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, 8 insertions, 18 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 1e547138f4fb..67fdd2523d92 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1898,13 +1898,6 @@ struct tcp6_pseudohdr {
 	__be32		protocol;	/* including padding */
 };
 
-union tcp_md5sum_block {
-	struct tcp4_pseudohdr ip4;
-#if IS_ENABLED(CONFIG_IPV6)
-	struct tcp6_pseudohdr ip6;
-#endif
-};
-
 /*
  * struct tcp_sigpool - per-CPU pool of ahash_requests
  * @scratch: per-CPU temporary area, that can be used between
@@ -1939,8 +1932,8 @@ int tcp_sigpool_start(unsigned int id, struct tcp_sigpool *c);
 void tcp_sigpool_end(struct tcp_sigpool *c);
 size_t tcp_sigpool_algo(unsigned int id, char *buf, size_t buf_len);
 /* - functions */
-int tcp_v4_md5_hash_skb(char *md5_hash, const struct tcp_md5sig_key *key,
-			const struct sock *sk, const struct sk_buff *skb);
+void tcp_v4_md5_hash_skb(char *md5_hash, const struct tcp_md5sig_key *key,
+			 const struct sock *sk, const struct sk_buff *skb);
 int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 		   int family, u8 prefixlen, int l3index, u8 flags,
 		   const u8 *newkey, u8 newkeylen);
@@ -1999,13 +1992,10 @@ static inline void tcp_md5_destruct_sock(struct sock *sk)
 }
 #endif
 
-int tcp_md5_alloc_sigpool(void);
-void tcp_md5_release_sigpool(void);
-void tcp_md5_add_sigpool(void);
-extern int tcp_md5_sigpool_id;
-
-int tcp_md5_hash_key(struct tcp_sigpool *hp,
-		     const struct tcp_md5sig_key *key);
+struct md5_ctx;
+void tcp_md5_hash_skb_data(struct md5_ctx *ctx, const struct sk_buff *skb,
+			   unsigned int header_len);
+void tcp_md5_hash_key(struct md5_ctx *ctx, const struct tcp_md5sig_key *key);
 
 /* From tcp_fastopen.c */
 void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
@@ -2355,7 +2345,7 @@ struct tcp_sock_af_ops {
 #ifdef CONFIG_TCP_MD5SIG
 	struct tcp_md5sig_key	*(*md5_lookup) (const struct sock *sk,
 						const struct sock *addr_sk);
-	int		(*calc_md5_hash)(char *location,
+	void		(*calc_md5_hash)(char *location,
 					 const struct tcp_md5sig_key *md5,
 					 const struct sock *sk,
 					 const struct sk_buff *skb);
@@ -2383,7 +2373,7 @@ struct tcp_request_sock_ops {
 #ifdef CONFIG_TCP_MD5SIG
 	struct tcp_md5sig_key *(*req_md5_lookup)(const struct sock *sk,
 						 const struct sock *addr_sk);
-	int		(*calc_md5_hash) (char *location,
+	void		(*calc_md5_hash) (char *location,
 					  const struct tcp_md5sig_key *md5,
 					  const struct sock *sk,
 					  const struct sk_buff *skb);