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Message-ID: <37ab45c67d630b4832912cb8403ce8235f52d9c0.camel@redhat.com>
Subject: Re: [PATCH] drivers/tty/vt: copy userspace arrays safely
From:   Philipp Stanner <pstanner@redhat.com>
To:     Al Viro <viro@zeniv.linux.org.uk>,
        David Airlie <airlied@redhat.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Tony Luck <tony.luck@intel.com>,
        Ard Biesheuvel <ardb@kernel.org>, linux-kernel@vger.kernel.org,
        linux-serial@vger.kernel.org
Date:   Thu, 02 Nov 2023 23:07:51 +0100
In-Reply-To: <20231102204903.GM1957730@ZenIV>
References: <20231102192134.53301-2-pstanner@redhat.com>
         <20231102201424.GL1957730@ZenIV>
         <CAMwc25rgdNt3oO3Yf=v05AWOSgN5W4j=ST=Y9qikeBrNTqqW2w@mail.gmail.com>
         <20231102204903.GM1957730@ZenIV>
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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=E2=80=AFAM Al Viro <viro@zeniv.linux.org.uk=
>
> > wrote:
> > >=20
> > > 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.
> > > >=20
> > > > Use the new wrappers memdup_array_user() and
> > > > vmemdup_array_user() to
> > > > copy the arrays more safely.
> > >=20
> > > > @@ -644,7 +644,7 @@ int con_set_unimap(struct vc_data *vc,
> > > > ushort ct, struct unipair __user *list)
> > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (!ct)
> > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 return 0;
> > >=20
> > > > -=C2=A0=C2=A0=C2=A0=C2=A0 unilist =3D vmemdup_user(list, array_size=
(sizeof(*unilist),
> > > > ct));
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0 unilist =3D vmemdup_array_user(list, ct, =
sizeof(*unilist));
> > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (IS_ERR(unilist))
> > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0 return PTR_ERR(unilist);
> > >=20
> > > a 16bit value times sizeof(something).
> >=20
> > 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?
>=20
> Sigh...=C2=A0 OK, if you want it spelled out, there we go.=C2=A0 I have n=
o
> 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.=C2=A0
> 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.
>=20

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/