From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Fri, 26 Jun 2020 13:56:23 +0000 (+0100)
Subject: x86/pv: Don't deliver #GP for a SYSENTER with NT set
X-Git-Tag: archive/raspbian/4.16.0+51-g0941d6cb-1+rpi1~2^2~42^2~1599
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=61d4a04349895edc5a5868274b906ba61ef24f47;p=xen.git

x86/pv: Don't deliver #GP for a SYSENTER with NT set

It is a matter of guest kernel policy what to do with offending userspace, and
terminating said userspace may not be the action chosen.

Linux explicitly tolerates this case.

Reported-by: Andy Lutomirski <luto@kernel.org>
Fixes: fdac951560 ("x86: clear EFLAGS.NT in SYSENTER entry path")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 71a00e846b..44a110b9c8 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -313,7 +313,6 @@ UNLIKELY_START(nz, sysenter_nt_set)
         pushfq
         andl  $~X86_EFLAGS_NT,(%rsp)
         popfq
-        xorl  %eax,%eax
 UNLIKELY_END(sysenter_nt_set)
         testq %rax,%rax
         leal  (,%rcx,TBF_INTERRUPT),%ecx