Re: [PATCH] vc_screen: don't clobber return value in vcs_read

[George Kennedy <george.kennedy@oracle.com>]

On 2/20/2023 11:34 AM, Thomas Weißschuh wrote:
> +Cc people who were involved in the original thread.
>
> On Mon, Feb 20, 2023 at 12:48:59PM +0100, Jiri Slaby wrote:
>> On 20. 02. 23, 7:46, linux@weissschuh.net wrote:
>>> From: Thomas Weißschuh <linux@weissschuh.net>
>>>
>>> Commit 226fae124b2d
>>> ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
>>> moved the call to vcs_vc() into the loop.
>>> While doing this it also moved the unconditional assignment of
>>> "ret = -ENXIO".
>>> This unconditional assignment was valid outside the loop but within it
>>> it clobbers the actual value of ret.
>>>
>>> To avoid this only assign "ret = -ENXIO" when actually needed.
>> Not sure -- I cannot find it -- but hasn't George fixed this yet?
> Indeed there was a proposed fix at
> https://lore.kernel.org/lkml/1675704844-17228-1-git-send-email-george.kennedy@oracle.com/
>
> Linus had some suggestions so it was not applied as is.
>
> I'm not sure what the current state is.
> George, do you have something in the pipeline?

Yes, that is in the pipeline:
https://lore.kernel.org/lkml/1675774098-17722-1-git-send-email-george.kennedy@oracle.com/

Linus suggested the fix, which was tested and submitted.

Jiri commented on the patch, which I believe was directed at Linus as he 
suggested the fix.

George
>
> I also tested the patch proposed by Linus as attachment and that works.
> (The small inline patch snippet doesn't)
>
>>> Reported-by: Storm Dragon <stormdragon2976@gmail.com>
>>> Link: https://lore.kernel.org/lkml/Y%2FKS6vdql2pIsCiI@hotmail.com/
>>> Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
>>> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
>>>
>>> ---
>>>
>>> @Storm Could you validate this patch?
>>> ---
>>>    drivers/tty/vt/vc_screen.c | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
>>> index f566eb1839dc..2ef519a40a87 100644
>>> --- a/drivers/tty/vt/vc_screen.c
>>> +++ b/drivers/tty/vt/vc_screen.c
>>> @@ -403,10 +403,11 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>>>    		unsigned int this_round, skip = 0;
>>>    		int size;
>>> -		ret = -ENXIO;
>>>    		vc = vcs_vc(inode, &viewed);
>>> -		if (!vc)
>>> +		if (!vc) {
>>> +			ret = -ENXIO;
>>>    			goto unlock_out;
>>> +		}
>>>    		/* Check whether we are above size each round,
>>>    		 * as copy_to_user at the end of this loop
>>>
>>> base-commit: c9c3395d5e3dcc6daee66c6908354d47bf98cb0c
>> -- 
>> js
>> suse labs
>>