evtchn: address races with evtchn_reset()

Neither d->evtchn_port_ops nor max_evtchns(d) may be used in an entirely
lock-less manner, as both may change by a racing evtchn_reset(). In the
common case, at least one of the domain's event lock or the per-channel
lock needs to be held. In the specific case of the inter-domain sending
by evtchn_send() and notify_via_xen_event_channel() holding the other
side's per-channel lock is sufficient, as the channel can't change state
without both per-channel locks held. Without such a channel changing
state, evtchn_reset() can't complete successfully.

Lock-free accesses continue to be permitted for the shim (calling some
otherwise internal event channel functions), as this happens while the
domain is in effectively single-threaded mode. Special care also needs
taking for the shim's marking of in-use ports as ECS_RESERVED (allowing
use of such ports in the shim case is okay because switching into and
hence also out of FIFO mode is impossihble there).

As a side effect, certain operations on Xen bound event channels which
were mistakenly permitted so far (e.g. unmask or poll) will be refused
now.

This is part of XSA-343.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/arch/x86/irq.c

@@ -2488,14 +2488,24 @@ static void dump_irqs(unsigned char key)
 
             for ( i = 0; i < action->nr_guests; )
             {
+                struct evtchn *evtchn;
+                unsigned int pending = 2, masked = 2;
+
                 d = action->guest[i++];
                 pirq = domain_irq_to_pirq(d, irq);
                 info = pirq_info(d, pirq);
+                evtchn = evtchn_from_port(d, info->evtchn);
+                local_irq_disable();
+                if ( spin_trylock(&evtchn->lock) )
+                {
+                    pending = evtchn_is_pending(d, evtchn);
+                    masked = evtchn_is_masked(d, evtchn);
+                    spin_unlock(&evtchn->lock);
+                }
+                local_irq_enable();
                 printk("d%d:%3d(%c%c%c)%c",
-                       d->domain_id, pirq,
-                       evtchn_port_is_pending(d, info->evtchn) ? 'P' : '-',
-                       evtchn_port_is_masked(d, info->evtchn) ? 'M' : '-',
-                       info->masked ? 'M' : '-',
+                       d->domain_id, pirq, "-P?"[pending],
+                       "-M?"[masked], info->masked ? 'M' : '-',
                        i < action->nr_guests ? ',' : '\n');
             }
         }

xen/arch/x86/pv/shim.c

@@ -660,8 +660,11 @@ void pv_shim_inject_evtchn(unsigned int port)
     if ( port_is_valid(guest, port) )
     {
         struct evtchn *chn = evtchn_from_port(guest, port);
+        unsigned long flags;
 
+        spin_lock_irqsave(&chn->lock, flags);
         evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
+        spin_unlock_irqrestore(&chn->lock, flags);
     }
 }
 

xen/common/event_2l.c

@@ -63,8 +63,10 @@ static void evtchn_2l_unmask(struct domain *d, struct evtchn *evtchn)
     }
 }
 
-static bool evtchn_2l_is_pending(const struct domain *d, evtchn_port_t port)
+static bool evtchn_2l_is_pending(const struct domain *d,
+                                 const struct evtchn *evtchn)
 {
+    evtchn_port_t port = evtchn->port;
     unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
 
     ASSERT(port < max_ports);
@@ -72,8 +74,10 @@ static bool evtchn_2l_is_pending(const struct domain *d, evtchn_port_t port)
             guest_test_bit(d, port, &shared_info(d, evtchn_pending)));
 }
 
-static bool evtchn_2l_is_masked(const struct domain *d, evtchn_port_t port)
+static bool evtchn_2l_is_masked(const struct domain *d,
+                                const struct evtchn *evtchn)
 {
+    evtchn_port_t port = evtchn->port;
     unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
 
     ASSERT(port < max_ports);

xen/common/event_channel.c

@@ -156,8 +156,9 @@ int evtchn_allocate_port(struct domain *d, evtchn_port_t port)
 
     if ( port_is_valid(d, port) )
     {
-        if ( evtchn_from_port(d, port)->state != ECS_FREE ||
-             evtchn_port_is_busy(d, port) )
+        const struct evtchn *chn = evtchn_from_port(d, port);
+
+        if ( chn->state != ECS_FREE || evtchn_is_busy(d, chn) )
             return -EBUSY;
     }
     else
@@ -774,6 +775,7 @@ void send_guest_vcpu_virq(struct vcpu *v, uint32_t virq)
     unsigned long flags;
     int port;
     struct domain *d;
+    struct evtchn *chn;
 
     ASSERT(!virq_is_global(virq));
 
@@ -784,7 +786,10 @@ void send_guest_vcpu_virq(struct vcpu *v, uint32_t virq)
         goto out;
 
     d = v->domain;
-    evtchn_port_set_pending(d, v->vcpu_id, evtchn_from_port(d, port));
+    chn = evtchn_from_port(d, port);
+    spin_lock(&chn->lock);
+    evtchn_port_set_pending(d, v->vcpu_id, chn);
+    spin_unlock(&chn->lock);
 
  out:
     spin_unlock_irqrestore(&v->virq_lock, flags);
@@ -813,7 +818,9 @@ void send_guest_global_virq(struct domain *d, uint32_t virq)
         goto out;
 
     chn = evtchn_from_port(d, port);
+    spin_lock(&chn->lock);
     evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
+    spin_unlock(&chn->lock);
 
  out:
     spin_unlock_irqrestore(&v->virq_lock, flags);
@@ -823,6 +830,7 @@ void send_guest_pirq(struct domain *d, const struct pirq *pirq)
 {
     int port;
     struct evtchn *chn;
+    unsigned long flags;
 
