tcp: Convert tcp-md5 to use MD5 library instead of crypto_ahash

Make tcp-md5 use the MD5 library API (added in 6.18) instead of the
crypto_ahash API.  This is much simpler and also more efficient:

- The library API just operates on struct md5_ctx.  Just allocate this
  struct on the stack instead of using a pool of pre-allocated
  crypto_ahash and ahash_request objects.

- The library API accepts standard pointers and doesn't require
  scatterlists.  So, for hashing the headers just use an on-stack buffer
  instead of a pool of pre-allocated kmalloc'ed scratch buffers.

- The library API never fails.  Therefore, checking for MD5 hashing
  errors is no longer necessary.  Update tcp_v4_md5_hash_skb(),
  tcp_v6_md5_hash_skb(), tcp_v4_md5_hash_hdr(), tcp_v6_md5_hash_hdr(),
  tcp_md5_hash_key(), tcp_sock_af_ops::calc_md5_hash, and
  tcp_request_sock_ops::calc_md5_hash to return void instead of int.

- The library API provides direct access to the MD5 code, eliminating
  unnecessary overhead such as indirect function calls and scatterlist
  management.  Microbenchmarks of tcp_v4_md5_hash_skb() on x86_64 show a
  speedup from 7518 to 7041 cycles (6% fewer) with skb->len == 1440, or
  from 1020 to 678 cycles (33% fewer) with skb->len == 140.

Since tcp_sigpool_hash_skb_data() can no longer be used, add a function
tcp_md5_hash_skb_data() which is specialized to MD5.  Of course, to the
extent that this duplicates any code, it's well worth it.

To preserve the existing behavior of TCP-MD5 support being disabled when
the kernel is booted with "fips=1", make tcp_md5_do_add() check
fips_enabled itself.  Previously it relied on the error from
crypto_alloc_ahash("md5") being bubbled up.  I don't know for sure that
this is actually needed, but this preserves the existing behavior.

Tested with bidirectional TCP-MD5, both IPv4 and IPv6, between a kernel
that includes this commit and a kernel that doesn't include this commit.

(Side note: please don't use TCP-MD5!  It's cryptographically weak.  But
as long as Linux supports it, it might as well be implemented properly.)

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20251014215836.115616-1-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 0 insertions, 2 deletions

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 2ec8c6f1cdcc..ded2cf1f6006 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -312,7 +312,6 @@ static void tcp_time_wait_init(struct sock *sk, struct tcp_timewait_sock *tcptw)
 			return;
 		if (!static_key_fast_inc_not_disabled(&tcp_md5_needed.key.key))
 			goto out_free;
-		tcp_md5_add_sigpool();
 	}
 	return;
 out_free:
@@ -406,7 +405,6 @@ void tcp_twsk_destructor(struct sock *sk)
 		if (twsk->tw_md5_key) {
 			kfree(twsk->tw_md5_key);
 			static_branch_slow_dec_deferred(&tcp_md5_needed);
-			tcp_md5_release_sigpool();
 		}
 	}
 #endif