linux-kernel - Re: [PATCH] USB: usbtmc: prevent kernel-infoleak

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Message-ID: <tencent_E8A0E926ED22CE20783A6EC02FC56C18950A@qq.com>
Date: Thu,  5 Sep 2024 22:16:49 +0800
From: Edward Adam Davis <eadavis@...com>
To: gregkh@...uxfoundation.org
Cc: eadavis@...com,
	linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org,
	syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] USB: usbtmc: prevent kernel-infoleak

On Thu, 5 Sep 2024 16:04:31 +0200, Greg KH wrote:
> On Thu, Sep 05, 2024 at 09:56:53PM +0800, Edward Adam Davis wrote:
> > On Wed, 4 Sep 2024 16:13:03 +0200, Greg KH wrote:
> > > On Wed, Sep 04, 2024 at 04:09:15PM +0200, Greg KH wrote:
> > > > On Wed, Sep 04, 2024 at 09:55:43PM +0800, Edward Adam Davis wrote:
> > > > > The syzbot reported a kernel-usb-infoleak in usbtmc_write,
> > > > > we need to clear the structure before filling fields.
> > > >
> > > > Really?
> > > >
> > > >
> > > > >
> > > > > Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
> > > > > Reported-and-tested-by: syzbot+9d34f80f841e948c3fdb@...kaller.appspotmail.com
> > > > > Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> > > > > Signed-off-by: Edward Adam Davis <eadavis@...com>
> > > > > ---
> > > > >  drivers/usb/class/usbtmc.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> > > > > index 6bd9fe565385..e9ddaa9b580d 100644
> > > > > --- a/drivers/usb/class/usbtmc.c
> > > > > +++ b/drivers/usb/class/usbtmc.c
> > > > > @@ -759,6 +759,7 @@ static struct urb *usbtmc_create_urb(void)
> > > > >  		usb_free_urb(urb);
> > > > >  		return NULL;
> > > > >  	}
> > > > > +	memset(dmabuf, 0, bufsize);
> > > >
> > > > To do this simpler, kzmalloc() above this would be nice.
> > > >
> > > > But, this feels odd, where is the data leaking from?  This is used for
> > > > both the read and write path, but where is the leak happening?  A short
> > > > read?  If so, we need to properly truncate the buffer being sent to
> > > > userspace and not send the unread data.  If a short write, that makes no
> > > > sense.
> > A short write.
> > >
> > > I looked at the report and this seems to be data sent to the device, so
> > > somehow we aren't setting the length to send to the device correctly.
> > The length of the data passed in by the user is 3 bytes, plus a TMC header
> > length of 12 bytes and an additional 3 bytes. The actual length of the
> > data sent to the device is 16 bytes((3 + 12 + 3)~3 = 16).
> >
> > Normally, when executing copy_from_user, the 3 bytes data passed in by
> > the user is written after the TMC header, which initializes the first 15
> > bytes of the 16 bytes. But it is not yet clear why the 15th byte is not
> > initialized. The kernel data leaked to user space reported by Syzbot
> > should be it.
> 
> But why are we sending 16 bytes to the device?  Is that the format of
> the message it expects?  If so, that's fine, just set that byte to 0.
Yes. I have set them to 0 before calling copy_from_user, and now still
running tests.
https://syzkaller.appspot.com/text?tag=Patch&x=14cb8f33980000
> 
> And as the device is the thing that is getting the kernel memory, that
> really isn't a big deal as we "trust" hardware once it is up and talking
> to the kernel.
:)

BR,
Edward