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[Xen-changelog] [xen stable-4.6] evtchn: avoid NULL derefs



commit d7a42bb64a6eda532519dc92a893ae393507319f
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Jun 20 16:40:25 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jun 20 16:40:25 2017 +0200

    evtchn: avoid NULL derefs
    
    Commit fbbd5009e6 ("evtchn: refactor low-level event channel port ops")
    added a de-reference of the struct evtchn pointer for a port without
    first making sure the bucket pointer is non-NULL. This de-reference is
    actually entirely unnecessary, as all relevant callers (beyond the
    problematic do_poll()) already hold the port number in their hands, and
    the actual leaf functions need nothing else.
    
    For FIFO event channels there's a second problem in that the ordering
    of reads and updates to ->num_evtchns and ->event_array[] was so far
    undefined (the read side isn't always holding the domain's event lock).
    Add respective barriers.
    
    This is XSA-221.
    
    Reported-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: e7719a0dfac7a20cb7da5529e09773d8271bb78b
    master date: 2017-06-20 14:37:47 +0200
---
 xen/arch/x86/irq.c         |  8 +++-----
 xen/common/event_2l.c      | 16 ++++++++++------
 xen/common/event_channel.c |  4 ++--
 xen/common/event_fifo.c    | 20 ++++++++++++++------
 xen/common/schedule.c      |  2 +-
 xen/include/xen/event.h    | 12 ++++++------
 6 files changed, 36 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index bf2e822..db69264 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1484,7 +1484,7 @@ int pirq_guest_unmask(struct domain *d)
         {
             pirq = pirqs[i]->pirq;
             if ( pirqs[i]->masked &&
-                 !evtchn_port_is_masked(d, evtchn_from_port(d, 
pirqs[i]->evtchn)) )
+                 !evtchn_port_is_masked(d, pirqs[i]->evtchn) )
                 pirq_guest_eoi(pirqs[i]);
         }
     } while ( ++pirq < d->nr_pirqs && n == ARRAY_SIZE(pirqs) );
@@ -2242,7 +2242,6 @@ static void dump_irqs(unsigned char key)
     int i, irq, pirq;
     struct irq_desc *desc;
     irq_guest_action_t *action;
-    struct evtchn *evtchn;
     struct domain *d;
     const struct pirq *info;
     unsigned long flags;
@@ -2285,11 +2284,10 @@ static void dump_irqs(unsigned char key)
                 d = action->guest[i];
                 pirq = domain_irq_to_pirq(d, irq);
                 info = pirq_info(d, pirq);
-                evtchn = evtchn_from_port(d, info->evtchn);
                 printk("%u:%3d(%c%c%c)",
                        d->domain_id, pirq,
-                       (evtchn_port_is_pending(d, evtchn) ? 'P' : '-'),
-                       (evtchn_port_is_masked(d, evtchn) ? 'M' : '-'),
+                       evtchn_port_is_pending(d, info->evtchn) ? 'P' : '-',
+                       evtchn_port_is_masked(d, info->evtchn) ? 'M' : '-',
                        (info->masked ? 'M' : '-'));
                 if ( i != action->nr_guests )
                     printk(",");
diff --git a/xen/common/event_2l.c b/xen/common/event_2l.c
index 5837ae8..42a5476 100644
--- a/xen/common/event_2l.c
+++ b/xen/common/event_2l.c
@@ -62,16 +62,20 @@ static void evtchn_2l_unmask(struct domain *d, struct 
evtchn *evtchn)
     }
 }
 
-static bool_t evtchn_2l_is_pending(struct domain *d,
-                                   const struct evtchn *evtchn)
+static bool_t evtchn_2l_is_pending(struct domain *d, evtchn_port_t port)
 {
-    return test_bit(evtchn->port, &shared_info(d, evtchn_pending));
+    unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
+
+    ASSERT(port < max_ports);
+    return port < max_ports && test_bit(port, &shared_info(d, evtchn_pending));
 }
 
