ida: Free correct IDA bitmap

There's a relatively rare race where we look at the per-cpu preallocated
IDA bitmap, see it's NULL, allocate a new one, and atomically update it.
If the kmalloc() happened to sleep and we were rescheduled to a different
CPU, or an interrupt came in at the exact right time, another task
might have successfully allocated a bitmap and already deposited it.
I forgot what the semantics of cmpxchg() were and ended up freeing the
wrong bitmap leading to KASAN reporting a use-after-free.

Dmitry found the bug with syzkaller & wrote the patch.  I wrote the test
case that will reproduce the bug without his patch being applied.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

1 files changed, 31 insertions, 3 deletions

diff --git a/tools/testing/radix-tree/idr-test.c b/tools/testing/radix-tree/idr-test.c
index 86de901fa5c6..30cd0b296f1a 100644
--- a/tools/testing/radix-tree/idr-test.c
+++ b/tools/testing/radix-tree/idr-test.c
@@ -363,7 +363,7 @@ void ida_check_random(void)
 {
         DEFINE_IDA(ida);
         DECLARE_BITMAP(bitmap, 2048);
-        int id;
+        int id, err;
         unsigned int i;
         time_t s = time(NULL);
 
@@ -377,8 +377,11 @@ void ida_check_random(void)
                         ida_remove(&ida, bit);
                 } else {
                         __set_bit(bit, bitmap);
-                        ida_pre_get(&ida, GFP_KERNEL);
-                        assert(!ida_get_new_above(&ida, bit, &id));
+                        do {
+                                ida_pre_get(&ida, GFP_KERNEL);
+                                err = ida_get_new_above(&ida, bit, &id);
+                        } while (err == -ENOMEM);
+                        assert(!err);
                         assert(id == bit);
                 }
         }
@@ -476,11 +479,36 @@ void ida_checks(void)
         radix_tree_cpu_dead(1);
 }
 
+static void *ida_random_fn(void *arg)
+{
+        rcu_register_thread();
+        ida_check_random();
+        rcu_unregister_thread();
+        return NULL;
+}
+
+void ida_thread_tests(void)
+{
+        pthread_t threads[10];
+        int i;
+
+        for (i = 0; i < ARRAY_SIZE(threads); i++)
+                if (pthread_create(&threads[i], NULL, ida_random_fn, NULL)) {
+                        perror("creating ida thread");
+                        exit(1);
+                }
+
+        while (i--)
+                pthread_join(threads[i], NULL);
+}
+
 int __weak main(void)
 {
         radix_tree_init();
         idr_checks();
         ida_checks();
+        ida_thread_tests();
+        radix_tree_cpu_dead(1);
         rcu_barrier();
         if (nr_allocated)
                 printf("nr_allocated = %d\n", nr_allocated);