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, 44 insertions, 93 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index b1fcf3e4e1ce..40a76da5364a 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -53,6 +53,7 @@
 #include <linux/module.h>
 #include <linux/random.h>
 #include <linux/cache.h>
+#include <linux/fips.h>
 #include <linux/jhash.h>
 #include <linux/init.h>
 #include <linux/times.h>
@@ -86,14 +87,13 @@
 #include <linux/btf_ids.h>
 #include <linux/skbuff_ref.h>
 
-#include <crypto/hash.h>
-#include <linux/scatterlist.h>
+#include <crypto/md5.h>
 
 #include <trace/events/tcp.h>
 
 #ifdef CONFIG_TCP_MD5SIG
-static int tcp_v4_md5_hash_hdr(char *md5_hash, const struct tcp_md5sig_key *key,
-			       __be32 daddr, __be32 saddr, const struct tcphdr *th);
+static void tcp_v4_md5_hash_hdr(char *md5_hash, const struct tcp_md5sig_key *key,
+				__be32 daddr, __be32 saddr, const struct tcphdr *th);
 #endif
 
 struct inet_hashinfo tcp_hashinfo;
@@ -754,7 +754,6 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
 	struct tcp_md5sig_key *key = NULL;
 	unsigned char newhash[16];
 	struct sock *sk1 = NULL;
-	int genhash;
 #endif
 	u64 transmit_time = 0;
 	struct sock *ctl_sk;
@@ -840,11 +839,9 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
 		if (!key)
 			goto out;
 
-
-		genhash = tcp_v4_md5_hash_skb(newhash, key, NULL, skb);
-		if (genhash || memcmp(md5_hash_location, newhash, 16) != 0)
+		tcp_v4_md5_hash_skb(newhash, key, NULL, skb);
+		if (memcmp(md5_hash_location, newhash, 16) != 0)
 			goto out;
-
 	}
 
 	if (key) {
@@ -1425,13 +1422,13 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (!rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk))) {
-		if (tcp_md5_alloc_sigpool())
-			return -ENOMEM;
+		if (fips_enabled) {
+			pr_warn_once("TCP-MD5 support is disabled due to FIPS\n");
+			return -EOPNOTSUPP;
+		}
 
-		if (tcp_md5sig_info_add(sk, GFP_KERNEL)) {
-			tcp_md5_release_sigpool();
+		if (tcp_md5sig_info_add(sk, GFP_KERNEL))
 			return -ENOMEM;
-		}
 
 		if (!static_branch_inc(&tcp_md5_needed.key)) {
 			struct tcp_md5sig_info *md5sig;
@@ -1439,7 +1436,6 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 			md5sig = rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk));
 			rcu_assign_pointer(tp->md5sig_info, NULL);
 			kfree_rcu(md5sig, rcu);
-			tcp_md5_release_sigpool();
 			return -EUSERS;
 		}
 	}
@@ -1456,12 +1452,9 @@ int tcp_md5_key_copy(struct sock *sk, const union tcp_md5_addr *addr,
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (!rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk))) {
-		tcp_md5_add_sigpool();
 
-		if (tcp_md5sig_info_add(sk, sk_gfp_mask(sk, GFP_ATOMIC))) {
-			tcp_md5_release_sigpool();
+		if (tcp_md5sig_info_add(sk, sk_gfp_mask(sk, GFP_ATOMIC)))
 			return -ENOMEM;
-		}
 
 		if (!static_key_fast_inc_not_disabled(&tcp_md5_needed.key.key)) {
 			struct tcp_md5sig_info *md5sig;
@@ -1470,7 +1463,6 @@ int tcp_md5_key_copy(struct sock *sk, const union tcp_md5_addr *addr,
 			net_warn_ratelimited("Too many TCP-MD5 keys in the system\n");
 			rcu_assign_pointer(tp->md5sig_info, NULL);
 			kfree_rcu(md5sig, rcu);
-			tcp_md5_release_sigpool();
 			return -EUSERS;
 		}
 	}
@@ -1578,66 +1570,44 @@ static int tcp_v4_parse_md5_keys(struct sock *sk, int optname,
 			      cmd.tcpm_key, cmd.tcpm_keylen);
 }
 
-static int tcp_v4_md5_hash_headers(struct tcp_sigpool *hp,
-				   __be32 daddr, __be32 saddr,
-				   const struct tcphdr *th, int nbytes)
+static void tcp_v4_md5_hash_headers(struct md5_ctx *ctx,
+				    __be32 daddr, __be32 saddr,
+				    const struct tcphdr *th, int nbytes)
 {
-	struct tcp4_pseudohdr *bp;
-	struct scatterlist sg;
-	struct tcphdr *_th;
-
-	bp = hp->scratch;
-	bp->saddr = saddr;
-	bp->daddr = daddr;
-	bp->pad = 0;
-	bp->protocol = IPPROTO_TCP;
-	bp->len = cpu_to_be16(nbytes);
-
-	_th = (struct tcphdr *)(bp + 1);
-	memcpy(_th, th, sizeof(*th));
-	_th->check = 0;
+	struct {
+		struct tcp4_pseudohdr ip;
+		struct tcphdr tcp;
+	} h;
 
