Re: [PATCH] vgacon: fix out of bounds write to the scrollback buffer
From: 张云海 <hidden>
Date: 2020-07-29 07:59:53
Also in:
dri-devel, lkml
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:quoted hunk ↗ jump to hunk
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/ (local) https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@huawei.com/ (local) 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 <redacted> Reported-and-debugged-by: 张云海 [off-list ref] Reported-and-debugged-by: Yang Yingliang [off-list ref] Reported-by: Kyungtae Kim <redacted> Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Greg KH <redacted> Cc: Solar Designer <redacted> Cc: "Srivatsa S. Bhat" <redacted> Cc: Anthony Liguori <redacted> Cc: Security Officers <security@kernel.org> Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang <redacted> Cc: Bartlomiej Zolnierkiewicz <redacted> Cc: dri-devel@lists.freedesktop.org Cc: linux-fbdev@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,
-- 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@nsfocus.com 网站：http://www.nsfocus.com