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Date:   Fri, 11 Oct 2019 17:06:46 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        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
Subject: Re: KMSAN: uninit-value in alauda_check_media
Message-ID: <20191011150646.GA1240544@kroah.com>
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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...

thanks,

greg k-h