execlog had the following comment:
> As we could have multiple threads trying to do this we need to
> serialise the expansion under a lock. Threads accessing already
> created entries can continue without issue even if the ptr array
> gets reallocated during resize.

However, when the ptr array gets reallocated, the other threads may have
a stale reference to the old buffer. This results in use-after-free.

Use GRWLock to properly fix this issue.

Fixes: 3d7caf145e ("contrib/plugins: add execlog to log instruction execution and memory access")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
---
 contrib/plugins/execlog.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/contrib/plugins/execlog.c b/contrib/plugins/execlog.c
index 7129d526f8..82dc2f584e 100644
--- a/contrib/plugins/execlog.c
+++ b/contrib/plugins/execlog.c
@@ -19,7 +19,7 @@ QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
 
 /* Store last executed instruction on each vCPU as a GString */
 static GPtrArray *last_exec;
-static GMutex expand_array_lock;
+static GRWLock expand_array_lock;
 
 static GPtrArray *imatches;
 static GArray *amatches;
@@ -28,18 +28,16 @@ static GArray *amatches;
  * Expand last_exec array.
  *
  * As we could have multiple threads trying to do this we need to
- * serialise the expansion under a lock. Threads accessing already
- * created entries can continue without issue even if the ptr array
- * gets reallocated during resize.
+ * serialise the expansion under a lock.
  */
 static void expand_last_exec(int cpu_index)
 {
-    g_mutex_lock(&expand_array_lock);
+    g_rw_lock_writer_lock(&expand_array_lock);
     while (cpu_index >= last_exec->len) {
         GString *s = g_string_new(NULL);
         g_ptr_array_add(last_exec, s);
     }
-    g_mutex_unlock(&expand_array_lock);
+    g_rw_lock_writer_unlock(&expand_array_lock);
 }
 
 /**
@@ -51,8 +49,10 @@ static void vcpu_mem(unsigned int cpu_index, qemu_plugin_meminfo_t info,
     GString *s;
 
     /* Find vCPU in array */
+    g_rw_lock_reader_lock(&expand_array_lock);
     g_assert(cpu_index < last_exec->len);
     s = g_ptr_array_index(last_exec, cpu_index);
+    g_rw_lock_reader_unlock(&expand_array_lock);
 
     /* Indicate type of memory access */
     if (qemu_plugin_mem_is_store(info)) {
@@ -80,10 +80,14 @@ static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
     GString *s;
 
     /* Find or create vCPU in array */
+    g_rw_lock_reader_lock(&expand_array_lock);
     if (cpu_index >= last_exec->len) {
+        g_rw_lock_reader_unlock(&expand_array_lock);
         expand_last_exec(cpu_index);
+        g_rw_lock_reader_lock(&expand_array_lock);
     }
     s = g_ptr_array_index(last_exec, cpu_index);
+    g_rw_lock_reader_unlock(&expand_array_lock);
 
     /* Print previous instruction in cache */
     if (s->len) {
-- 
2.42.0