linux-kernel - Re: [PATCH] drivers/tty/vt: copy userspace arrays safely

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Date:   Thu, 2 Nov 2023 20:49:03 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Airlie <airlied@...hat.com>
Cc:     Philipp Stanner <pstanner@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Tony Luck <tony.luck@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH] drivers/tty/vt: copy userspace arrays safely

On Fri, Nov 03, 2023 at 06:24:09AM +1000, David Airlie wrote:
> On Fri, Nov 3, 2023 at 6:14 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Thu, Nov 02, 2023 at 08:21:35PM +0100, Philipp Stanner wrote:
> > > The functions (v)memdup_user() are utilized to copy userspace arrays.
> > > This is done without overflow checks.
> > >
> > > Use the new wrappers memdup_array_user() and vmemdup_array_user() to
> > > copy the arrays more safely.
> >
> > > @@ -644,7 +644,7 @@ int con_set_unimap(struct vc_data *vc, ushort ct, struct unipair __user *list)
> > >       if (!ct)
> > >               return 0;
> >
> > > -     unilist = vmemdup_user(list, array_size(sizeof(*unilist), ct));
> > > +     unilist = vmemdup_array_user(list, ct, sizeof(*unilist));
> > >       if (IS_ERR(unilist))
> > >               return PTR_ERR(unilist);
> >
> > a 16bit value times sizeof(something).
> 
> So since it's already using array_size here, moving it to a new helper
> for consistency just makes things clearer, and so you are fine with
> the patch?

Sigh...  OK, if you want it spelled out, there we go.  I have no objections
to the contents of patches; e.g. in case of ppp ioctl it saves the reader
a grep in search of structure definitions, which is a good thing.  The one
and only suggestion I have for those patches is that such patches might be
better off with explicit "in this case the overflow is avoided due to
<reasons>, but use of this helper makes it obviously safe" - or, in case
of real bugs, "the overflow is, indeed, possible here", in which case
Fixes: ... and Cc: stable might be in order.