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

[Thomas Weißschuh]

On Tue, Feb 21, 2023 at 08:30:11AM -0500, George Kennedy wrote:
> 
> 
> 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@xxxxxxxxxxxxxx wrote:
> > > > From: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> > > > 
> > > > 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@xxxxxxxxxx/
> > 
> > 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@xxxxxxxxxx/
> 
> 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.

Thanks for the pointer!

I searched for it by its Fixes: tag.
The v2 has a different one than the v1.
To me the v1 Fixes: seems more correct, was the change
intentional?

> 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@xxxxxxxxx>
> > > > Link: https://lore.kernel.org/lkml/Y%2FKS6vdql2pIsCiI@xxxxxxxxxxx/
> > > > Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
> > > > Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> > > > 
> > > > ---
> > > > 
> > > > @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
> > > 
>