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From: Jan Beulich <jbeulich@suse.com>
Subject: x86/HVM: guard against emulator driving ioreq state in weird ways
In the case where hvm_wait_for_io() calls wait_on_xen_event_channel(),
p->state ends up being read twice in succession: once to determine that
state != p->state, and then again at the top of the loop. This gives a
compromised emulator a chance to change the state back between the two
reads, potentially keeping Xen in a loop indefinitely.
Instead:
* Read p->state once in each of the wait_on_xen_event_channel() tests,
* re-use that value the next time around,
* and insist that the states continue to transition "forward" (with the
exception of the transition to STATE_IOREQ_NONE).
This is XSA-262.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
--- a/xen/arch/x86/hvm/ioreq.c
+++ b/xen/arch/x86/hvm/ioreq.c
@@ -87,14 +87,17 @@ static void hvm_io_assist(struct hvm_ior
static bool hvm_wait_for_io(struct hvm_ioreq_vcpu *sv, ioreq_t *p)
{
+ unsigned int prev_state = STATE_IOREQ_NONE;
+
while ( sv->pending )
{
unsigned int state = p->state;
- rmb();
- switch ( state )
+ smp_rmb();
+
+ recheck:
+ if ( unlikely(state == STATE_IOREQ_NONE) )
{
- case STATE_IOREQ_NONE:
/*
* The only reason we should see this case is when an
* emulator is dying and it races with an I/O being
@@ -102,14 +105,30 @@ static bool hvm_wait_for_io(struct hvm_i
*/
hvm_io_assist(sv, ~0ul);
break;
+ }
+
+ if ( unlikely(state < prev_state) )
+ {
+ gdprintk(XENLOG_ERR, "Weird HVM ioreq state transition %u -> %u\n",
+ prev_state, state);
+ sv->pending = false;
+ domain_crash(sv->vcpu->domain);
+ return false; /* bail */
+ }
+
+ switch ( prev_state = state )
+ {
case STATE_IORESP_READY: /* IORESP_READY -> NONE */
p->state = STATE_IOREQ_NONE;
hvm_io_assist(sv, p->data);
break;
case STATE_IOREQ_READY: /* IOREQ_{READY,INPROCESS} -> IORESP_READY */
case STATE_IOREQ_INPROCESS:
- wait_on_xen_event_channel(sv->ioreq_evtchn, p->state != state);
- break;
+ wait_on_xen_event_channel(sv->ioreq_evtchn,
+ ({ state = p->state;
+ smp_rmb();
+ state != prev_state; }));
+ goto recheck;
default:
gdprintk(XENLOG_ERR, "Weird HVM iorequest state %u\n", state);
sv->pending = false;
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