[Xen-devel] [PATCH v2] Fix ioreq-server event channel vulnerability

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

The code in hvm_send_assist_req_to_ioreq_server() and hvm_do_resume() uses
an event channel port number taken from the page of memory shared with the
emulator. This allows an emulator to corrupt values that are then blindly
used by Xen, leading to assertion failures in some cases. Moreover, in the
case of the default ioreq server the page remains in the guest p2m so a
malicious guest could similarly corrupt those values.

This patch changes the afforementioned functions to get the event channel
port number from an internal structure and also adds an extra check to
hvm_send_assist_req_to_ioreq_server() which will crash the domain should the
guest or an emulator corrupt the port number in the shared page.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Reported-by: Wen Congyang <wency@xxxxxxxxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---

v2:
 - hvm_wait_for_io() now returns a bool_t as suggested by Andrew Cooper

 xen/arch/x86/hvm/hvm.c |  113 +++++++++++++++++++++++++++++++++---------------
 1 file changed, 77 insertions(+), 36 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index ef2411c..a573c59 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -392,6 +392,33 @@ bool_t hvm_io_pending(struct vcpu *v)
     return 0;
 }
 
+static bool_t hvm_wait_for_io(struct hvm_ioreq_vcpu *sv, ioreq_t *p)
+{
+    /* NB. Optimised for common case (p->state == STATE_IOREQ_NONE). */
+    while ( p->state != STATE_IOREQ_NONE )
+    {
+        switch ( p->state )
+        {
+        case STATE_IORESP_READY: /* IORESP_READY -> NONE */
+            rmb(); /* see IORESP_READY /then/ read contents of ioreq */
+            hvm_io_assist(p);
+            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_IOREQ_READY) &&
+                                      (p->state != STATE_IOREQ_INPROCESS));
+            break;
+        default:
+            gdprintk(XENLOG_ERR, "Weird HVM iorequest state %d.\n", p->state);
+            domain_crash(sv->vcpu->domain);
+            return 0; /* bail */
+        }
+    }
+
+    return 1;
+}
+
 void hvm_do_resume(struct vcpu *v)
 {
     struct domain *d = v->domain;
@@ -406,27 +433,18 @@ void hvm_do_resume(struct vcpu *v)
                           &d->arch.hvm_domain.ioreq_server.list,
                           list_entry )
     {
-        ioreq_t *p = get_ioreq(s, v);
+        struct hvm_ioreq_vcpu *sv;
 
-        /* NB. Optimised for common case (p->state == STATE_IOREQ_NONE). */
-        while ( p->state != STATE_IOREQ_NONE )
+        list_for_each_entry ( sv,
+                              &s->ioreq_vcpu_list,
+                              list_entry )
         {
-            switch ( p->state )
+            if ( sv->vcpu == v )
             {
-            case STATE_IORESP_READY: /* IORESP_READY -> NONE */
-                rmb(); /* see IORESP_READY /then/ read contents of ioreq */
-                hvm_io_assist(p);
-                break;
-            case STATE_IOREQ_READY:  /* IOREQ_{READY,INPROCESS} -> 
IORESP_READY */
-            case STATE_IOREQ_INPROCESS:
-                wait_on_xen_event_channel(p->vp_eport,
-                                          (p->state != STATE_IOREQ_READY) &&
-                                          (p->state != STATE_IOREQ_INPROCESS));
+                if ( !hvm_wait_for_io(sv, get_ioreq(s, v)) )
+                    return;
+
                 break;
-            default:
-                gdprintk(XENLOG_ERR, "Weird HVM iorequest state %d.\n", 
p->state);
-                domain_crash(d);
-                return; /* bail */
             }
         }
     }
@@ -2545,35 +2563,58 @@ bool_t hvm_send_assist_req_to_ioreq_server(struct 
hvm_ioreq_server *s,
 {
     struct vcpu *curr = current;
     struct domain *d = curr->domain;
-    ioreq_t *p;
+    struct hvm_ioreq_vcpu *sv;
 
     if ( unlikely(!vcpu_start_shutdown_deferral(curr)) )
         return 0; /* implicitly bins the i/o operation */
 
-    p = get_ioreq(s, curr);
-
-    if ( unlikely(p->state != STATE_IOREQ_NONE) )
+    list_for_each_entry ( sv,
+                          &s->ioreq_vcpu_list,
+                          list_entry )
     {
-        /* This indicates a bug in the device model. Crash the domain. */
-        gdprintk(XENLOG_ERR, "Device model set bad IO state %d.\n", p->state);
-        domain_crash(d);
-        return 0;
-    }
+        if ( sv->vcpu == curr )
+        {
+            evtchn_port_t port = sv->ioreq_evtchn;
+            ioreq_t *p = get_ioreq(s, curr);
+
+            if ( unlikely(p->state != STATE_IOREQ_NONE) )
+            {
+                gdprintk(XENLOG_ERR,
+                         "Device model set bad IO state %d.\n",
+                         p->state);
+                goto crash;
+            }
+
+            if ( unlikely(p->vp_eport != port) )
+            {
+                gdprintk(XENLOG_ERR,
+                         "Device model set bad event channel %d.\n",
+                         p->vp_eport);
+                goto crash;
+            }
 
-    proto_p->state = STATE_IOREQ_NONE;
-    proto_p->vp_eport = p->vp_eport;
-    *p = *proto_p;
+            proto_p->state = STATE_IOREQ_NONE;
+            proto_p->vp_eport = port;
+            *p = *proto_p;
 
-    prepare_wait_on_xen_event_channel(p->vp_eport);
+            prepare_wait_on_xen_event_channel(port);
 
-    /*
-     * Following happens /after/ blocking and setting up ioreq contents.
-     * prepare_wait_on_xen_event_channel() is an implicit barrier.
-     */
-    p->state = STATE_IOREQ_READY;
-    notify_via_xen_event_channel(d, p->vp_eport);
+            /*
+             * Following happens /after/ blocking and setting up ioreq
+             * contents. prepare_wait_on_xen_event_channel() is an implicit
+             * barrier.
+             */
+            p->state = STATE_IOREQ_READY;
+            notify_via_xen_event_channel(d, port);
+            break;
+        }
+    }
 
     return 1;
+
+ crash:
+    domain_crash(d);
+    return 0;
 }
 
 bool_t hvm_send_assist_req(ioreq_t *p)
-- 
1.7.10.4


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