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SubjectRe: fbcon scrollback broken was Re: Linux 5.9-rc6
Hi!

> > The one thing that does show up in the diffstat is the softscroll
> > removal (both fbcon and vgacon), and there are people who want to save
> > that, but we'll see if some maintainer steps up. I'm not willing to
> > resurrect it in the broken form it was in, so I doubt that will happen
> > in 5.9, but we'll see what happens.
>
> Could you... like try not to make the job harder by merging miriad of
> "let's kill the documentation" patches?
>
> I needed the feature twice already this month, once during fsck, and
> then five minutes ago, when oops scrolled by me during system
> shutdown. Unfortunately, shift-pageup was not available to take a look
> what is going on there.
>
> Unfortunately, Greg backported your "fix" to all the stable releases,
> so resulting damage will not be easy to repair.
>
> For the record, making the security problem unexploitable should be as
> simple as patch below.
>
> Unfortunately, Linus' changelog does not exactly tell me what all the
> known problems are. Willy Tarreau pointed me to
> https://www.openwall.com/lists/oss-security/2020/09/15/2 , which has
> some information, but all I can reproduce are transient screen
> artefacts. If someone has more detailed information, that would be
> helpful.

I can now reproduce it. C code from
https://www.openwall.com/lists/oss-security/2020/09/15/2 + this patch
makes the issue easy to see.

Best regards,
Pavel

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 5f1183b0b89d..5a8a43c0b546 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1209,6 +1209,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
rlth = min(old_row_size, new_row_size);
rrem = new_row_size - rlth;
old_origin = vc->vc_origin;
+ vc->vc_origin = NULL;
new_origin = (long) newscreen;
new_scr_end = new_origin + new_screen_size;

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 75ebbbf0a1fb..6ceae7ea1ccc 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2818,6 +2818,11 @@ static void fbcon_scrolldelta(struct vc_data *vc, int lines)
struct display *disp = &fb_display[fg_console];
int offset, limit, scrollback_old;

+ if (!vc->vc_origin) {
+ printk("scrolldelta races with resize?!\n");
+ return;
+ }
+
if (softback_top) {
if (vc->vc_num != fg_console)
return;
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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