[PATCH] vgacon: fix out of bounds write to the scrollback buffer

[Bartlomiej Zolnierkiewicz]

Hi Jiri,

On 7/29/20 9:02 AM, Jiri Slaby wrote:
> The current vgacon's scroll up implementation uses a circural buffer
> in vgacon_scrollback_cur. It always advances tail to prepare it for the
> next write and caps it to zero if the next ->vc_size_row bytes won't fit.
> 
> But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new
> line might not fit to the end of the scrollback buffer in the next
> attempt to scroll. This leads to various crashes as
> vgacon_scrollback_update writes out of the buffer:
>  BUG: unable to handle page fault for address: ffffc900001752a0
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  RIP: 0010:mutex_unlock+0x13/0x30
> ...
>  Call Trace:
>   n_tty_write+0x1a0/0x4d0
>   tty_write+0x1a0/0x2e0
> 
> Or to KASAN reports:
> BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
> 
> So check whether the line fits in the buffer and wrap if needed. Do it
> before the loop as console_sem is held and ->vc_size_row cannot change
> during the execution of vgacon_scrollback_cur. If it does change, we
> need to ensure it does not change elsewhere, not here.
> 
> Also, we do not split the write of a line into chunks as that would
> break the consumers of the buffer. They expect ->cnt, ->tail and ->size
> to be in harmony and advanced by ->vc_size_row.
> 
> I found few reports of this in the past, some with patches included,
> some even 2 years old:
> https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X49akg@mail.gmail.com/

Sorry but I don't work on fixing fbdev/console KASAN/syzbot/etc.
reports (-ENORESOURCES).  This has been made official in the past.

I'm happy to review/apply patches though.

> https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@huawei.com/

This was the first time the patch for issue was submitted.

I tried to apply it to drm-misc but then I have noticed that
it has not been posted to linux-fbdev / dri-devel MLs (so it
was not possible to merge it using dim tool) and thus I've
requested the author to resend it:

https://lore.kernel.org/lkml/62544bd9-e47d-e7f9-92f2-49b8dbb132c1@samsung.com/

which he did:

https://lore.kernel.org/lkml/20200713105730.550334-1-yangyingliang@huawei.com/

and the patch is currently under review period (to give people
chance to comment on it) and in my "to apply if no objections"
folder.

I see that your/Yunhai patch addresses the root source of
the issue so I'll be happy to apply/ACK it instead of Yang's
patch once the final version is posted.

Thank you for working on this.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics

> This fixes CVE-2020-14331.
> 
> Big thanks to guys mentioned in the Reported-and-debugged-by lines below
> who actually found the root cause.
> 
> Signed-off-by: Jiri Slaby <jslaby at suse.cz>
> Reported-and-debugged-by: 张云海 <zhangyunhai at nsfocus.com>
> Reported-and-debugged-by: Yang Yingliang <yangyingliang at huawei.com>
> Reported-by: Kyungtae Kim <kt0755 at gmail.com>
> Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback)
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: Greg KH <greg at kroah.com>
> Cc: Solar Designer <solar at openwall.com>
> Cc: "Srivatsa S. Bhat" <srivatsa at csail.mit.edu>
> Cc: Anthony Liguori <aliguori at amazon.com>
> Cc: Security Officers <security at kernel.org>
> Cc: linux-distros at vs.openwall.org
> Cc: Yang Yingliang <yangyingliang at huawei.com>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie at samsung.com>
> Cc: dri-devel at lists.freedesktop.org
> Cc: linux-fbdev at vger.kernel.org
> ---
>  drivers/video/console/vgacon.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> index f0f3d573f848..13194bb246f8 100644
> --- a/drivers/video/console/vgacon.c
> +++ b/drivers/video/console/vgacon.c
> @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
>  
>  	p = (void *) (c->vc_origin + t * c->vc_size_row);
>  
> +	/* vc_size_row might have changed by VT_RESIZE in the meantime */
> +	if ((vgacon_scrollback_cur->tail + c->vc_size_row) >=
> +			vgacon_scrollback_cur->size)
> +		vgacon_scrollback_cur->tail = 0;
> +
>  	while (count--) {
>  		scr_memcpyw(vgacon_scrollback_cur->data +
>  			    vgacon_scrollback_cur->tail,
>