linux-kernel - Re: BUG: bad usercopy in ld_usb

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Message-ID: <201908101120.BE5034521A@keescook>
Date:   Sat, 10 Aug 2019 11:23:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        syzbot <syzbot+45b2f40f0778cfa7634e@...kaller.appspotmail.com>,
        Michael Hund <mhund@...didactic.de>, akpm@...ux-foundation.org,
        andreyknvl@...gle.com, cai@....pw, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-usb@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, tglx@...utronix.de
Subject: Re: BUG: bad usercopy in ld_usb_read

On Fri, Aug 09, 2019 at 11:13:00AM -0400, Alan Stern wrote:
> In fact, I don't see why any of the computations here should overflow
> or wrap around, or even give rise to a negative value.  If syzbot had a
> reproducer we could get more debugging output -- but it doesn't.

Yeah, this is odd. The only thing I could see here with more study was
that ring_tail is used/updated outside of the rbsl lock in
ld_usb_read(). I couldn't convince myself there wasn't a race against
the interrupt, but I also couldn't think of a way it could break...

-- 
Kees Cook