linux-kernel - Re: [syzbot] [usb?] BUG: sleeping function called from invalid context in dummy

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Message-ID: <0d6d8778-a45e-498f-9e31-1d926f582d7e@rowland.harvard.edu>
Date: Mon, 25 Aug 2025 10:18:52 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: syzbot <syzbot+8baacc4139f12fa77909@...kaller.appspotmail.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Yunseong Kim <ysk@...lloc.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [usb?] BUG: sleeping function called from invalid
 context in dummy_dequeue

On Mon, Aug 25, 2025 at 01:51:29AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    8d245acc1e88 Merge tag 'char-misc-6.17-rc3' of git://git.k..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11e42062580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e1e1566c7726877e
> dashboard link: https://syzkaller.appspot.com/bug?extid=8baacc4139f12fa77909
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13317062580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14d5ea34580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/096739d8f0ec/disk-8d245acc.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/83a21aa9b978/vmlinux-8d245acc.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/7e7f165a3b29/bzImage-8d245acc.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8baacc4139f12fa77909@...kaller.appspotmail.com
> 
> BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 6118, name: syz.0.17
> preempt_count: 0, expected: 0
> RCU nest depth: 0, expected: 0
> 1 lock held by syz.0.17/6118:
>  #0: ffff888144724058 (&dum_hcd->dum->lock){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
>  #0: ffff888144724058 (&dum_hcd->dum->lock){+.+.}-{3:3}, at: dummy_dequeue+0x164/0x480 drivers/usb/gadget/udc/dummy_hcd.c:769

Here's the solution that we settled on.

Alan Stern

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ 8d245acc1e88

Index: usb-devel/drivers/usb/gadget/udc/dummy_hcd.c
===================================================================
--- usb-devel.orig/drivers/usb/gadget/udc/dummy_hcd.c
+++ usb-devel/drivers/usb/gadget/udc/dummy_hcd.c
@@ -765,8 +765,7 @@ static int dummy_dequeue(struct usb_ep *
 	if (!dum->driver)
 		return -ESHUTDOWN;
 
-	local_irq_save(flags);
-	spin_lock(&dum->lock);
+	spin_lock_irqsave(&dum->lock, flags);
 	list_for_each_entry(iter, &ep->queue, queue) {
 		if (&iter->req != _req)
 			continue;
@@ -776,15 +775,16 @@ static int dummy_dequeue(struct usb_ep *
 		retval = 0;
 		break;
 	}
-	spin_unlock(&dum->lock);
 
 	if (retval == 0) {
 		dev_dbg(udc_dev(dum),
 				"dequeued req %p from %s, len %d buf %p\n",
 				req, _ep->name, _req->length, _req->buf);
+		spin_unlock(&dum->lock);
 		usb_gadget_giveback_request(_ep, _req);
+		spin_lock(&dum->lock);
 	}
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&dum->lock, flags);
 	return retval;
 }