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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Mon, 14 Oct 2019 14:56:36 +0200
Message-ID: <CAAeHK+yCdTUtTMbH6r_r=hmS+Tk5mk6=nyRVuOFz6yvYSySKPA@mail.gmail.com>
Subject: Re: KMSAN: uninit-value in alauda_check_media
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Jaskaran Singh <jaskaransingh7654321@gmail.com>,
        syzbot <syzbot+e7d46eb426883fb97efd@syzkaller.appspotmail.com>,
        Alexander Potapenko <glider@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        usb-storage@lists.one-eyed-alien.net
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On Fri, Oct 11, 2019 at 5:06 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Fri, Oct 11, 2019 at 10:53:47AM -0400, Alan Stern wrote:
> > On Fri, 11 Oct 2019, Andrey Konovalov wrote:
> >
> > > On Fri, Oct 11, 2019 at 4:08 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > > > Now yes, it's true that defining status as an array on the stack is
> > > > also a bug, since USB transfer buffers are not allowed to be stack
> > > > variables.
> > >
> > > Hi Alan,
> > >
> > > I'm curious, what is the reason for disallowing that? Should we try to
> > > somehow detect such cases automatically?
> >
> > Transfer buffers are read and written by DMA.  On systems that don't
> > have cache-coherent DMA controllers, it is essential that the CPU does
> > not access any cache line involved in a DMA transfer while the transfer
> > is in progress.  Otherwise the data in the cache would be different
> > from the data in the buffer, leading to corruption.
> >
> > (In theory it would be okay for the CPU to read (not write!) a cache
> > line assigned to a buffer for a DMA write (not read!) transfer.  But
> > even doing that isn't really a good idea.)
> >
> > (Also, this isn't an issue for x86 architectures, because x86 has
> > cache-coherent DMA.  But it is an issue on other architectures.)
> >
> > In practice, this means transfer buffers have to be allocated by
> > something like kmalloc, so that they occupies their own separate set of
> > cache lines.  Buffers on the stack obviously don't satisfy this
> > requirement.
> >
> > At some point there was a discussion about automatically detecting when
> > on-stack (or otherwise invalid) buffers are used for DMA transfers.  I
> > don't recall what the outcome was.
>
> A patchset from Kees was sent, but it needs a bit more work...

Hi Greg,

Could you send a link to the patchset?

Thanks!