[PATCH net v4 0/2] net/rds: scope-aware sendmsg connection cache

[PATCH net v4 0/2] net/rds: scope-aware sendmsg connection cache

[Allison Henderson]

Hi all,

This is v4 of the sendmsg connection-cache scope fix (v1 at [1], v2 at
[2], v3 at [3]).

  Patch 1 gives the connected peer's link-local scope its own field.
  rds_connect() used to store it in rs_bound_scope_id, the scope of
  the socket's own bound address, which a later bind() to a
  non-link-local address resets to 0 and which a later connect()
  pollutes for a globally bound socket.  A send without a destination,
  getpeername() and recvmsg() now take the connected peer's scope from
  that field.

  Patch 2 compares c_dev_if with the request's scope in the cache-hit
  test, so the cache uses the same key as rds_conn_lookup().

Both were found by inspection while reworking this cache for the
connection-lifetime series on net-next; neither has been observed in
the field.  The misdelivery in patch 2 needs a socket that sends to
the same link-local address through two interfaces, and the stall in
patch 1 a connect()/bind() order that loses the peer's scope.

Changes since v3 [3]:
 - Patch 1: a destination-less send falls back to the bound scope for a
   non-link-local peer, so send() and sendto() to the connected peer
   compute the same scope (review of v3: a socket bound link-local and
   connected to a global peer stalled otherwise); getpeername() and
   recvmsg() report the connected peer's scope instead of the bound
   scope (review of v3).
 - Commit messages say how the issues were found.

Changes since v2 [2]:
 - The v2 "scope 0 matches anything" wildcard is gone; the
   destination-less send carries the connected peer's scope instead
   (new patch 1), and the comparison is exact.

[1] https://lore.kernel.org/netdev/20260921215046.174745-1-achender@kernel.org/
[2] https://lore.kernel.org/netdev/20260927062312.169097-1-achender@kernel.org/
[3] https://lore.kernel.org/netdev/20261003163526.250805-1-achender@kernel.org/

Thank you,
Allison


Allison Henderson (2):
  net/rds: keep the connected peer's scope id apart from the bound one
  net/rds: include the scope id in the sendmsg connection cache check

 net/rds/af_rds.c | 15 +++++++++------
 net/rds/bind.c   |  4 ++--
 net/rds/rds.h    |  2 ++
 net/rds/recv.c   |  3 ++-
 net/rds/send.c   |  9 +++++++--
 5 files changed, 22 insertions(+), 11 deletions(-)

-- 
2.25.1

[PATCH net v4 1/2] net/rds: keep the connected peer's scope id apart from the bound one

[Allison Henderson]

rds_connect() has nowhere to keep the scope of a link-local peer, so
it stores it in rs_bound_scope_id, the scope of the socket's own
bound address, for rds_bind() to check a later link-local bind
against.  That field is then also what a send without a destination
uses as the request's scope, what getpeername() and recvmsg() report
as the peer's scope, and what a later bind() overwrites:
rds_add_bound() stores the bound address's scope unconditionally,
which for a non-link-local address is 0.

So after connect(fe80::x%ifA) followed by bind(global), a send() with
no destination asks for fe80::x with scope 0.  rds_conn_lookup() keys
on the interface, so that finds or creates a connection with c_dev_if
0, which the TCP transport then tries to connect through
sin6_scope_id 0 and tcp_v6_connect() rejects for a link-local peer:
the connected socket's data is queued on a connection that can never
come up.  In the other order, bind(global) then connect(fe80::x%ifB),
the connect leaves a global-bound socket with rs_bound_scope_id ifB,
so sends to other link-local peers are refused as off-link and sends
to global peers inherit a meaningless interface.

Give the connected peer its own rs_conn_scope_id.  rds_connect()
records it there and leaves the bound scope alone, rds_bind()'s
connected-socket check compares against it, and the destination-less
send takes its scope from it - falling back to the bound scope for a
non-link-local peer, so that send() and sendto() to the connected
peer compute the same scope, and so the same connection.
getpeername() reports the connected peer with its own scope, and
recvmsg() reports a sender with the connected peer's scope when there
is one and the bound scope otherwise, as before.

Found by inspection while reworking the sendmsg connection cache; not
observed in the field.

Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
 net/rds/af_rds.c | 15 +++++++++------
 net/rds/bind.c   |  4 ++--
 net/rds/rds.h    |  2 ++
 net/rds/recv.c   |  3 ++-
 net/rds/send.c   |  6 +++++-
 5 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/net/rds/af_rds.c b/net/rds/af_rds.c
index d5defe9172e3..e84d00bbe9a1 100644
--- a/net/rds/af_rds.c
+++ b/net/rds/af_rds.c
@@ -136,8 +136,7 @@ static int rds_getname(struct socket *sock, struct sockaddr *uaddr,
 			sin6->sin6_port = rs->rs_conn_port;
 			sin6->sin6_addr = rs->rs_conn_addr;
 			sin6->sin6_flowinfo = 0;
-			/* scope_id is the same as in the bound address. */
-			sin6->sin6_scope_id = rs->rs_bound_scope_id;
+			sin6->sin6_scope_id = rs->rs_conn_scope_id;
 			uaddr_len = sizeof(*sin6);
 		}
 	} else {
@@ -572,6 +571,7 @@ static int rds_connect(struct socket *sock, struct sockaddr_unsized *uaddr,
 		}
 		ipv6_addr_set_v4mapped(sin->sin_addr.s_addr, &rs->rs_conn_addr);
 		rs->rs_conn_port = sin->sin_port;
+		rs->rs_conn_scope_id = 0;
 		break;
 
 #if IS_ENABLED(CONFIG_IPV6)
@@ -616,11 +616,14 @@ static int rds_connect(struct socket *sock, struct sockaddr_unsized *uaddr,
 				ret = -EINVAL;
 				break;
 			}
-			/* Remember the connected address scope ID.  It will
-			 * be checked against the binding local address when
-			 * the socket is bound.
+			/* Remember the connected address scope ID.  It is
+			 * checked against the binding local address when
+			 * the socket is bound, and gives a send without a
+			 * destination its scope.
 			 */
-			rs->rs_bound_scope_id = sin6->sin6_scope_id;
+			rs->rs_conn_scope_id = sin6->sin6_scope_id;
+		} else {
+			rs->rs_conn_scope_id = 0;
 		}
 		rs->rs_conn_addr = sin6->sin6_addr;
 		rs->rs_conn_port = sin6->sin6_port;
diff --git a/net/rds/bind.c b/net/rds/bind.c
index f800d920d969..3ac59cd512a2 100644
--- a/net/rds/bind.c
+++ b/net/rds/bind.c
@@ -233,8 +233,8 @@ int rds_bind(struct socket *sock, struct sockaddr_unsized *uaddr, int addr_len)
 	 * non-link local address (scope_id is 0).
 	 */
 	if (!ipv6_addr_any(&rs->rs_conn_addr) && scope_id &&
-	    rs->rs_bound_scope_id &&
-	    scope_id != rs->rs_bound_scope_id) {
+	    rs->rs_conn_scope_id &&
+	    scope_id != rs->rs_conn_scope_id) {
 		ret = -EINVAL;
 		goto out;
 	}
diff --git a/net/rds/rds.h b/net/rds/rds.h
index 2db49573dacd..9b1ffc49c0a6 100644
--- a/net/rds/rds.h
+++ b/net/rds/rds.h
@@ -646,6 +646,8 @@ struct rds_sock {
 	struct in6_addr		rs_conn_addr;
 #define rs_conn_addr_v4		rs_conn_addr.s6_addr32[3]
 	__be16			rs_conn_port;
+	/* scope of rs_conn_addr when it is link-local, 0 otherwise */
+	__u32			rs_conn_scope_id;
 	struct rds_transport    *rs_transport;
 
 	/*
diff --git a/net/rds/recv.c b/net/rds/recv.c
index 6204e577a90a..9d81c76320ea 100644
--- a/net/rds/recv.c
+++ b/net/rds/recv.c
@@ -789,7 +789,8 @@ int rds_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 				sin6->sin6_port = inc->i_hdr.h_sport;
 				sin6->sin6_addr = inc->i_saddr;
 				sin6->sin6_flowinfo = 0;
-				sin6->sin6_scope_id = rs->rs_bound_scope_id;
+				sin6->sin6_scope_id = rs->rs_conn_scope_id ?:
+						      rs->rs_bound_scope_id;
 				msg->msg_namelen = sizeof(*sin6);
 			}
 		}
diff --git a/net/rds/send.c b/net/rds/send.c
index 1afa981e5c06..7b525a6f7eac 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -1256,7 +1256,11 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
 		lock_sock(sk);
 		daddr = rs->rs_conn_addr;
 		dport = rs->rs_conn_port;
-		scope_id = rs->rs_bound_scope_id;
+		/* The connected peer's scope for a link-local peer, else
+		 * the bound scope, as an explicit send to that peer would
+		 * compute below.
+		 */
+		scope_id = rs->rs_conn_scope_id ?: rs->rs_bound_scope_id;
 		release_sock(sk);
 	}
 
