* [PATCH 1/2] evtchn/fifo: don't spin indefinitely when setting LINK
2013-11-12 11:38 [PATCHv4 0/2] Xen: FIFO-based event channel ABI fixes David Vrabel
@ 2013-11-12 11:38 ` David Vrabel
2013-11-20 15:19 ` David Vrabel
2013-11-12 11:38 ` [PATCH 2/2] evtchn/fifo: don't corrupt queues if an old tail moves queues David Vrabel
1 sibling, 1 reply; 6+ messages in thread
From: David Vrabel @ 2013-11-12 11:38 UTC (permalink / raw)
To: xen-devel; +Cc: Keir Fraser, David Vrabel, Jan Beulich
From: David Vrabel <david.vrabel@citrix.com>
A malicious or buggy guest can cause another domain to spin
indefinitely by repeatedly writing to an event word when the other
guest is trying to link a new event. The cmpxchg() in
evtchn_fifo_set_link() will repeatedly fail and the loop may never
terminate.
Fixing this requires a change to the ABI which is documented in draft
H of the design.
http://xenbits.xen.org/people/dvrabel/event-channels-H.pdf
Since a well-behaved guest only makes a limited set of state changes,
the loop can terminate early if the guest makes an invalid state
transition.
The guest may:
- clear LINKED and LINK.
- clear PENDING
- set MASKED
- clear MASKED
It is valid for the guest to mask and unmask an event at any time so
specify that it is not valid for a guest to clear MASKED if Xen is
trying to update LINK. Indicate this to the guest with an additional
BUSY bit in the event word. The guest must not clear MASKED if BUSY
is set and it should spin until BUSY is cleared.
The remaining valid writes (clear LINKED, clear PENDING, set MASKED,
clear MASKED by Xen) will limit the number of failures of the
cmpxchg() to at most 4. A clear of LINKED will also terminate the
loop early. Therefore, the loop can then be limited to at most 4
iterations.
If the buggy or malicious guest does cause the loop to exit with
LINKED set and LINK unset then that buggy guest will lose events.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Anthony Liguori <aliguori@amazon.com>
---
xen/common/event_fifo.c | 75 +++++++++++++++++++++++++++++------
xen/include/public/event_channel.h | 1 +
2 files changed, 63 insertions(+), 13 deletions(-)
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 64dbfff..9106c55 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -34,19 +34,67 @@ static inline event_word_t *evtchn_fifo_word_from_port(struct domain *d,
return d->evtchn_fifo->event_array[p] + w;
}
-static bool_t evtchn_fifo_set_link(event_word_t *word, uint32_t link)
+static int try_set_link(event_word_t *word, event_word_t *w, uint32_t link)
{
- event_word_t n, o, w;
+ event_word_t new, old;
- w = *word;
+ if ( !(*w & (1 << EVTCHN_FIFO_LINKED)) )
+ return 0;
- do {
- if ( !(w & (1 << EVTCHN_FIFO_LINKED)) )
- return 0;
- o = w;
- n = (w & ~EVTCHN_FIFO_LINK_MASK) | link;
- } while ( (w = cmpxchg(word, o, n)) != o );
+ old = *w;
+ new = (old & ~((1 << EVTCHN_FIFO_BUSY) | EVTCHN_FIFO_LINK_MASK)) | link;
+ *w = cmpxchg(word, old, new);
+ if ( *w == old )
+ return 1;
+ return -EAGAIN;
+}
+
+/*
+ * Atomically set the LINK field iff it is still LINKED.
+ *
+ * The guest is only permitted to make the following changes to a
+ * LINKED event.
+ *
+ * - set MASKED
+ * - clear MASKED
+ * - clear PENDING
+ * - clear LINKED (and LINK)
+ *
+ * We block unmasking by the guest by marking the tail word as BUSY,
+ * therefore, the cmpxchg() may fail at most 4 times.
+ */
+static bool_t evtchn_fifo_set_link(const struct domain *d, event_word_t *word,
+ uint32_t link)
+{
+ event_word_t w;
+ unsigned int try;
+ int ret;
+
+ w = read_atomic(word);
+
+ ret = try_set_link(word, &w, link);
+ if ( ret >= 0 )
+ return ret;
+
+ /* Lock the word to prevent guest unmasking. */
+ set_bit(EVTCHN_FIFO_BUSY, word);
+
+ w = read_atomic(word);
+
+ for ( try = 0; try < 4; try++ )
+ {
+ ret = try_set_link(word, &w, link);
+ if ( ret >= 0 )
+ {
+ if ( ret == 0 )
+ clear_bit(EVTCHN_FIFO_BUSY, word);
+ return ret;
+ }
+ }
+ gdprintk(XENLOG_WARNING, "domain %d, port %d not linked\n",
+ d->domain_id, link);
+ clear_bit(EVTCHN_FIFO_BUSY, word);
return 1;
}
@@ -105,7 +153,7 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
if ( port != q->tail )
{
tail_word = evtchn_fifo_word_from_port(d, q->tail);
- linked = evtchn_fifo_set_link(tail_word, port);
+ linked = evtchn_fifo_set_link(d, tail_word, port);
}
if ( !linked )
write_atomic(q->head, port);
@@ -202,11 +250,12 @@ static void evtchn_fifo_print_state(struct domain *d,
word = evtchn_fifo_word_from_port(d, evtchn->port);
if ( !word )
- printk("? ");
+ printk("? ");
else if ( test_bit(EVTCHN_FIFO_LINKED, word) )
- printk("%-4u", *word & EVTCHN_FIFO_LINK_MASK);
+ printk("%c %-4u", test_bit(EVTCHN_FIFO_BUSY, word) ? 'B' : ' ',
