sctp: Convert cookie authentication to use HMAC-SHA256

Convert SCTP cookies to use HMAC-SHA256, instead of the previous choice
of the legacy algorithms HMAC-MD5 and HMAC-SHA1.  Simplify and optimize
the code by using the HMAC-SHA256 library instead of crypto_shash, and
by preparing the HMAC key when it is generated instead of per-operation.

This doesn't break compatibility, since the cookie format is an
implementation detail, not part of the SCTP protocol itself.

Note that the cookie size doesn't change either.  The HMAC field was
already 32 bytes, even though previously at most 20 bytes were actually
compared.  32 bytes exactly fits an untruncated HMAC-SHA256 value.  So,
although we could safely truncate the MAC to something slightly shorter,
for now just keep the cookie size the same.

I also considered SipHash, but that would generate only 8-byte MACs.  An
8-byte MAC *might* suffice here.  However, there's quite a lot of
information in the SCTP cookies: more than in TCP SYN cookies.  So
absent an analysis that occasional forgeries of all that information is
okay in SCTP, I errored on the side of caution.

Remove HMAC-MD5 and HMAC-SHA1 as options, since the new HMAC-SHA256
option is just better.  It's faster as well as more secure.  For
example, benchmarking on x86_64, cookie authentication is now nearly 3x
as fast as the previous default choice and implementation of HMAC-MD5.

Also just make the kernel always support cookie authentication if SCTP
is supported at all, rather than making it optional in the build.  (It
was sort of optional before, but it didn't really work properly.  E.g.,
a kernel with CONFIG_SCTP_COOKIE_HMAC_MD5=n still supported HMAC-MD5
cookie authentication if CONFIG_CRYPTO_HMAC and CONFIG_CRYPTO_MD5
happened to be enabled in the kconfig for other reasons.)

Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250818205426.30222-5-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 4 insertions, 7 deletions

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 9756d16e3df1..3d6782683eee 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -3508,16 +3508,13 @@ cookie_hmac_alg - STRING
 	a listening sctp socket to a connecting client in the INIT-ACK chunk.
 	Valid values are:
 
-	* md5
-	* sha1
+	* sha256
 	* none
 
-	Ability to assign md5 or sha1 as the selected alg is predicated on the
-	configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
-	CONFIG_CRYPTO_SHA1).
+	md5 and sha1 are also accepted for backwards compatibility, but cause
+	sha256 to be selected.
 
-	Default: Dependent on configuration.  MD5 if available, else SHA1 if
-	available, else none.
+	Default: sha256
 
 rcvbuf_policy - INTEGER
 	Determines if the receive buffer is attributed to the socket or to