- CVE-2022-48781
+- CVE-2019-8912
+- CVE-2018-14619
+- CVE-2017-18075
+- CVE-2017-17806
+- CVE-2017-17805
+- CVE-2016-10147
+- CVE-2015-8970
+- CVE-2015-3331
+- CVE-2014-9644
+- CVE-2013-7421
+- CVE-2011-4081
+
+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
+can also be used to restrict the use of AF_ALG to trusted programs.
+
+The remainder of this documentation provides the historical documentation for
+the deprecated AF_ALG interface.
 
 User Space API General Remarks
 ------------------------------