app_chanspy: Fix occasional deadlock with ChanSpy and Local channels.

Channel masquerading had a conflict with autochannel locking.

When locking autochannel->channel, the channel is fetched from the
autochannel and then locked. During the fetch, the autochannel -- which
has no locks itself -- can be modified by someone who owns the channel
lock. That means that the value of autochan->channel cannot be trusted
until you hold the lock.

In practice, this caused problems with Local channels getting
masqueraded away while the ChanSpy attempted to get info from that
channel. The old channel which was about to get removed got locked, but
the new (replaced) channel got unlocked (no-op). Because the replaced
channel was now locked (and would never get unlocked), it couldn't get
removed from the channel list in a timely manner, and would now cause
deadlocks when iterating over the channel list.

This change checks the autochannel after locking the channel for changes
to the autochannel. If the channel had been changed, the lock is
reobtained on the new channel.

In theory it seems possible that after this fix, the lock attempt on the
old (wrong) channel can be on an already destroyed lock, maybe causing
a crash. But that hasn't been observed in the wild and is harder induce
than the current deadlock.

Thanks go to Filip Frank for suggesting a fix similar to this and
especially to IRC user hexanol for pointing out why this deadlock was
possible and testing this fix. And to Richard for catching my rookie
while loop mistake ;)

ASTERISK-25321 #close

Change-Id: I293ae0014e531cd0e675c3f02d1d118a98683def

diff --git a/apps/app_chanspy.c b/apps/app_chanspy.c
index 31a1e18ca26f534db13c1efe1f81ff8dd7834614..78733ef7f2a3d9a21f1b48ca5b556e53e9b5341c 100644 (file)
--- a/apps/app_chanspy.c
+++ b/apps/app_chanspy.c
@@ -532,12 +532,12 @@ static int attach_barge(struct ast_autochan *spyee_autochan,
                return -1;
        }
 
-       ast_channel_lock(internal_bridge_autochan->chan);
+       ast_autochan_channel_lock(internal_bridge_autochan);
        if (start_spying(internal_bridge_autochan, spyer_name, bridge_whisper_audiohook)) {
                ast_log(LOG_WARNING, "Unable to attach barge audiohook on spyee '%s'. Barge mode disabled.\n", name);
                retval = -1;
        }
-       ast_channel_unlock(internal_bridge_autochan->chan);
+       ast_autochan_channel_unlock(internal_bridge_autochan);
 
        *spyee_bridge_autochan = internal_bridge_autochan;
 
@@ -569,9 +569,9 @@ static int channel_spy(struct ast_channel *chan, struct ast_autochan *spyee_auto
                return 0;
        }
 
-       ast_channel_lock(spyee_autochan->chan);
+       ast_autochan_channel_lock(spyee_autochan);
        name = ast_strdupa(ast_channel_name(spyee_autochan->chan));
-       ast_channel_unlock(spyee_autochan->chan);
+       ast_autochan_channel_unlock(spyee_autochan);
 
        ast_verb(2, "Spying on channel %s\n", name);
        /*** DOCUMENTATION

diff --git a/include/asterisk/autochan.h b/include/asterisk/autochan.h
index 2ace4f2c2d639bfb37324df08d638b143f00a713..2421e7b3840917bf68c3a6703fb63e215a86b861 100644 (file)
--- a/include/asterisk/autochan.h
+++ b/include/asterisk/autochan.h
@@ -56,8 +56,28 @@ struct ast_autochan {
  * to save off the pointer using ast_channel_ref and to unref the channel when you
  * are finished with the pointer. If you do not do this and a masquerade occurs on
  * the channel, then it is possible that your saved pointer will become invalid.
+ *
+ * 3. If you want to lock the autochan->chan channel, be sure to use
+ * ast_autochan_channel_lock and ast_autochan_channel_unlock. An attempt to lock
+ * the autochan->chan directly may result in it being changed after you've
+ * retrieved the value of chan, but before you've had a chance to lock it.
+ * First when chan is locked, the autochan structure is guaranteed to keep the
+ * same channel.
  */
 
+#define ast_autochan_channel_lock(autochan) \
+       do { \
+               struct ast_channel *autochan_chan = autochan->chan; \
+               ast_channel_lock(autochan_chan); \
+               if (autochan->chan == autochan_chan) { \
+                       break; \
+               } \
+               ast_channel_unlock(autochan_chan); \
+       } while (1)
+
+#define ast_autochan_channel_unlock(autochan) \
+       ast_channel_unlock(autochan->chan)
+
 /*!
  * \brief set up a new ast_autochan structure
  *

diff --git a/main/autochan.c b/main/autochan.c
index 7da249a136358568a58f1ccef3698820330f6151..c1e8b83f363f2263ca65130a33f43d0056859a20 100644 (file)
--- a/main/autochan.c
+++ b/main/autochan.c
@@ -51,7 +51,7 @@ struct ast_autochan *ast_autochan_setup(struct ast_channel *chan)
 
        autochan->chan = ast_channel_ref(chan);
 
-       ast_channel_lock(autochan->chan);
+       ast_channel_lock(autochan->chan); /* autochan is still private, no need for ast_autochan_channel_lock() */
        AST_LIST_INSERT_TAIL(ast_channel_autochans(autochan->chan), autochan, list);
        ast_channel_unlock(autochan->chan);
 
@@ -64,7 +64,7 @@ void ast_autochan_destroy(struct ast_autochan *autochan)
 {
        struct ast_autochan *autochan_iter;
 
-       ast_channel_lock(autochan->chan);
+       ast_autochan_channel_lock(autochan);
        AST_LIST_TRAVERSE_SAFE_BEGIN(ast_channel_autochans(autochan->chan), autochan_iter, list) {
                if (autochan_iter == autochan) {
                        AST_LIST_REMOVE_CURRENT(list);
@@ -73,7 +73,7 @@ void ast_autochan_destroy(struct ast_autochan *autochan)
                }
        }
        AST_LIST_TRAVERSE_SAFE_END;
-       ast_channel_unlock(autochan->chan);
+       ast_autochan_channel_unlock(autochan);
 
        autochan->chan = ast_channel_unref(autochan->chan);