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SubjectRe: [PATCH] vgacon: fix out of bounds write to the scrollback buffer
Update the patch, add CC list, sample output, and Jiri's PoC.

On 2020/7/30 14:46, Jiri Slaby wrote:
> Hi, OTOH, you should have CCed all the (public) lists.

From ad143ede24ff4e61292cc9c96000100aacd97259 Mon Sep 17 00:00:00 2001
From: Yunhai Zhang <>
Date: Tue, 28 Jul 2020 09:58:03 +0800
Subject: [PATCH] Fix for missing check in vgacon scrollback handling

vgacon_scrollback_update() always leaves enbough room in the scrollback
buffer for the next call, but if the console size changed that room
might not actually be enough, and so we need to re-check.

The check should be in the loop since vgacon_scrollback_cur->tail is
updated in the loop and count may be more than 1 when triggered by CSI M,
as Jiri's PoC:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>

int main(int argc, char** argv)
int fd = open("/dev/tty1", O_RDWR);
unsigned short size[3] = {25, 200, 0};
ioctl(fd, 0x5609, size); // VT_RESIZE

write(fd, "\e[1;1H", 6);
for (int i = 0; i < 30; i++)
write(fd, "\e[10M", 5);

It 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:

Or to KASAN reports:
BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed

This fixes CVE-2020-14331.

Reported-and-debugged-by: 张云海 <>
Reported-and-debugged-by: Yang Yingliang <>
Reported-by: Kyungtae Kim <>
Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback)
Cc: Linus Torvalds <>
Cc: Greg KH <>
Cc: Solar Designer <>
Cc: "Srivatsa S. Bhat" <>
Cc: Anthony Liguori <>
Cc: Yang Yingliang <>
Cc: Bartlomiej Zolnierkiewicz <>
Signed-off-by: Yunhai Zhang <>
drivers/video/console/vgacon.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 998b0de1812f..37b5711cd958 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -251,6 +251,10 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
p = (void *) (c->vc_origin + t * c->vc_size_row);

while (count--) {
+ if ((vgacon_scrollback_cur->tail + c->vc_size_row) >
+ vgacon_scrollback_cur->size)
+ vgacon_scrollback_cur->tail = 0;
scr_memcpyw(vgacon_scrollback_cur->data +
p, c->vc_size_row);
 \ /
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