[Xen-changelog] [xen stable-4.4] x86/HVM: avoid reading ioreq state more than once

[patchbot@xxxxxxx]

commit 4dacb5d7429913430895fe38cb58308ea4fa4a05
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Dec 17 14:31:28 2015 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Dec 17 14:31:28 2015 +0100

    x86/HVM: avoid reading ioreq state more than once
    
    Otherwise, especially when the compiler chooses to translate the
    switch() to a jump table, unpredictable behavior (and in the jump table
    case arbitrary code execution) can result.
    
    This is XSA-166.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    master commit: b452430a4cdfc801fa4bc391aed7522365e1deb6
    master date: 2015-12-17 14:22:46 +0100
---
 xen/arch/x86/hvm/hvm.c |   11 ++++++-----
 1 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 321f8bf..f3f6c61 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -348,6 +348,7 @@ void hvm_migrate_pirqs(struct vcpu *v)
 void hvm_do_resume(struct vcpu *v)
 {
     ioreq_t *p;
+    unsigned int state;
 
     check_wakeup_from_wait();
 
@@ -358,9 +359,10 @@ void hvm_do_resume(struct vcpu *v)
     if ( !(p = get_ioreq(v)) )
         goto check_inject_trap;
 
-    while ( p->state != STATE_IOREQ_NONE )
+    while ( (state = p->state) != STATE_IOREQ_NONE )
     {
-        switch ( p->state )
+        rmb();
+        switch ( state )
         {
         case STATE_IORESP_READY: /* IORESP_READY -> NONE */
             hvm_io_assist(p);
@@ -368,11 +370,10 @@ void hvm_do_resume(struct vcpu *v)
         case STATE_IOREQ_READY:  /* IOREQ_{READY,INPROCESS} -> IORESP_READY */
         case STATE_IOREQ_INPROCESS:
             wait_on_xen_event_channel(v->arch.hvm_vcpu.xen_port,
-                                      (p->state != STATE_IOREQ_READY) &&
-                                      (p->state != STATE_IOREQ_INPROCESS));
+                                      p->state != state);
             break;
         default:
-            gdprintk(XENLOG_ERR, "Weird HVM iorequest state %d.\n", p->state);
+            gdprintk(XENLOG_ERR, "Weird HVM iorequest state %u\n", state);
             domain_crash(v->domain);
             return; /* bail */
         }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.4

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