-	sg_init_one(&sg, bp, sizeof(*bp) + sizeof(*th));
-	ahash_request_set_crypt(hp->req, &sg, NULL,
-				sizeof(*bp) + sizeof(*th));
-	return crypto_ahash_update(hp->req);
+	h.ip.saddr = saddr;
+	h.ip.daddr = daddr;
+	h.ip.pad = 0;
+	h.ip.protocol = IPPROTO_TCP;
+	h.ip.len = cpu_to_be16(nbytes);
+	h.tcp = *th;
+	h.tcp.check = 0;
+	md5_update(ctx, (const u8 *)&h, sizeof(h.ip) + sizeof(h.tcp));
 }
 
-static int tcp_v4_md5_hash_hdr(char *md5_hash, const struct tcp_md5sig_key *key,
-			       __be32 daddr, __be32 saddr, const struct tcphdr *th)
+static noinline_for_stack void
+tcp_v4_md5_hash_hdr(char *md5_hash, const struct tcp_md5sig_key *key,
+		    __be32 daddr, __be32 saddr, const struct tcphdr *th)
 {
-	struct tcp_sigpool hp;
+	struct md5_ctx ctx;
 
-	if (tcp_sigpool_start(tcp_md5_sigpool_id, &hp))
-		goto clear_hash_nostart;
-
-	if (crypto_ahash_init(hp.req))
-		goto clear_hash;
-	if (tcp_v4_md5_hash_headers(&hp, daddr, saddr, th, th->doff << 2))
-		goto clear_hash;
-	if (tcp_md5_hash_key(&hp, key))
-		goto clear_hash;
-	ahash_request_set_crypt(hp.req, NULL, md5_hash, 0);
-	if (crypto_ahash_final(hp.req))
-		goto clear_hash;
-
-	tcp_sigpool_end(&hp);
-	return 0;
-
-clear_hash:
-	tcp_sigpool_end(&hp);
-clear_hash_nostart:
-	memset(md5_hash, 0, 16);
-	return 1;
+	md5_init(&ctx);
+	tcp_v4_md5_hash_headers(&ctx, daddr, saddr, th, th->doff << 2);
+	tcp_md5_hash_key(&ctx, key);
+	md5_final(&ctx, md5_hash);
 }
 
-int tcp_v4_md5_hash_skb(char *md5_hash, const struct tcp_md5sig_key *key,
-			const struct sock *sk,
-			const struct sk_buff *skb)
+noinline_for_stack void
+tcp_v4_md5_hash_skb(char *md5_hash, const struct tcp_md5sig_key *key,
+		    const struct sock *sk, const struct sk_buff *skb)
 {
 	const struct tcphdr *th = tcp_hdr(skb);
-	struct tcp_sigpool hp;
 	__be32 saddr, daddr;
+	struct md5_ctx ctx;
 
 	if (sk) { /* valid for establish/request sockets */
 		saddr = sk->sk_rcv_saddr;
@@ -1648,30 +1618,11 @@ int tcp_v4_md5_hash_skb(char *md5_hash, const struct tcp_md5sig_key *key,
 		daddr = iph->daddr;
 	}
 
-	if (tcp_sigpool_start(tcp_md5_sigpool_id, &hp))
-		goto clear_hash_nostart;
-
-	if (crypto_ahash_init(hp.req))
-		goto clear_hash;
-
-	if (tcp_v4_md5_hash_headers(&hp, daddr, saddr, th, skb->len))
-		goto clear_hash;
-	if (tcp_sigpool_hash_skb_data(&hp, skb, th->doff << 2))
-		goto clear_hash;
-	if (tcp_md5_hash_key(&hp, key))
-		goto clear_hash;
-	ahash_request_set_crypt(hp.req, NULL, md5_hash, 0);
-	if (crypto_ahash_final(hp.req))
-		goto clear_hash;
-
-	tcp_sigpool_end(&hp);
-	return 0;
-
-clear_hash:
-	tcp_sigpool_end(&hp);
-clear_hash_nostart:
-	memset(md5_hash, 0, 16);
-	return 1;
+	md5_init(&ctx);
+	tcp_v4_md5_hash_headers(&ctx, daddr, saddr, th, skb->len);
+	tcp_md5_hash_skb_data(&ctx, skb, th->doff << 2);
+	tcp_md5_hash_key(&ctx, key);
+	md5_final(&ctx, md5_hash);
 }
 EXPORT_IPV6_MOD(tcp_v4_md5_hash_skb);