KASAN: use-after-free Write in percpu_ref_switch_to_percpu

KASAN: use-after-free Write in percpu_ref_switch_to_percpu

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    754beeec Merge tag 'char-misc-5.6-rc1-2' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15fe4511e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=99db4e42d047be3
dashboard link: https://syzkaller.appspot.com/bug?extid=7caeaea49c2c8a591e3d
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7caeaea49c2c8a591e3d@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in percpu_ref_switch_to_percpu+0x69/0x70 lib/percpu-refcount.c:314
Write of size 1 at addr ffff888095e3b430 by task kworker/1:31/2772

CPU: 1 PID: 2772 Comm: kworker/1:31 Not tainted 5.5.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events io_ring_file_ref_switch
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x197/0x210 lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
 __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
 kasan_report+0x12/0x20 mm/kasan/common.c:641
 __asan_report_store1_noabort+0x17/0x20 mm/kasan/generic_report.c:137
 percpu_ref_switch_to_percpu+0x69/0x70 lib/percpu-refcount.c:314
 io_ring_file_ref_switch+0x791/0xac0 fs/io_uring.c:5499
 process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264
 worker_thread+0x98/0xe40 kernel/workqueue.c:2410
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 26829:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 __kasan_kmalloc mm/kasan/common.c:515 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488
 kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
 __do_kmalloc mm/slab.c:3656 [inline]
 __kmalloc+0x163/0x770 mm/slab.c:3665
 kmalloc include/linux/slab.h:561 [inline]
 kzalloc.constprop.0+0x1b/0x20 include/linux/slab.h:670
 io_sqe_files_register fs/io_uring.c:5528 [inline]
 __io_uring_register+0xad7/0x2f40 fs/io_uring.c:6875
 __do_sys_io_uring_register fs/io_uring.c:6955 [inline]
 __se_sys_io_uring_register fs/io_uring.c:6937 [inline]
 __x64_sys_io_uring_register+0x1a1/0x560 fs/io_uring.c:6937
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 26826:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 kasan_set_free_info mm/kasan/common.c:337 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:476
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x10a/0x2c0 mm/slab.c:3757
 io_sqe_files_unregister+0x2e2/0x4d0 fs/io_uring.c:5250
 io_ring_ctx_free fs/io_uring.c:6229 [inline]
 io_ring_ctx_wait_and_kill+0x447/0x9b0 fs/io_uring.c:6310
 io_uring_release+0x42/0x50 fs/io_uring.c:6318
 __fput+0x2ff/0x890 fs/file_table.c:280
 ____fput+0x16/0x20 fs/file_table.c:313
 task_work_run+0x145/0x1c0 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x316/0x380 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x676/0x790 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff888095e3b400
 which belongs to the cache kmalloc-256 of size 256
The buggy address is located 48 bytes inside of
 256-byte region [ffff888095e3b400, ffff888095e3b500)
The buggy address belongs to the page:
page:ffffea0002578ec0 refcount:1 mapcount:0 mapping:ffff8880aa4008c0 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00025c09c8 ffffea00027f8308 ffff8880aa4008c0
raw: 0000000000000000 ffff888095e3b000 0000000100000008 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888095e3b300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888095e3b380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888095e3b400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                     ^
 ffff888095e3b480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888095e3b500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
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Re: KASAN: use-after-free Write in percpu_ref_switch_to_percpu

[Jens Axboe]

On 2/4/20 2:06 PM, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    754beeec Merge tag 'char-misc-5.6-rc1-2' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15fe4511e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=99db4e42d047be3
> dashboard link: https://syzkaller.appspot.com/bug?extid=7caeaea49c2c8a591e3d
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.

I can't reproduce this one, but I think we've seen it internally
as well. Testing the below fix.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 87f8655656b5..f204593b4f1a 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -753,6 +753,7 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
 				 struct io_uring_files_update *ip,
 				 unsigned nr_args);
 static int io_grab_files(struct io_kiocb *req);
+static void io_ring_file_ref_flush(struct fixed_file_data *data);
 
 static struct kmem_cache *req_cachep;
 
@@ -5261,15 +5262,10 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 	if (!data)
 		return -ENXIO;
 
-	/* protect against inflight atomic switch, which drops the ref */
-	percpu_ref_get(&data->refs);
-	/* wait for existing switches */
-	flush_work(&data->ref_work);
 	percpu_ref_kill_and_confirm(&data->refs, io_file_ref_kill);
-	wait_for_completion(&data->done);
-	percpu_ref_put(&data->refs);
-	/* flush potential new switch */
 	flush_work(&data->ref_work);
+	io_ring_file_ref_flush(data);
+	wait_for_completion(&data->done);
 	percpu_ref_exit(&data->refs);
 
 	__io_sqe_files_unregister(ctx);
@@ -5507,14 +5503,11 @@ struct io_file_put {
 	struct completion *done;
 };
 
-static void io_ring_file_ref_switch(struct work_struct *work)
+static void io_ring_file_ref_flush(struct fixed_file_data *data)
 {
 	struct io_file_put *pfile, *tmp;
-	struct fixed_file_data *data;
 	struct llist_node *node;
 
-	data = container_of(work, struct fixed_file_data, ref_work);
-
 	while ((node = llist_del_all(&data->put_llist)) != NULL) {
 		llist_for_each_entry_safe(pfile, tmp, node, llist) {
 			io_ring_file_put(data->ctx, pfile->file);
@@ -5524,7 +5517,14 @@ static void io_ring_file_ref_switch(struct work_struct *work)
 				kfree(pfile);
 		}
 	}
+}
 
+static void io_ring_file_ref_switch(struct work_struct *work)
+{
+	struct fixed_file_data *data;
+
+	data = container_of(work, struct fixed_file_data, ref_work);
+	io_ring_file_ref_flush(data);
 	percpu_ref_get(&data->refs);
 	percpu_ref_switch_to_percpu(&data->refs);
 }
@@ -5535,8 +5535,14 @@ static void io_file_data_ref_zero(struct percpu_ref *ref)
 
 	data = container_of(ref, struct fixed_file_data, refs);
 
-	/* we can't safely switch from inside this context, punt to wq */
-	queue_work(system_wq, &data->ref_work);
+	/*
+	 * We can't safely switch from inside this context, punt to wq. If
+	 * the table ref is going away, the table is being unregistered.
+	 * Don't queue up the async work for that case, the caller will
+	 * handle it.
+	 */
+	if (!percpu_ref_is_dying(&data->refs))
+		queue_work(system_wq, &data->ref_work);
 }
 
 static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,

-- 
Jens Axboe