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=+X4... https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang...
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@suse.cz Reported-and-debugged-by: 张云海 zhangyunhai@nsfocus.com Reported-and-debugged-by: Yang Yingliang yangyingliang@huawei.com Reported-by: Kyungtae Kim kt0755@gmail.com Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Greg KH greg@kroah.com Cc: Solar Designer solar@openwall.com Cc: "Srivatsa S. Bhat" srivatsa@csail.mit.edu Cc: Anthony Liguori aliguori@amazon.com Cc: Security Officers security@kernel.org Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang yangyingliang@huawei.com Cc: Bartlomiej Zolnierkiewicz b.zolnierkie@samsung.com 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,