linux-kernel - Re: Is usb_hcd_giveback_urb() allowed in task context?

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Message-ID: <20201005152218.GF376584@rowland.harvard.edu>
Date:   Mon, 5 Oct 2020 11:22:18 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Nazime Hande Harputluoglu <handeharputlu@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: Is usb_hcd_giveback_urb() allowed in task context?

On Mon, Oct 05, 2020 at 05:08:11PM +0200, Andrey Konovalov wrote:
> Dear USB and USB/IP maintainers,
> 
> While fuzzing the USB/IP stack with syzkaller we've stumbled upon an issue.
> 
> Currently kcov (the subsystem that is used for coverage collection)
> USB-related callbacks assume that usb_hcd_giveback_urb() can only be
> called from interrupt context, as indicated by the comment before the
> function definition.

The primary reason for this restriction (as far as I'm aware) is because 
the routine uses spin_lock/spin_unlock rather than the 
_irqsave/_irqrestore variants.  There's also a small efficiency issue: 
In the vast majority of cases involving real host controllers, the 
routine _will_ be called in interrupt context.  So we optimized for that 
case.

>  In the USB/IP code, however, it's called from the
> task context (see the stack trace below).
> 
> Is this something that is allowed and we need to fix kcov? Or is this
> a bug in USB/IP?

It's a bug in USB/IP.  Interrupts should be disabled when it calls 
usb_hcd_giveback_urb().

Alan Stern