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

[Thomas Weißschuh]

+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?

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
>