+ *word & EVTCHN_FIFO_LINK_MASK);
else
- printk("- ");
+ printk("%c - ", test_bit(EVTCHN_FIFO_BUSY, word) ? 'B' : ' ');
}
static const struct evtchn_port_ops evtchn_port_ops_fifo =
diff --git a/xen/include/public/event_channel.h b/xen/include/public/event_channel.h
index 4a53484..b78ee32 100644
--- a/xen/include/public/event_channel.h
+++ b/xen/include/public/event_channel.h
@@ -343,6 +343,7 @@ typedef uint32_t event_word_t;
#define EVTCHN_FIFO_PENDING 31
#define EVTCHN_FIFO_MASKED 30
#define EVTCHN_FIFO_LINKED 29
+#define EVTCHN_FIFO_BUSY 28
#define EVTCHN_FIFO_LINK_BITS 17
#define EVTCHN_FIFO_LINK_MASK ((1 << EVTCHN_FIFO_LINK_BITS) - 1)
--
1.7.2.5
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 2/2] evtchn/fifo: don't corrupt queues if an old tail moves queues
2013-11-12 11:38 [PATCHv4 0/2] Xen: FIFO-based event channel ABI fixes David Vrabel
2013-11-12 11:38 ` [PATCH 1/2] evtchn/fifo: don't spin indefinitely when setting LINK David Vrabel
@ 2013-11-12 11:38 ` David Vrabel
2013-11-15 13:15 ` David Vrabel
1 sibling, 1 reply; 6+ messages in thread
From: David Vrabel @ 2013-11-12 11:38 UTC (permalink / raw)
To: xen-devel; +Cc: Keir Fraser, David Vrabel, Jan Beulich
From: David Vrabel <david.vrabel@citrix.com>
An event may still be the tail of a queue even if the queue is now
empty (an 'old tail' event). There is logic to handle the case when
this old tail event needs to be added to the now empty queue (by
checking for q->tail == port).
However, if the old tail event on queue A is moved to a different
queue B (by changing its VCPU or priority), the event may then be
linked onto queue B. When another event is linked onto queue A it
will check the old tail, see that it is linked (but on queue B) and
overwrite the LINK field, corrupting both queues.
When an event is linked, save the vcpu id and priority of thee queue
it is being linked onto. Use this when linking an event to check if
it is an unlinked old tail event. i.e., a) it has moved queues; b) it
is currently unlinked; and c) it's the tail of the old queue. If it
is an unlinked, old tail event, the old queue is empty and old_q->tail
is invalidated to ensure adding another event to old_q will update
HEAD. The tail is invalidated by setting it to 0 since the event 0 is
never linked.
The old_q->lock is held while setting LINKED to avoid a race with the
test of LINKED in evtchn_fifo_set_link().
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
xen/common/event_fifo.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
xen/include/xen/sched.h | 2 ++
2 files changed, 44 insertions(+), 1 deletions(-)
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 9106c55..8e126d6 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -98,6 +98,47 @@ static bool_t evtchn_fifo_set_link(const struct domain *d, event_word_t *word,
return 1;
}
+static bool_t test_and_set_linked(const struct domain *d,
+ struct evtchn *evtchn,
+ struct evtchn_fifo_queue *q,
+ event_word_t *word)
+{
+ struct vcpu *old_v;
+ struct evtchn_fifo_queue *old_q;
+ bool_t was_linked;
+ unsigned long flags;
+
+ old_v = d->vcpu[evtchn->last_vcpu_id];
+ old_q = &old_v->evtchn_fifo->queue[evtchn->last_priority];
+
+ evtchn->last_vcpu_id = evtchn->notify_vcpu_id;
+ evtchn->last_priority = evtchn->priority;
+
+ if ( q == old_q )
+ return test_and_set_bit(EVTCHN_FIFO_LINKED, word);
+
+ /*
+ * This event is now on a different queue.
+ *
+ * If the event is still linked in the old queue it won't be moved
+ * yet.
+ *
+ * If this event is unlinked /and/ it's the old queue's tail, the
+ * old queue is empty and its tail must be invalidated to prevent
+ * adding an event to the old queue from corrupting the new queue.
+ */
+ spin_lock_irqsave(&old_q->lock, flags);
+
+ was_linked = test_and_set_bit(EVTCHN_FIFO_LINKED, word);
+
+ if ( !was_linked && old_q->tail == evtchn->port )
+ old_q->tail = 0;
+
+ spin_unlock_irqrestore(&old_q->lock, flags);
+
+ return was_linked;
+}
+
static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
{
struct domain *d = v->domain;
@@ -133,7 +174,7 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
* Link the event if it unmasked and not already linked.
*/
if ( !test_bit(EVTCHN_FIFO_MASKED, word)
- && !test_and_set_bit(EVTCHN_FIFO_LINKED, word) )
+ && !test_and_set_linked(d, evtchn, q, word) )
{
event_word_t *tail_word;
bool_t linked = 0;
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 2397537..3714c37 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -98,6 +98,8 @@ struct evtchn
} u;
u8 priority;
u8 pending:1;
+ u16 last_vcpu_id;
+ u8 last_priority;
#ifdef FLASK_ENABLE
void *ssid;
#endif
--
1.7.2.5
^ permalink raw reply related [flat|nested] 6+ messages in thread