[PATCH 3/6] plugins/cache: Fixed a use-after-free bug with multithreaded usermode

[Mahmoud Mandour <ma.mandourr@gmail.com>]

Since callbacks may be interleaved because of multithreaded execution,
we should not make assumptions about `plugin_exit` either. The problem
with `plugin_exit` is that it frees shared data structures (caches and
`miss_ht` hash table). It should not be assumed that callbacks will not
be called after it and hence use already-freed data structures.

This is mitigated in this commit by synchronizing the call to
`plugin_exit` through locking to ensure execlusive access to data
structures, and NULL-ifying those data structures so that subsequent
callbacks can check whether the data strucutres are freed, and if so,
immediately exit.

It's okay to immediately exit and don't account for those callbacks
since they won't be accounted for anyway since `plugin_exit` is already
called once and reported the statistics.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
---
 contrib/plugins/cache.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/contrib/plugins/cache.c b/contrib/plugins/cache.c
index 695fb969dc..a452aba01c 100644
--- a/contrib/plugins/cache.c
+++ b/contrib/plugins/cache.c
@@ -363,6 +363,11 @@ static void vcpu_mem_access(unsigned int vcpu_index, qemu_plugin_meminfo_t info,
     effective_addr = hwaddr ? qemu_plugin_hwaddr_phys_addr(hwaddr) : vaddr;
 
     g_mutex_lock(&mtx);
+    if (dcache == NULL) {
+        g_mutex_unlock(&mtx);
+        return;
+    }
+
     if (!access_cache(dcache, effective_addr)) {
         insn = (InsnData *) userdata;
         insn->dmisses++;
@@ -380,6 +385,11 @@ static void vcpu_insn_exec(unsigned int vcpu_index, void *userdata)
     g_mutex_lock(&mtx);
     insn_addr = ((InsnData *) userdata)->addr;
 
+    if (icache == NULL) {
+        g_mutex_unlock(&mtx);
+        return;
+    }
+
     if (!access_cache(icache, insn_addr)) {
         insn = (InsnData *) userdata;
         insn->imisses++;
@@ -406,12 +416,24 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
             effective_addr = (uint64_t) qemu_plugin_insn_vaddr(insn);
         }
 
+        g_mutex_lock(&mtx);
+
+        /*
+         * is the plugin_exit callback called? If so, any further callback
+         * registration is useless as it won't get accounted for after calling
+         * plugin_exit once already, and also will use miss_ht after it's freed
+         */
+        if (miss_ht == NULL) {
+            g_mutex_unlock(&mtx);
+            return;
+        }
+
         /*
          * Instructions might get translated multiple times, we do not create
          * new entries for those instructions. Instead, we fetch the same
          * entry from the hash table and register it for the callback again.
          */
-        g_mutex_lock(&mtx);
+
         data = g_hash_table_lookup(miss_ht, GUINT_TO_POINTER(effective_addr));
         if (data == NULL) {
             data = g_new0(InsnData, 1);
@@ -527,13 +549,20 @@ static void log_top_insns()
 
 static void plugin_exit(qemu_plugin_id_t id, void *p)
 {
+    g_mutex_lock(&mtx);
     log_stats();
     log_top_insns();
 
     cache_free(dcache);
+    dcache = NULL;
+
     cache_free(icache);
+    icache = NULL;
 
     g_hash_table_destroy(miss_ht);
+    miss_ht = NULL;
+
+    g_mutex_unlock(&mtx);
 }
 
 static void policy_init()
-- 
2.25.1