     /*
      * PV guests: It should not be possible to race with __evtchn_close(). The
@@ -837,7 +845,9 @@ void send_guest_pirq(struct domain *d, const struct pirq *pirq)
     }
 
     chn = evtchn_from_port(d, port);
+    spin_lock_irqsave(&chn->lock, flags);
     evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
+    spin_unlock_irqrestore(&chn->lock, flags);
 }
 
 static struct domain *global_virq_handlers[NR_VIRQS] __read_mostly;
@@ -1034,12 +1044,15 @@ int evtchn_unmask(unsigned int port)
 {
     struct domain *d = current->domain;
     struct evtchn *evtchn;
+    unsigned long flags;
 
     if ( unlikely(!port_is_valid(d, port)) )
         return -EINVAL;
 
     evtchn = evtchn_from_port(d, port);
+    spin_lock_irqsave(&evtchn->lock, flags);
     evtchn_port_unmask(d, evtchn);
+    spin_unlock_irqrestore(&evtchn->lock, flags);
 
     return 0;
 }
@@ -1449,8 +1462,8 @@ static void domain_dump_evtchn_info(struct domain *d)
 
         printk("    %4u [%d/%d/",
                port,
-               evtchn_port_is_pending(d, port),
-               evtchn_port_is_masked(d, port));
+               evtchn_is_pending(d, chn),
+               evtchn_is_masked(d, chn));
         evtchn_port_print_state(d, chn);
         printk("]: s=%d n=%d x=%d",
                chn->state, chn->notify_vcpu_id, chn->xen_consumer);

xen/common/event_fifo.c

@@ -296,23 +296,26 @@ static void evtchn_fifo_unmask(struct domain *d, struct evtchn *evtchn)
         evtchn_fifo_set_pending(v, evtchn);
 }
 
-static bool evtchn_fifo_is_pending(const struct domain *d, evtchn_port_t port)
+static bool evtchn_fifo_is_pending(const struct domain *d,
+                                   const struct evtchn *evtchn)
 {
-    const event_word_t *word = evtchn_fifo_word_from_port(d, port);
+    const event_word_t *word = evtchn_fifo_word_from_port(d, evtchn->port);
 
     return word && guest_test_bit(d, EVTCHN_FIFO_PENDING, word);
 }
 
-static bool_t evtchn_fifo_is_masked(const struct domain *d, evtchn_port_t port)
+static bool_t evtchn_fifo_is_masked(const struct domain *d,
+                                    const struct evtchn *evtchn)
 {
-    const event_word_t *word = evtchn_fifo_word_from_port(d, port);
+    const event_word_t *word = evtchn_fifo_word_from_port(d, evtchn->port);
 
     return !word || guest_test_bit(d, EVTCHN_FIFO_MASKED, word);
 }
 
-static bool_t evtchn_fifo_is_busy(const struct domain *d, evtchn_port_t port)
+static bool_t evtchn_fifo_is_busy(const struct domain *d,
+                                  const struct evtchn *evtchn)
 {
-    const event_word_t *word = evtchn_fifo_word_from_port(d, port);
+    const event_word_t *word = evtchn_fifo_word_from_port(d, evtchn->port);
 
     return word && guest_test_bit(d, EVTCHN_FIFO_LINKED, word);
 }

xen/include/asm-x86/event.h

@@ -47,4 +47,10 @@ static inline bool arch_virq_is_global(unsigned int virq)
     return true;
 }
 
+#ifdef CONFIG_PV_SHIM
+# include <asm/pv/shim.h>
+# define arch_evtchn_is_special(chn) \
+             (pv_shim && (chn)->port && (chn)->state == ECS_RESERVED)
+#endif
+
 #endif

xen/include/xen/event.h

@@ -133,6 +133,24 @@ static inline struct evtchn *evtchn_from_port(struct domain *d, unsigned int p)
     return bucket_from_port(d, p) + (p % EVTCHNS_PER_BUCKET);
 }
 
+/*
+ * "usable" as in "by a guest", i.e. Xen consumed channels are assumed to be
+ * taken care of separately where used for Xen's internal purposes.
+ */
+static bool evtchn_usable(const struct evtchn *evtchn)
+{
+    if ( evtchn->xen_consumer )
+        return false;
+
+#ifdef arch_evtchn_is_special
+    if ( arch_evtchn_is_special(evtchn) )
+        return true;
+#endif
+
+    BUILD_BUG_ON(ECS_FREE > ECS_RESERVED);
+    return evtchn->state > ECS_RESERVED;
+}
+
 /* Wait on a Xen-attached event channel. */
 #define wait_on_xen_event_channel(port, condition)                      \
     do {                                                                \
@@ -165,19 +183,24 @@ int evtchn_reset(struct domain *d);
 
