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Date:   Thu, 4 Aug 2022 10:44:51 +0200
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Subject: Re: [PATCH] tty: vt: selection: Add check for valid tiocl_selection
 values
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From:   Helge Deller <deller@gmx.de>
To:     Jiri Slaby <jirislaby@kernel.org>,
        Khalid Masum <khalid.masum.92@gmail.com>,
        syzbot <syzbot+14b0e8f3fd1612e35350@syzkaller.appspotmail.com>,
        dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On 8/4/22 09:15, Helge Deller wrote:
> Hello Jiri,
>
> Thanks for looking into this patch!
>
> On 8/4/22 07:47, Jiri Slaby wrote:
>> On 30. 07. 22, 20:49, Helge Deller wrote:
>>> The line and column numbers for the selection need to start at 1.
>>> Add the checks to prevent invalid input.
>>>
>>> Signed-off-by: Helge Deller <deller@gmx.de>
>>> Reported-by: syzbot+14b0e8f3fd1612e35350@syzkaller.appspotmail.com
>>>
>>> diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
>>> index f7755e73696e..58692a9b4097 100644
>>> --- a/drivers/tty/vt/selection.c
>>> +++ b/drivers/tty/vt/selection.c
>>> @@ -326,6 +326,9 @@ static int vc_selection(struct vc_data *vc, struct=
 tiocl_selection *v,
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return 0;
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>>>
>>> +=C2=A0=C2=A0=C2=A0 if (!v->xs || !v->ys || !v->xe || !v->ye)
>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return -EINVAL;
>>
>> Hmm, I'm not sure about this. It potentially breaks userspace (by
>> returning EINVAL now).
>
> Right.
> According to the code below, my interpretation is that all xs/ys/xe/ye v=
alues
> should be > 0. But of course I might be wrong on this, as I didn't find =
any
> documentation for TIOCL_SETSEL.
>
> And if userspace tries to set an invalid selection (e.g. by selecting ro=
w 0),
> my patch now returns -EINVAL, while it returned success before.
>
>> And the code below should handle this just fine, right:
>>>       v->xs =3D min_t(u16, v->xs - 1, vc->vc_cols - 1);
>>>       v->ys =3D min_t(u16, v->ys - 1, vc->vc_rows - 1);
>>>       v->xe =3D min_t(u16, v->xe - 1, vc->vc_cols - 1);
>
> It "handles it fine" in the sense that it can cope with the
> input and will not crash.
> But it returns (maybe?) unexpected results...

After some more thinking maybe you are right.
In case a user provided invalid values in the past, simply an unexpected
selection was set, but nothing broke.
Since the patch doesn't fix any critical issue, we could just drop this pa=
tch
and leave it as is.

Helge