From 274a1f6dab3e9f4ac339831211c6f779bc33e777 Mon Sep 17 00:00:00 2001
From: Jan Beulich <jbeulich@suse.com>
Date: Tue, 22 Nov 2016 14:28:40 +0100
Subject: [PATCH 4/8] x86/PV: writes of %fs and %gs base MSRs require canonical
 addresses

Commit c42494acb2 ("x86: fix FS/GS base handling when using the
fsgsbase feature") replaced the use of wrmsr_safe() on these paths
without recognizing that wr{f,g}sbase() use just wrmsrl() and that the
WR{F,G}SBASE instructions also raise #GP for non-canonical input.

Similarly arch_set_info_guest() needs to prevent non-canonical
addresses from getting stored into state later to be loaded by context
switch code. For consistency also check stack pointers and LDT base.
DR0..3, otoh, already get properly checked in set_debugreg() (albeit
we discard the error there).

The SHADOW_GS_BASE check isn't strictly necessary, but I think we
better avoid trying the WRMSR if we know it's going to fail.

This is CVE-2016-9385 / XSA-193.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: f3fa3abf3e61fb1f25ce721e14ac324dda67311f
master date: 2016-11-22 13:46:28 +0100
---
 xen/arch/x86/domain.c | 8 +++++++-
 xen/arch/x86/traps.c  | 6 +++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index b1d46c0..2596bff 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -741,7 +741,13 @@ int arch_set_info_guest(
     {
         if ( !compat )
         {
-            if ( !is_canonical_address(c.nat->user_regs.eip) ||
+            if ( !is_canonical_address(c.nat->user_regs.rip) ||
+                 !is_canonical_address(c.nat->user_regs.rsp) ||
+                 !is_canonical_address(c.nat->kernel_sp) ||
+                 (c.nat->ldt_ents && !is_canonical_address(c.nat->ldt_base)) ||
+                 !is_canonical_address(c.nat->fs_base) ||
+                 !is_canonical_address(c.nat->gs_base_kernel) ||
+                 !is_canonical_address(c.nat->gs_base_user) ||
                  !is_canonical_address(c.nat->event_callback_eip) ||
                  !is_canonical_address(c.nat->syscall_callback_eip) ||
                  !is_canonical_address(c.nat->failsafe_callback_eip) )
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 592037b..dc949f2 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2439,19 +2439,19 @@ static int emulate_privileged_op(struct cpu_user_regs *regs)
         switch ( (u32)regs->ecx )
         {
         case MSR_FS_BASE:
-            if ( is_pv_32on64_vcpu(v) )
+            if ( is_pv_32on64_vcpu(v) || !is_canonical_address(msr_content) )
                 goto fail;
             wrfsbase(msr_content);
             v->arch.pv_vcpu.fs_base = msr_content;
             break;
         case MSR_GS_BASE:
-            if ( is_pv_32on64_vcpu(v) )
+            if ( is_pv_32on64_vcpu(v) || !is_canonical_address(msr_content) )
                 goto fail;
             wrgsbase(msr_content);
             v->arch.pv_vcpu.gs_base_kernel = msr_content;
             break;
         case MSR_SHADOW_GS_BASE:
-            if ( is_pv_32on64_vcpu(v) )
+            if ( is_pv_32on64_vcpu(v) || !is_canonical_address(msr_content) )
                 goto fail;
             if ( wrmsr_safe(MSR_SHADOW_GS_BASE, msr_content) )
                 goto fail;
-- 
2.10.2