From 2dc8401e2377e3ce7bee5c57bfd504f71c406cd9 Mon Sep 17 00:00:00 2001
From: David du Colombier <0intro@gmail.com>
Date: Tue, 20 May 1997 00:00:00 +0000
Subject: [PATCH] Plan 9 from Bell Labs 1997-05-20

---
 pc/mem.h  |  4 ++--
 pc/trap.c | 16 ++++++++++++----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/pc/mem.h b/pc/mem.h
index a7dc3e85e77cb28ab9061f0349b4edef4bd99594..76f4c6e2f499bac0e29c5ae0bfd4e6231a061ed1 100644
--- a/pc/mem.h
+++ b/pc/mem.h
@@ -73,8 +73,8 @@
 #define	UESEG	4	/* user executable */
 #define TSSSEG	5	/* task segment */
 
-#define SELGDT	(0<<3)	/* selector is in gdt */
-#define	SELLDT	(1<<3)	/* selector is in ldt */
+#define SELGDT	(0<<2)	/* selector is in gdt */
+#define	SELLDT	(1<<2)	/* selector is in ldt */
 
 #define SELECTOR(i, t, p)	(((i)<<3) | (t) | (p))
 
diff --git a/pc/trap.c b/pc/trap.c
index 8c399e957e3692d1792c757b2df475a9b6c49110..8a2950f3d35df091be686473905b08a74c0c406b 100644
--- a/pc/trap.c
+++ b/pc/trap.c
@@ -483,14 +483,22 @@ noted(Ureg* ureg, ulong arg0)
 	/* sanity clause */
 	oureg = (ulong)nureg;
 	if(!okaddr((ulong)oureg-BY2WD, BY2WD+sizeof(Ureg), 0)){
-		pprint("bad ureg in noted or call to noted() when not notified\n");
+		pprint("bad ureg in noted or call to noted when not notified\n");
 		qunlock(&up->debug);
 		pexit("Suicide", 0);
 	}
 
-	/* don't let user change text or stack segments */
-	nureg->cs = ureg->cs;
-	nureg->ss = ureg->ss;
+	/*
+	 * Check the segment selectors are all valid, otherwise
+	 * a fault will be taken on attempting to return to the
+	 * user process.
+	 */
+	if(nureg->cs != UESEL || nureg->ss != UDSEL || nureg->ds != UDSEL
+	  || nureg->es != UDSEL || nureg->fs != UDSEL || nureg->gs != UDSEL){
+		pprint("bad segement selector in noted\n");
+		qunlock(&up->debug);
+		pexit("Suicide", 0);
+	}
 
 	/* don't let user change system flags */
 	nureg->flags = (ureg->flags & ~0xCD5) | (nureg->flags & 0xCD5);