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

On Thu, May 11, 2023 at 3:12 PM Marcelo Ricardo Leitner
[off-list ref] wrote:

Hi,

Hi Marcelo,

thanks! Fixed in -v3.

Kind regards,
Alex

Two things:

On Thu, May 11, 2023 at 02:31:48PM +0200, Alexander Mikhalitsyn wrote:

quoted

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@canonical.com/ (local)

Cool, but the description is mentioning the CONNECTX3 sockopt.

quoted

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Stanislav Fomichev <redacted>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: linux-sctp@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Suggested-by: Stanislav Fomichev <redacted>
Acked-by: Stanislav Fomichev <redacted>
Signed-off-by: Alexander Mikhalitsyn <redacted>
---
 net/sctp/socket.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index cda8c2874691..a211a203003c 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c

@@ -8281,6 +8281,35 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
      return retval;
 }

+static bool sctp_bpf_bypass_getsockopt(int level, int optname)
+{
+     if (level == SOL_SCTP) {
+             switch (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.
+              */

This and..

quoted

+             case SCTP_SOCKOPT_PEELOFF:
+             case SCTP_SOCKOPT_PEELOFF_FLAGS:
+             /*
+              * As pointed by Marcelo Ricardo Leitner it seems reasonable to skip
+              * bpf getsockopt hook for this sockopt too. Because internaly, it
+              * triggers connect() and if error will be masked userspace can be confused.
+              */

..this comments can be removed, as they are easily visible on the
description later on for who is interested on why such lines were
added.

Thanks,
Marcelo

quoted

+             case SCTP_SOCKOPT_CONNECTX3:
+                     return true;
+             default:
+                     return false;
+             }
+     }
+
+     return false;
+}
+
 static int sctp_hash(struct sock *sk)
 {
      /* STUB */

@@ -9650,6 +9679,7 @@ 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 +9735,7 @@ 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