Patch "USB: core: Avoid race of async_completed() w/ usbdev_release()" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

USB: core: Avoid race of async_completed() w/ usbdev_release()

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
usb-core-avoid-race-of-async_completed-w-usbdev_release.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Douglas Anderson <[email protected]>
Date: Thu, 10 Aug 2017 15:42:22 -0700
Subject: USB: core: Avoid race of async_completed() w/ usbdev_release()

From: Douglas Anderson <[email protected]>

commit ed62ca2f4f51c17841ea39d98c0c409cb53a3e10 upstream.

While running reboot tests w/ a specific set of USB devices (and slub_debug enabled), I found that once every few hours my device would be crashed with a stack that looked like this:

[   14.012445] BUG: spinlock bad magic on CPU#0, modprobe/2091
[   14.012460]  lock: 0xffffffc0cb055978, .magic: ffffffc0, .owner: cryption contexts: %lu/%lu
[   14.012460] /1025536097, .owner_cpu: 0
[   14.012466] CPU: 0 PID: 2091 Comm: modprobe Not tainted 4.4.79 #352
[   14.012468] Hardware name: Google Kevin (DT)
[   14.012471] Call trace:
[   14.012483] [<....>] dump_backtrace+0x0/0x160
[   14.012487] [<....>] show_stack+0x20/0x28
[   14.012494] [<....>] dump_stack+0xb4/0xf0
[   14.012500] [<....>] spin_dump+0x8c/0x98
[   14.012504] [<....>] spin_bug+0x30/0x3c
[   14.012508] [<....>] do_raw_spin_lock+0x40/0x164
[   14.012515] [<....>] _raw_spin_lock_irqsave+0x64/0x74
[   14.012521] [<....>] __wake_up+0x2c/0x60
[   14.012528] [<....>] async_completed+0x2d0/0x300
[   14.012534] [<....>] __usb_hcd_giveback_urb+0xc4/0x138
[   14.012538] [<....>] usb_hcd_giveback_urb+0x54/0xf0
[   14.012544] [<....>] xhci_irq+0x1314/0x1348
[   14.012548] [<....>] usb_hcd_irq+0x40/0x50
[   14.012553] [<....>] handle_irq_event_percpu+0x1b4/0x3f0
[   14.012556] [<....>] handle_irq_event+0x4c/0x7c
[   14.012561] [<....>] handle_fasteoi_irq+0x158/0x1c8
[   14.012564] [<....>] generic_handle_irq+0x30/0x44
[   14.012568] [<....>] __handle_domain_irq+0x90/0xbc
[   14.012572] [<....>] gic_handle_irq+0xcc/0x18c

Investigation using kgdb() found that the wait queue that was passed into wake_up() had been freed (it was filled with slub_debug poison).

I analyzed and instrumented the code and reproduced. My current belief is that this is happening:

1. async_completed() is called (from IRQ). Moves "as" onto the completed list.
2. On another CPU, proc_reapurbnonblock_compat() calls async_getcompleted(). Blocks on spinlock.
3. async_completed() releases the lock; keeps running; gets blocked midway through wake_up().
4. proc_reapurbnonblock_compat() => async_getcompleted() gets the lock; removes "as" from completed list and frees it.
5. usbdev_release() is called. Frees "ps".
6. async_completed() finally continues running wake_up(). ...but wake_up() has a pointer to the freed "ps".

The instrumentation that led me to believe this was based on adding some trace_printk() calls in a select few functions and then using kdb's "ftdump" at crash time. The trace follows (NOTE: in the trace below I cheated a little bit and added a udelay(1000) in async_completed() after releasing the spinlock because I wanted it to trigger quicker):

<...>-2104   0d.h2 13759034us!: async_completed at start: as=ffffffc0cc638200
mtpd-2055    3.... 13759356us : async_getcompleted before spin_lock_irqsave
mtpd-2055    3d..1 13759362us : async_getcompleted after list_del_init: as=ffffffc0cc638200
mtpd-2055    3.... 13759371us+: proc_reapurbnonblock_compat: free_async(ffffffc0cc638200)
mtpd-2055    3.... 13759422us+: async_getcompleted before spin_lock_irqsave
mtpd-2055    3.... 13759479us : usbdev_release at start: ps=ffffffc0cc042080
mtpd-2055    3.... 13759487us : async_getcompleted before spin_lock_irqsave
mtpd-2055    3.... 13759497us!: usbdev_release after kfree(ps): ps=ffffffc0cc042080
<...>-2104   0d.h2 13760294us : async_completed before wake_up(): as=ffffffc0cc638200

To fix this problem we can just move the wake_up() under the ps->lock. There should be no issues there that I'm aware of.

---
 drivers/usb/core/devio.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -623,6 +623,8 @@ static void async_completed(struct urb *
 	if (as->status < 0 && as->bulk_addr && as->status != -ECONNRESET &&
 			as->status != -ENOENT)
 		cancel_bulk_urbs(ps, as->bulk_addr);
+
+	wake_up(&ps->wait);
 	spin_unlock(&ps->lock);
 
 	if (signr) {

@@ -630,8 +632,6 @@ static void async_completed(struct urb *
 		put_pid(pid);
 		put_cred(cred);
 	}
-
-	wake_up(&ps->wait);
 }
 
 static void destroy_async(struct usb_dev_state *ps, struct list_head *list)

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/usb-core-avoid-race-of-async_completed-w-usbdev_release.patch