path: root/fs/io_uring.c
diff options
authorJens Axboe <axboe@kernel.dk>2020-03-04 07:25:50 -0700
committerJens Axboe <axboe@kernel.dk>2020-03-06 10:15:21 -0700
commitc1e2148f8ecb26863b899d402a823dab8e26efd1 (patch)
treeb67decc33dda36a01f749303327b1a03e06c14ec /fs/io_uring.c
parentio-wq: remove io_wq_flush and IO_WQ_WORK_INTERNAL (diff)
io_uring: free fixed_file_data after RCU grace period
The percpu refcount protects this structure, and we can have an atomic switch in progress when exiting. This makes it unsafe to just free the struct normally, and can trigger the following KASAN warning: BUG: KASAN: use-after-free in percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 Read of size 1 at addr ffff888181a19a30 by task swapper/0/0 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.0-rc4+ #5747 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014 Call Trace: <IRQ> dump_stack+0x76/0xa0 print_address_description.constprop.0+0x3b/0x60 ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 __kasan_report.cold+0x1a/0x3d ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 rcu_core+0x370/0x830 ? percpu_ref_exit+0x50/0x50 ? rcu_note_context_switch+0x7b0/0x7b0 ? run_rebalance_domains+0x11d/0x140 __do_softirq+0x10a/0x3e9 irq_exit+0xd5/0xe0 smp_apic_timer_interrupt+0x86/0x200 apic_timer_interrupt+0xf/0x20 </IRQ> RIP: 0010:default_idle+0x26/0x1f0 Fix this by punting the final exit and free of the struct to RCU, then we know that it's safe to do so. Jann suggested the approach of using a double rcu callback to achieve this. It's important that we do a nested call_rcu() callback, as otherwise the free could be ordered before the atomic switch, even if the latter was already queued. Reported-by: syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com Suggested-by: Jann Horn <jannh@google.com> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to '')
1 files changed, 22 insertions, 2 deletions
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6a595c13e108..68050b61ad0e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -191,6 +191,7 @@ struct fixed_file_data {
struct llist_head put_llist;
struct work_struct ref_work;
struct completion done;
+ struct rcu_head rcu;
struct io_ring_ctx {
@@ -5329,6 +5330,26 @@ static void io_file_ref_kill(struct percpu_ref *ref)
+static void __io_file_ref_exit_and_free(struct rcu_head *rcu)
+ struct fixed_file_data *data = container_of(rcu, struct fixed_file_data,
+ rcu);
+ percpu_ref_exit(&data->refs);
+ kfree(data);
+static void io_file_ref_exit_and_free(struct rcu_head *rcu)
+ /*
+ * We need to order our exit+free call against the potentially
+ * existing call_rcu() for switching to atomic. One way to do that
+ * is to have this rcu callback queue the final put and free, as we
+ * could otherwise have a pre-existing atomic switch complete _after_
+ * the free callback we queued.
+ */
+ call_rcu(rcu, __io_file_ref_exit_and_free);
static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
struct fixed_file_data *data = ctx->file_data;
@@ -5341,14 +5362,13 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
- percpu_ref_exit(&data->refs);
nr_tables = DIV_ROUND_UP(ctx->nr_user_files, IORING_MAX_FILES_TABLE);
for (i = 0; i < nr_tables; i++)
- kfree(data);
+ call_rcu(&data->rcu, io_file_ref_exit_and_free);
ctx->file_data = NULL;
ctx->nr_user_files = 0;
return 0;