-- 
2.25.1

[PATCH net v4 2/2] net/rds: include the scope id in the sendmsg connection cache check

[Allison Henderson]

rds_sendmsg() reuses the connection cached in rs->rs_conn when its
peer address and ToS match the request.  The interface index is part
of a connection's identity as well: rds_conn_create_outgoing() passes
the request's scope_id down as dev_if, and rds_conn_lookup() compares
c_dev_if, so sends to the same link-local address through two
interfaces are two different connections.  The cache-hit test never
looked at it.

A socket bound to a non-link-local address has rs_bound_scope_id 0,
and the scope check at the top of rds_sendmsg() accepts any non-zero
destination scope for such a socket.  So after a send to fe80::x%ifA,
a send to fe80::x%ifB hits the cached ifA connection and the datagram
leaves through ifA, to whichever peer answers to that address there.

Compare c_dev_if with the request's scope_id in the cache test, so
that such a send takes the lookup path and finds, or creates, the ifB
connection - the same key the lookup uses.  With the previous patch a
send without a destination carries the connected peer's scope, or the
bound scope for a non-link-local peer, which is what an explicit send
to that peer computes, so the comparison is exact for it too.

Found by inspection while reworking this cache for the connection
lifetime series on net-next; not observed in the field.

Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
 net/rds/send.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/rds/send.c b/net/rds/send.c
index 7b525a6f7eac..f38967f65af1 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -1346,7 +1346,8 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
 	/* rds_conn_create has a spinlock that runs with IRQ off.
 	 * Caching the conn in the socket helps a lot. */
 	if (rs->rs_conn && ipv6_addr_equal(&rs->rs_conn->c_faddr, &daddr) &&
-	    rs->rs_tos == rs->rs_conn->c_tos) {
+	    rs->rs_tos == rs->rs_conn->c_tos &&
+	    rs->rs_conn->c_dev_if == scope_id) {
 		conn = rs->rs_conn;
 	} else {
 		conn = rds_conn_create_outgoing(sock_net(sock->sk),
-- 
2.25.1

Re: [PATCH net v4 2/2] net/rds: include the scope id in the sendmsg connection cache check

[sashiko-bot]

> rds_sendmsg() reuses the connection cached in rs->rs_conn when its
> peer address and ToS match the request.  The interface index is part
> of a connection's identity as well: rds_conn_create_outgoing() passes
> the request's scope_id down as dev_if, and rds_conn_lookup() compares
> c_dev_if, so sends to the same link-local address through two
> interfaces are two different connections.  The cache-hit test never
> looked at it.
> 
> A socket bound to a non-link-local address has rs_bound_scope_id 0,
> and the scope check at the top of rds_sendmsg() accepts any non-zero
> destination scope for such a socket.  So after a send to fe80::x%ifA,
> a send to fe80::x%ifB hits the cached ifA connection and the datagram
> leaves through ifA, to whichever peer answers to that address there.
> 
> Compare c_dev_if with the request's scope_id in the cache test, so
> that such a send takes the lookup path and finds, or creates, the ifB
> connection - the same key the lookup uses.  With the previous patch a
> send without a destination carries the connected peer's scope, or the
> bound scope for a non-link-local peer, which is what an explicit send
> to that peer computes, so the comparison is exact for it too.
> 
> Found by inspection while reworking this cache for the connection
> lifetime series on net-next; not observed in the field.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261008031426.1142344-1-achender@kernel.org?part=2

Re: [PATCH net v4 1/2] net/rds: keep the connected peer's scope id apart from the bound one

[sashiko-bot]

