Hi All,
This patch dosen't fix the issue, the check should be in the loop.
The change of the VT sze is before vgacon_scrollback_update, not in the
meantime.
Let's consider the following situation:
suppose:
vgacon_scrollback_cur->size is 65440
vgacon_scrollback_cur->tail is 64960
c->vc_size_row is 160
count is 5
Reset c->vc_size_row to 200 by VT_RESIZE, then call
vgacon_scrollback_update.
This will pass the check, since (vgacon_scrollback_cur->tail +
c->vc_size_row)
is 65160 which is less then vgacon_scrollback_cur->size(65440).
However, in the 3rd iteration of the loop, vgacon_scrollback_cur->tail
is update
to 65360, the memcpy will overflow.
To avoid overflow, the check should be
if ((vgacon_scrollback_cur->tail + c->vc_size_row * count) >=
However, this will break the circular of the buffer, since all 5 lines
will be copy at the beginning.
To avoid break circular, we have to detect if wrap occurs, use a loop to
copy lines before
wrap, reset vgacon_scrollback_cur->tail to 0, then use another loop to
copy lines after wrap.
Of course the 2 loop can be combine into 2 memcpy, that will be similar
to Linus's patch.
Thus, I think the check should be in the loop.
The 2 check in the loop seems to be redundancy, Zhang Xiao from
Windriver suggest that the check after the memcpy can be remove.
I think he was right, but not very sure.
Thus, I suggest we discuss that too.
Regards,
Yunhai Zhang / NSFOCUS Security Team
On 2020/7/29 15:02, 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@xxxxxxxxxxxxxx/
> https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@xxxxxxxxxx/
>
> 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@xxxxxxx>
> Reported-and-debugged-by: 张云海 <zhangyunhai@xxxxxxxxxxx>
> Reported-and-debugged-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
> Reported-by: Kyungtae Kim <kt0755@xxxxxxxxx>
> Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback)
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Greg KH <greg@xxxxxxxxx>
> Cc: Solar Designer <solar@xxxxxxxxxxxx>
> Cc: "Srivatsa S. Bhat" <srivatsa@xxxxxxxxxxxxx>
> Cc: Anthony Liguori <aliguori@xxxxxxxxxx>
> Cc: Security Officers <security@xxxxxxxxxx>
> Cc: linux-distros@xxxxxxxxxxxxxxx
> Cc: Yang Yingliang <yangyingliang@xxxxxxxxxx>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-fbdev@xxxxxxxxxxxxxxx
> ---
> 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,
>
--
张云海
绿盟科技 安全研究部 研究员
地址：北京市海淀区北洼路4号益泰大厦三层
邮编：100089
电话：(010)68438880-8510
传真：(010)68437328
手机：13691192782
邮箱：zhangyunhai@xxxxxxxxxxx
网站：http://www.nsfocus.com
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/dri-devel
