[PATCH net-next v2] sctp: add bpf_bypass_getsockopt proto callback

[Alexander Mikhalitsyn]

Add bpf_bypass_getsockopt proto callback and filter out
SCTP_SOCKOPT_PEELOFF and SCTP_SOCKOPT_PEELOFF_FLAGS socket options
from running eBPF hook on them.

These options do fd_install(), and if BPF_CGROUP_RUN_PROG_GETSOCKOPT
hook returns an error after success of the original handler
sctp_getsockopt(...), userspace will receive an error from getsockopt
syscall and will be not aware that fd was successfully installed into fdtable.

This patch was born as a result of discussion around a new SCM_PIDFD interface:
https://lore.kernel.org/all/20230413133355.350571-3-aleksandr.mikhalitsyn@xxxxxxxxxxxxx/

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Cc: Xin Long <lucien.xin@xxxxxxxxx>
Cc: linux-sctp@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Suggested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
---
 net/sctp/socket.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index cda8c2874691..fed6057beb60 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -8281,6 +8281,29 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
 	return retval;
 }
 
+static bool sctp_bpf_bypass_getsockopt(int level, int optname)
+{
+	/*
+	 * These options do fd_install(), and if BPF_CGROUP_RUN_PROG_GETSOCKOPT
+	 * hook returns an error after success of the original handler
+	 * sctp_getsockopt(...), userspace will receive an error from getsockopt
+	 * syscall and will be not aware that fd was successfully installed into fdtable.
+	 *
+	 * Let's prevent bpf cgroup hook from running on them.
+	 */
+	if (level == SOL_SCTP) {
+		switch (optname) {
+		case SCTP_SOCKOPT_PEELOFF:
+		case SCTP_SOCKOPT_PEELOFF_FLAGS:
+			return true;
+		default:
+			return false;
+		}
+	}
+
+	return false;
+}
+
 static int sctp_hash(struct sock *sk)
 {
 	/* STUB */
@@ -9650,6 +9673,7 @@ struct proto sctp_prot = {
 	.shutdown    =	sctp_shutdown,
 	.setsockopt  =	sctp_setsockopt,
 	.getsockopt  =	sctp_getsockopt,
+	.bpf_bypass_getsockopt	= sctp_bpf_bypass_getsockopt,
 	.sendmsg     =	sctp_sendmsg,
 	.recvmsg     =	sctp_recvmsg,
 	.bind        =	sctp_bind,
@@ -9705,6 +9729,7 @@ struct proto sctpv6_prot = {
 	.shutdown	= sctp_shutdown,
 	.setsockopt	= sctp_setsockopt,
 	.getsockopt	= sctp_getsockopt,
+	.bpf_bypass_getsockopt	= sctp_bpf_bypass_getsockopt,
 	.sendmsg	= sctp_sendmsg,
 	.recvmsg	= sctp_recvmsg,
 	.bind		= sctp_bind,
-- 
2.34.1