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Subject: Re: [PATCH] vgacon: fix out of bounds write to the scrollback buffer
To:     Jiri Slaby <jslaby@suse.cz>, b.zolnierkie@samsung.com
Cc:     linux-kernel@vger.kernel.org,
        Yang Yingliang <yangyingliang@huawei.com>,
        Kyungtae Kim <kt0755@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <greg@kroah.com>, Solar Designer <solar@openwall.com>,
        "Srivatsa S. Bhat" <srivatsa@csail.mit.edu>,
        Anthony Liguori <aliguori@amazon.com>,
        Security Officers <security@kernel.org>,
        linux-distros@vs.openwall.org, dri-devel@lists.freedesktop.org,
        linux-fbdev@vger.kernel.org
References: <20200729070249.20892-1-jslaby@suse.cz>
From:   =?UTF-8?B?5byg5LqR5rW3?= <zhangyunhai@nsfocus.com>
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Message-ID: <55075898-bf95-1805-3358-b0d1438feaa9@nsfocus.com>
Date:   Wed, 29 Jul 2020 15:53:02 +0800
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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@mail.gmail.com/
> https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@huawei.com/
> 
> 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,
> 

-- 
张云海
绿盟科技 安全研究部 研究员
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