To avoid a kernel panic provoked by a NULL pointer dereference,

do not clear the `sb_sel' member of the sockbuf structure
while invalidating the receive sockbuf in sorflush(), called
from soshutdown().

The panic was reproduceable from user land by attaching a knote
with EVFILT_READ filters to a socket, disabling further reads
from it using shutdown(2), and then closing it.  knote_remove()
was called to remove all knotes from the socket file descriptor
by detaching each using its associated filterops' detach call-
back function, sordetach() in this case, which tried to remove
itself from the invalidated sockbuf's klist (sb_sel.si_note).

PR:	kern/54331

sys/kern/uipc_socket.c

@@ -1133,8 +1133,14 @@ sorflush(so)
 	socantrcvmore(so);
 	sbunlock(sb);
 	asb = *sb;
-	bzero(sb, sizeof (*sb));
+	/*
+	 * Invalidate/clear most of the sockbuf structure, but keep
+	 * its selinfo structure valid.
+	 */
+	bzero(&sb->sb_startzero,
+	    sizeof(*sb) - offsetof(struct sockbuf, sb_startzero));
 	splx(s);
+
 	if (pr->pr_flags & PR_RIGHTS && pr->pr_domain->dom_dispose)
 		(*pr->pr_domain->dom_dispose)(asb.sb_mb);
 	sbrelease(&asb, so);

sys/sys/socketvar.h

@@ -100,6 +100,7 @@ struct socket {
  */
 	struct sockbuf {
 		struct	selinfo sb_sel;	/* process selecting read/write */
+#define	sb_startzero	sb_mb
 		struct	mbuf *sb_mb;	/* the mbuf chain */
 		u_int	sb_cc;		/* actual chars in buffer */
 		u_int	sb_hiwat;	/* max actual char count */