From ef2fa499124934626b1182bb236872f40a841b19 Mon Sep 17 00:00:00 2001
From: William Pitcock <nenolod@dereferenced.org>
Date: Thu, 15 Feb 2018 04:32:16 +0000
Subject: [PATCH] x86: trampoline: fix GOT clobbering, pointed out by fabled

---
 arch/x86/startcontext.S | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/startcontext.S b/arch/x86/startcontext.S
index 2bb87ee..3304863 100644
--- a/arch/x86/startcontext.S
+++ b/arch/x86/startcontext.S
@@ -15,15 +15,26 @@ __start_context:
 	/* get the proper context into position and test for NULL */
 	leal	(%esp,%ebx,4), %esp
 	cmpl	$0, (%esp)
-	je	hosed
+
+	/* restore global offset table, exit@plt and __setcontext@plt need this */
+	call	__i686.get_pc_thunk.bx
+	addl	$_GLOBAL_OFFSET_TABLE_, %ebx
+
+	/* if we have no linked context, lets get out of here */
+	je	no_linked_context
 
 	/* call setcontext to switch to the linked context */
 	call	__setcontext@plt
 	movl	%eax, (%esp)
 
-hosed:
+no_linked_context:
 	/* we are returning into a null context, it seems, so maybe we should exit */
 	call	exit@plt
 
 	/* something is really hosed, call hlt to force termination */
 	hlt
+
+
+__i686.get_pc_thunk.bx:
+	mov	(%esp), %ebx
+	ret