Re: [PATCH 16/36] tty/vt: consolemap: check put_user() in con_get_unimap()

Jiri Slaby
2022-06-09T08:54:10Z

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@suse.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