[Qemu-devel] [PATCH v2] qemu-coroutine: segfault when restarting co_queue

[Bin Wu <wu.wubin@huawei.com>]

From: Bin Wu <wu.wubin@huawei.com>

We tested VMs migration with their disk images by drive_mirror. With
migration, two VMs copyed large files between each other. During the
test, a segfault occured. The stack was as follow:

(gdb) bt
qemu-coroutine-lock.c:66
to=0x7fa5a1798648) at qemu-coroutine.c:97
request=0x7fa28c2ffa10, reply=0x7fa28c2ffa30, qiov=0x0, offset=0) at
block/nbd-client.c:165
sector_num=8552704, nb_sectors=2040, qiov=0x7fa5a1757468, offset=0) at
block/nbd-client.c:262
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468) at
block/nbd-client.c:296
nb_sectors=2048, qiov=0x7fa5a1757468) at block/nbd.c:291
req=0x7fa28c2ffbb0, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
nb_sectors=2048, qiov=0x7fa5a1757468) at block.c:3480
nb_sectors=2048, qiov=0x7fa5a1757468) at block/raw_bsd.c:62
req=0x7fa28c2ffe30, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
coroutine-ucontext.c:121

After analyzing the stack and reviewing the code, we find the
qemu_co_queue_run_restart should not be put in the coroutine_swap function which
can be invoked by qemu_coroutine_enter or qemu_coroutine_yield. Only
qemu_coroutine_enter needs to restart the co_queue.

The error scenario is as follow: coroutine C1 enters C2, C2 yields
back to C1, then C1 ternimates and the related coroutine memory
becomes invalid. After a while, the C2 coroutine is entered again.
At this point, C1 is used as a parameter passed to
qemu_co_queue_run_restart. Therefore, qemu_co_queue_run_restart
accesses an invalid memory and a segfault error ocurrs.

The qemu_co_queue_run_restart function re-enters coroutines waiting
in the co_queue. However, this function should be only used int the
qemu_coroutine_enter context. Only in this context, when the current
coroutine gets execution control again(after the execution of
qemu_coroutine_switch), we can restart the target coutine because the
target coutine has yielded back to the current coroutine or it has
terminated.

First we want to put qemu_co_queue_run_restart in qemu_coroutine_enter,
but we find we can not access the target coroutine if it terminates.

Signed-off-by: Bin Wu <wu.wubin@huawei.com>
---
 qemu-coroutine.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/qemu-coroutine.c b/qemu-coroutine.c
index 525247b..cc0bdfa 100644
--- a/qemu-coroutine.c
+++ b/qemu-coroutine.c
@@ -99,29 +99,31 @@ static void coroutine_delete(Coroutine *co)
     qemu_coroutine_delete(co);
 }
 
-static void coroutine_swap(Coroutine *from, Coroutine *to)
+static CoroutineAction coroutine_swap(Coroutine *from, Coroutine *to)
 {
     CoroutineAction ret;
 
     ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
 
-    qemu_co_queue_run_restart(to);
-
     switch (ret) {
     case COROUTINE_YIELD:
-        return;
+        break;
     case COROUTINE_TERMINATE:
         trace_qemu_coroutine_terminate(to);
+        qemu_co_queue_run_restart(to);
         coroutine_delete(to);
-        return;
+        break;
     default:
         abort();
     }
+
+    return ret;
 }
 
 void qemu_coroutine_enter(Coroutine *co, void *opaque)
 {
     Coroutine *self = qemu_coroutine_self();
+    CoroutineAction ret;
 
     trace_qemu_coroutine_enter(self, co, opaque);
 
@@ -132,7 +134,9 @@ void qemu_coroutine_enter(Coroutine *co, void *opaque)
 
     co->caller = self;
     co->entry_arg = opaque;
-    coroutine_swap(self, co);
+    ret = coroutine_swap(self, co);
+    if (ret == COROUTINE_YIELD)
+        qemu_co_queue_run_restart(co);
 }
 
 void coroutine_fn qemu_coroutine_yield(void)
-- 
1.7.12.4