Hi Jan,
On 05/01/2021 13:09, Jan Beulich wrote:
Neither evtchn_status() nor domain_dump_evtchn_info() nor
flask_get_peer_sid() need to hold the per-domain lock - they all only
read a single channel's state (at a time, in the dump case).
Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
v4: New.
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -968,15 +968,16 @@ int evtchn_status(evtchn_status_t *statu
if ( d == NULL )
return -ESRCH;
- spin_lock(&d->event_lock);
-
if ( !port_is_valid(d, port) )
There is one issue that is now becoming more apparent. To be clear, the
problem is not in this patch, but I think it is the best place to
discuss it as d->event_lock may be part of the solution.
After XSA-344, evtchn_destroy() will end up to decrement d->valid_evtchns.
Given that evtchn_status() can work on the non-current domain, it would
be possible to run it concurrently with evtchn_destroy(). As a
consequence, port_is_valid() will be unstable as a valid event channel
may turn invalid.
AFAICT, we are getting away so far, as the memory is not freed until the
domain is fully destroyed. However, we re-introduced XSA-338 in a
different way.
To be clear this is not the fault of this patch. But I don't think this
is sane to re-introduce a behavior that lead us to an XSA.
I can see two solutions:
1) Use d->event_lock to protect port_is_valid() when d !=
current->domain. This would require evtchn_destroy() to grab the lock
when updating d->valid_evtchns.
2) Never decrement d->valid_evtchns and use a different field for
closing ports
I am not a big fan of 1) because this is muddying the already complex
locking situation in the event channel code. But I suggested it because
I wasn't sure whether you would be happy with 2).
If you are happy with 2), then the lock can be dropped here. I would be
happy to send the patch for either 1) or 2) depending on the agreement here.
Cheers,
{
- rc = -EINVAL;
- goto out;
+ rcu_unlock_domain(d);
+ return -EINVAL;
}
chn = evtchn_from_port(d, port);
+
+ evtchn_read_lock(chn);
+
if ( consumer_is_xen(chn) )
{
rc = -EACCES;
@@ -1021,7 +1022,7 @@ int evtchn_status(evtchn_status_t *statu
status->vcpu = chn->notify_vcpu_id;
out:
- spin_unlock(&d->event_lock);
+ evtchn_read_unlock(chn);
rcu_unlock_domain(d);
return rc;
@@ -1576,22 +1577,32 @@ void evtchn_move_pirqs(struct vcpu *v)
static void domain_dump_evtchn_info(struct domain *d)
{
unsigned int port;
- int irq;
printk("Event channel information for domain %d:\n"
"Polling vCPUs: {%*pbl}\n"
" port [p/m/s]\n", d->domain_id, d->max_vcpus, d->poll_mask);
- spin_lock(&d->event_lock);
-
for ( port = 1; port_is_valid(d, port); ++port )
{
- const struct evtchn *chn;
+ struct evtchn *chn;
char *ssid;
+ if ( !(port & 0x3f) )
+ process_pending_softirqs();
+
chn = evtchn_from_port(d, port);
+
+ if ( !evtchn_read_trylock(chn) )
+ {
+ printk(" %4u in flux\n", port);
+ continue;
+ }
+
if ( chn->state == ECS_FREE )
+ {
+ evtchn_read_unlock(chn);
continue;
+ }
printk(" %4u [%d/%d/",
port,
@@ -1601,26 +1612,49 @@ static void domain_dump_evtchn_info(stru
printk("]: s=%d n=%d x=%d",
chn->state, chn->notify_vcpu_id, chn->xen_consumer);
+ ssid = xsm_show_security_evtchn(d, chn);
+
switch ( chn->state )
{
case ECS_UNBOUND:
printk(" d=%d", chn->u.unbound.remote_domid);
break;
+
case ECS_INTERDOMAIN:
printk(" d=%d p=%d",
chn->u.interdomain.remote_dom->domain_id,
chn->u.interdomain.remote_port);
break;
- case ECS_PIRQ:
- irq = domain_pirq_to_irq(d, chn->u.pirq.irq);
- printk(" p=%d i=%d", chn->u.pirq.irq, irq);
+
+ case ECS_PIRQ: {
+ unsigned int pirq = chn->u.pirq.irq;
+
+ /*
+ * The per-channel locks nest inside the per-domain one, so we
+ * can't acquire the latter without first letting go of the former.
+ */
+ evtchn_read_unlock(chn);
+ chn = NULL;
+ if ( spin_trylock(&d->event_lock) )
+ {
+ int irq = domain_pirq_to_irq(d, pirq);
+
+ spin_unlock(&d->event_lock);
+ printk(" p=%u i=%d", pirq, irq);
+ }
+ else
+ printk(" p=%u i=?", pirq);
break;
+ }
+
case ECS_VIRQ:
printk(" v=%d", chn->u.virq);
break;
}
- ssid = xsm_show_security_evtchn(d, chn);
+ if ( chn )
+ evtchn_read_unlock(chn);
+
if (ssid) {
printk(" Z=%s\n", ssid);
xfree(ssid);
@@ -1628,8 +1662,6 @@ static void domain_dump_evtchn_info(stru
printk("\n");
}
}
-
- spin_unlock(&d->event_lock);
}
static void dump_evtchn_info(unsigned char key)
--- a/xen/xsm/flask/flask_op.c
+++ b/xen/xsm/flask/flask_op.c
@@ -555,12 +555,13 @@ static int flask_get_peer_sid(struct xen
struct evtchn *chn;
struct domain_security_struct *dsec;
- spin_lock(&d->event_lock);
-
if ( !port_is_valid(d, arg->evtchn) )
- goto out;
+ return -EINVAL;
chn = evtchn_from_port(d, arg->evtchn);
+
+ evtchn_read_lock(chn);
+
if ( chn->state != ECS_INTERDOMAIN )
goto out;
@@ -573,7 +574,7 @@ static int flask_get_peer_sid(struct xen
rv = 0;
out:
- spin_unlock(&d->event_lock);
+ evtchn_read_unlock(chn);
return rv;
}
--
Julien Grall