-static bool_t evtchn_2l_is_masked(struct domain *d,
-                                  const struct evtchn *evtchn)
+static bool_t evtchn_2l_is_masked(struct domain *d, evtchn_port_t port)
 {
-    return test_bit(evtchn->port, &shared_info(d, evtchn_mask));
+    unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
+
+    ASSERT(port < max_ports);
+    return port >= max_ports || test_bit(port, &shared_info(d, evtchn_mask));
 }
 
 static void evtchn_2l_print_state(struct domain *d,
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index 584a5ff..014d88c 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1386,8 +1386,8 @@ static void domain_dump_evtchn_info(struct domain *d)
 
         printk("    %4u [%d/%d/",
                port,
-               !!evtchn_port_is_pending(d, chn),
-               !!evtchn_port_is_masked(d, chn));
+               evtchn_port_is_pending(d, port),
+               evtchn_port_is_masked(d, port));
         evtchn_port_print_state(d, chn);
         printk("]: s=%d n=%d x=%d",
                chn->state, chn->notify_vcpu_id, chn->xen_consumer);
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 79c36ff..0be7fb3 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -28,6 +28,12 @@ static inline event_word_t 
*evtchn_fifo_word_from_port(struct domain *d,
     if ( unlikely(port >= d->evtchn_fifo->num_evtchns) )
         return NULL;
 
+    /*
+     * Callers aren't required to hold d->event_lock, so we need to synchronize
+     * with add_page_to_event_array().
+     */
+    smp_rmb();
+
     p = port / EVTCHN_FIFO_EVENT_WORDS_PER_PAGE;
     w = port % EVTCHN_FIFO_EVENT_WORDS_PER_PAGE;
 
@@ -288,24 +294,22 @@ static void evtchn_fifo_unmask(struct domain *d, struct 
evtchn *evtchn)
         evtchn_fifo_set_pending(v, evtchn);
 }
 
-static bool_t evtchn_fifo_is_pending(struct domain *d,
-                                     const struct evtchn *evtchn)
+static bool_t evtchn_fifo_is_pending(struct domain *d, evtchn_port_t port)
 {
     event_word_t *word;
 
-    word = evtchn_fifo_word_from_port(d, evtchn->port);
+    word = evtchn_fifo_word_from_port(d, port);
     if ( unlikely(!word) )
         return 0;
 
     return test_bit(EVTCHN_FIFO_PENDING, word);
 }
 
-static bool_t evtchn_fifo_is_masked(struct domain *d,
-                                    const struct evtchn *evtchn)
+static bool_t evtchn_fifo_is_masked(struct domain *d, evtchn_port_t port)
 {
     event_word_t *word;
 
-    word = evtchn_fifo_word_from_port(d, evtchn->port);
+    word = evtchn_fifo_word_from_port(d, port);
     if ( unlikely(!word) )
         return 1;
 
@@ -594,6 +598,10 @@ static int add_page_to_event_array(struct domain *d, 
unsigned long gfn)
         return rc;
 
     d->evtchn_fifo->event_array[slot] = virt;
+
+    /* Synchronize with evtchn_fifo_word_from_port(). */
+    smp_wmb();
+
     d->evtchn_fifo->num_evtchns += EVTCHN_FIFO_EVENT_WORDS_PER_PAGE;
 
     /*
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index fddcd52..2a515b2 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -870,7 +870,7 @@ static long do_poll(struct sched_poll *sched_poll)
             goto out;
 
         rc = 0;
-        if ( evtchn_port_is_pending(d, evtchn_from_port(d, port)) )
+        if ( evtchn_port_is_pending(d, port) )
             goto out;
     }
 
diff --git a/xen/include/xen/event.h b/xen/include/xen/event.h
index ef22f44..ed3a170 100644
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -134,8 +134,8 @@ struct evtchn_port_ops {
     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_t (*is_pending)(struct domain *d, const struct evtchn *evtchn);
-    bool_t (*is_masked)(struct domain *d, const struct evtchn *evtchn);
+    bool_t (*is_pending)(struct domain *d, evtchn_port_t port);
+    bool_t (*is_masked)(struct domain *d, evtchn_port_t port);
     /*
      * Is the port unavailable because it's still being cleaned up
      * after being closed?
@@ -172,15 +172,15 @@ static inline void evtchn_port_unmask(struct domain *d,
 }
 
 static inline bool_t evtchn_port_is_pending(struct domain *d,
-                                            const struct evtchn *evtchn)
+                                            evtchn_port_t port)
 {
-    return d->evtchn_port_ops->is_pending(d, evtchn);
+    return d->evtchn_port_ops->is_pending(d, port);
 }
 
 static inline bool_t evtchn_port_is_masked(struct domain *d,
-                                           const struct evtchn *evtchn)
+                                           evtchn_port_t port)
 {
-    return d->evtchn_port_ops->is_masked(d, evtchn);
+    return d->evtchn_port_ops->is_masked(d, port);
 }
 
 static inline bool_t evtchn_port_is_busy(struct domain *d, evtchn_port_t port)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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