 /*
  * Low-level event channel port ops.
+ *
+ * All hooks have to be called with a lock held which prevents the channel
+ * from changing state. This may be the domain event lock, the per-channel
+ * lock, or in the case of sending interdomain events also the other side's
+ * per-channel lock. Exceptions apply in certain cases for the PV shim.
  */
 struct evtchn_port_ops {
     void (*init)(struct domain *d, struct evtchn *evtchn);
     void (*set_pending)(struct vcpu *v, struct evtchn *evtchn);
     void (*clear_pending)(struct domain *d, struct evtchn *evtchn);
     void (*unmask)(struct domain *d, struct evtchn *evtchn);
-    bool (*is_pending)(const struct domain *d, evtchn_port_t port);
-    bool (*is_masked)(const struct domain *d, evtchn_port_t port);
+    bool (*is_pending)(const struct domain *d, const struct evtchn *evtchn);
+    bool (*is_masked)(const struct domain *d, const struct evtchn *evtchn);
     /*
      * Is the port unavailable because it's still being cleaned up
      * after being closed?
      */
-    bool (*is_busy)(const struct domain *d, evtchn_port_t port);
+    bool (*is_busy)(const struct domain *d, const struct evtchn *evtchn);
     int (*set_priority)(struct domain *d, struct evtchn *evtchn,
                         unsigned int priority);
     void (*print_state)(struct domain *d, const struct evtchn *evtchn);
@@ -193,38 +216,67 @@ static inline void evtchn_port_set_pending(struct domain *d,
                                            unsigned int vcpu_id,
                                            struct evtchn *evtchn)
 {
-    d->evtchn_port_ops->set_pending(d->vcpu[vcpu_id], evtchn);
+    if ( evtchn_usable(evtchn) )
+        d->evtchn_port_ops->set_pending(d->vcpu[vcpu_id], evtchn);
 }
 
 static inline void evtchn_port_clear_pending(struct domain *d,
                                              struct evtchn *evtchn)
 {
-    d->evtchn_port_ops->clear_pending(d, evtchn);
+    if ( evtchn_usable(evtchn) )
+        d->evtchn_port_ops->clear_pending(d, evtchn);
 }
 
 static inline void evtchn_port_unmask(struct domain *d,
                                       struct evtchn *evtchn)
 {
-    d->evtchn_port_ops->unmask(d, evtchn);
+    if ( evtchn_usable(evtchn) )
+        d->evtchn_port_ops->unmask(d, evtchn);
 }
 
-static inline bool evtchn_port_is_pending(const struct domain *d,
-                                          evtchn_port_t port)
+static inline bool evtchn_is_pending(const struct domain *d,
+                                     const struct evtchn *evtchn)
 {
-    return d->evtchn_port_ops->is_pending(d, port);
+    return evtchn_usable(evtchn) && d->evtchn_port_ops->is_pending(d, evtchn);
 }
 
-static inline bool evtchn_port_is_masked(const struct domain *d,
-                                         evtchn_port_t port)
+static inline bool evtchn_port_is_pending(struct domain *d, evtchn_port_t port)
 {
-    return d->evtchn_port_ops->is_masked(d, port);
+    struct evtchn *evtchn = evtchn_from_port(d, port);
+    bool rc;
+    unsigned long flags;
+
+    spin_lock_irqsave(&evtchn->lock, flags);
+    rc = evtchn_is_pending(d, evtchn);
+    spin_unlock_irqrestore(&evtchn->lock, flags);
+
+    return rc;
 }
 
-static inline bool evtchn_port_is_busy(const struct domain *d,
-                                       evtchn_port_t port)
+static inline bool evtchn_is_masked(const struct domain *d,
+                                    const struct evtchn *evtchn)
+{
+    return !evtchn_usable(evtchn) || d->evtchn_port_ops->is_masked(d, evtchn);
+}
+
+static inline bool evtchn_port_is_masked(struct domain *d, evtchn_port_t port)
+{
+    struct evtchn *evtchn = evtchn_from_port(d, port);
+    bool rc;
+    unsigned long flags;
+
+    spin_lock_irqsave(&evtchn->lock, flags);
+    rc = evtchn_is_masked(d, evtchn);
+    spin_unlock_irqrestore(&evtchn->lock, flags);
+
+    return rc;
+}
+
+static inline bool evtchn_is_busy(const struct domain *d,
+                                  const struct evtchn *evtchn)
 {
     return d->evtchn_port_ops->is_busy &&
-           d->evtchn_port_ops->is_busy(d, port);
+           d->evtchn_port_ops->is_busy(d, evtchn);
 }
 
 static inline int evtchn_port_set_priority(struct domain *d,
@@ -233,6 +285,8 @@ static inline int evtchn_port_set_priority(struct domain *d,
 {
     if ( !d->evtchn_port_ops->set_priority )
         return -ENOSYS;
+    if ( !evtchn_usable(evtchn) )
+        return -EACCES;
     return d->evtchn_port_ops->set_priority(d, evtchn, priority);
 }