From 84203fed6bace55a9e7f89d83cf74bd81603e91e Mon Sep 17 00:00:00 2001
From: kib <kib@FreeBSD.org>
Date: Tue, 5 Feb 2019 17:49:27 +0000
Subject: [PATCH] amd64: clear callee-preserved registers on syscall exit.

%r8, %r10, and on non-KPTI configuration %r9 were not restored on fast
return from a syscall.

Reviewed by:	markj
Approved by:	so
Security:	CVE-2019-5595
Sponsored by:	The FreeBSD Foundation
MFC after:	0 minutes
---
 sys/amd64/amd64/exception.S | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sys/amd64/amd64/exception.S b/sys/amd64/amd64/exception.S
index 91767094954b..c0f57ad4babc 100644
--- a/sys/amd64/amd64/exception.S
+++ b/sys/amd64/amd64/exception.S
@@ -521,12 +521,14 @@ fast_syscall_common:
 	movq	TF_RFLAGS(%rsp),%r11	/* original %rflags */
 	movq	TF_RIP(%rsp),%rcx	/* original %rip */
 	movq	TF_RSP(%rsp),%rsp	/* user stack pointer */
+	xorl	%r8d,%r8d		/* zero the rest of GPRs */
+	xorl	%r10d,%r10d
 	cmpq	$~0,PCPU(UCR3)
 	je	2f
 	movq	PCPU(UCR3),%r9
 	movq	%r9,%cr3
-	xorl	%r9d,%r9d
-2:	swapgs
+2:	xorl	%r9d,%r9d
+	swapgs
 	sysretq
 
 3:	/* AST scheduled. */