> rds_connect() has nowhere to keep the scope of a link-local peer, so
> it stores it in rs_bound_scope_id, the scope of the socket's own
> bound address, for rds_bind() to check a later link-local bind
> against.  That field is then also what a send without a destination
> uses as the request's scope, what getpeername() and recvmsg() report
> as the peer's scope, and what a later bind() overwrites:
> rds_add_bound() stores the bound address's scope unconditionally,
> which for a non-link-local address is 0.
> 
> So after connect(fe80::x%ifA) followed by bind(global), a send() with
> no destination asks for fe80::x with scope 0.  rds_conn_lookup() keys
> on the interface, so that finds or creates a connection with c_dev_if
> 0, which the TCP transport then tries to connect through
> sin6_scope_id 0 and tcp_v6_connect() rejects for a link-local peer:
> the connected socket's data is queued on a connection that can never
> [ ... ]
> observed in the field.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261008031426.1142344-1-achender@kernel.org?part=1

Re: [PATCH net v4 1/2] net/rds: keep the connected peer's scope id apart from the bound one

[Simon Horman]

On Wed, Oct 07, 2026 at 08:14:25PM -0700, Allison Henderson wrote:
> rds_connect() has nowhere to keep the scope of a link-local peer, so
> it stores it in rs_bound_scope_id, the scope of the socket's own
> bound address, for rds_bind() to check a later link-local bind
> against.  That field is then also what a send without a destination
> uses as the request's scope, what getpeername() and recvmsg() report
> as the peer's scope, and what a later bind() overwrites:
> rds_add_bound() stores the bound address's scope unconditionally,
> which for a non-link-local address is 0.
> 
> So after connect(fe80::x%ifA) followed by bind(global), a send() with
> no destination asks for fe80::x with scope 0.  rds_conn_lookup() keys
> on the interface, so that finds or creates a connection with c_dev_if
> 0, which the TCP transport then tries to connect through
> sin6_scope_id 0 and tcp_v6_connect() rejects for a link-local peer:
> the connected socket's data is queued on a connection that can never
> come up.  In the other order, bind(global) then connect(fe80::x%ifB),
> the connect leaves a global-bound socket with rs_bound_scope_id ifB,
> so sends to other link-local peers are refused as off-link and sends
> to global peers inherit a meaningless interface.
> 
> Give the connected peer its own rs_conn_scope_id.  rds_connect()
> records it there and leaves the bound scope alone, rds_bind()'s
> connected-socket check compares against it, and the destination-less
> send takes its scope from it - falling back to the bound scope for a
> non-link-local peer, so that send() and sendto() to the connected
> peer compute the same scope, and so the same connection.
> getpeername() reports the connected peer with its own scope, and
> recvmsg() reports a sender with the connected peer's scope when there
> is one and the bound scope otherwise, as before.
> 
> Found by inspection while reworking the sendmsg connection cache; not
> observed in the field.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Reviewed-by: Simon Horman <horms@kernel.org>

Re: [PATCH net v4 2/2] net/rds: include the scope id in the sendmsg connection cache check

[Simon Horman]

On Wed, Oct 07, 2026 at 08:14:26PM -0700, Allison Henderson wrote:
> rds_sendmsg() reuses the connection cached in rs->rs_conn when its
> peer address and ToS match the request.  The interface index is part
> of a connection's identity as well: rds_conn_create_outgoing() passes
> the request's scope_id down as dev_if, and rds_conn_lookup() compares
> c_dev_if, so sends to the same link-local address through two
> interfaces are two different connections.  The cache-hit test never
> looked at it.
> 
> A socket bound to a non-link-local address has rs_bound_scope_id 0,
> and the scope check at the top of rds_sendmsg() accepts any non-zero
> destination scope for such a socket.  So after a send to fe80::x%ifA,
> a send to fe80::x%ifB hits the cached ifA connection and the datagram
> leaves through ifA, to whichever peer answers to that address there.
> 
> Compare c_dev_if with the request's scope_id in the cache test, so
> that such a send takes the lookup path and finds, or creates, the ifB
> connection - the same key the lookup uses.  With the previous patch a
> send without a destination carries the connected peer's scope, or the
> bound scope for a non-link-local peer, which is what an explicit send
> to that peer computes, so the comparison is exact for it too.
> 
> Found by inspection while reworking this cache for the connection
> lifetime series on net-next; not observed in the field.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Reviewed-by: Simon Horman <horms@kernel.org>