linux-kernel - Re: [PATCH 16/36] tty/vt: consolemap: check put_user() in con_get

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Message-ID: <926a95c0-3d0b-4dec-4894-3fd756d565a4@kernel.org>
Date:   Thu, 9 Jun 2022 10:51:44 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/36] tty/vt: consolemap: check put_user() in
 con_get_unimap()

On 08. 06. 22, 10:11, Jiri Slaby wrote:
> On 08. 06. 22, 10:02, David Laight wrote:
>> From: Jiri Slaby
>>> Sent: 07 June 2022 11:49
>>>
>>> Only the return value of copy_to_user() is checked in con_get_unimap().
>>> Do the same for put_user() of the count too.
>>>
>>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>>> ---
>>>   drivers/tty/vt/consolemap.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/tty/vt/consolemap.c b/drivers/tty/vt/consolemap.c
>>> index 831450f2bfd1..92b5dddb00d9 100644
>>> --- a/drivers/tty/vt/consolemap.c
>>> +++ b/drivers/tty/vt/consolemap.c
>>> @@ -813,7 +813,8 @@ int con_get_unimap(struct vc_data *vc, ushort ct, 
>>> ushort __user *uct,
>>>       console_unlock();
>>>       if (copy_to_user(list, unilist, min(ect, ct) * sizeof(*unilist)))
>>>           ret = -EFAULT;
>>> -    put_user(ect, uct);
>>> +    if (put_user(ect, uct))
>>> +        ret = -EFAULT;
>>>       kvfree(unilist);
>>>       return ret ? ret : (ect <= ct) ? 0 : -ENOMEM;
>>>   }
>>
>> How is the user expected to check the result of this code?
>>
>> AFAICT -ENOMEM is returned if either kmalloc() fails or
>> the user buffer is too short?
>> Looks pretty hard to detect which.
> 
> Agreed. The code is far from perfect. We might try to return ENOSPC and 
> watch what breaks.

brltty and kbd (see below) would break at least:
https://sources.debian.org/src/brltty/6.4-6/Drivers/Screen/Linux/screen.c/#L875

brltty apparently relies exactly on ENOMEM, increases buffer if that 
error is returned, and retries.

So I don't think we can change that ENOMEM to anything else.

>> I've not looked at the effect of all the patches, but setting
>> 'ret = -ENOMEM' and breaking the loop when the array is too
>> small would simplify things.

That would break kbd for example:
https://sources.debian.org/src/kbd/2.3.0-3/src/libkfont/kdmapop.c/?hl=154#L159

GIO_UNIMAP is called with zero count to actually find out the count...

So apart from the original patch which checks the return value of 
put_user, we cannot do anything else. (Except decoupling the "?:" to 
make it more readable.)

thanks,
-- 
js
suse labs