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

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Date:   Thu, 02 Nov 2023 23:07:51 +0100
From:   Philipp Stanner <pstanner@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        David Airlie <airlied@...hat.com>
Cc:     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 Thu, 2023-11-02 at 20:49 +0000, Al Viro wrote:
> 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.
> 

So if you agree the content is improving things a little bit then it
seems the only critical thing is the commit message :)

So let's get that fixed, shifting the focus from security to
readability and general usefulness.

Do you have a proposal for a good wording?

Personally, I would have gone with something minimalistic like here in
my other commit, where the irrelevance of the overflow-aspect was more
obvious for me to see [1]
I can also add a sentence clarifying that it's about improving
readability or sth if you think that's better

Kind regards,
P.

[1] https://lore.kernel.org/all/20231102192402.53721-2-pstanner@redhat.com/