linux-kernel - Re: [PATCH] tty: vt: selection: Add check for valid tiocl

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Date:   Thu, 4 Aug 2022 11:22:26 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Helge Deller <deller@....de>,
        Khalid Masum <khalid.masum.92@...il.com>,
        syzbot <syzbot+14b0e8f3fd1612e35350@...kaller.appspotmail.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] tty: vt: selection: Add check for valid tiocl_selection
 values

On 04. 08. 22, 10:44, Helge Deller wrote:
> 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@....de>
>>>> Reported-by: syzbot+14b0e8f3fd1612e35350@...kaller.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,
>>>>            return 0;
>>>>        }
>>>>
>>>> +    if (!v->xs || !v->ys || !v->xe || !v->ye)
>>>> +        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 values
>> 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 row 0),
>> my patch now returns -EINVAL, while it returned success before.
>>
>>> And the code below should handle this just fine, right:
>>>>        v->xs = min_t(u16, v->xs - 1, vc->vc_cols - 1);
>>>>        v->ys = min_t(u16, v->ys - 1, vc->vc_rows - 1);
>>>>        v->xe = 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 patch
> and leave it as is.

We can still do a trial and revert it if something breaks... It's just 
that _noone_ knows with all this undocumented stuff ;).

But in fact, 0 currently means full row/column. Isn't it on purpose?

Today, we are out of luck, codesearch.debian.net gives no clue about users:
https://codesearch.debian.net/search?q=%5CbTIOCL_SETSEL%5Cb&literal=0

thanks,
-- 
js
suse labs