linux-kernel - Re: [v2] vt: fix use after free in function "vc_do

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Date:   Tue, 3 Mar 2020 10:49:53 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Ye Bin <yebin10@...wei.com>, gregkh@...uxfoundation.org
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [v2] vt: fix use after free in function "vc_do_resize"

Hi,

On 02. 03. 20, 12:28, Ye Bin wrote:
> Fix CVE-2020-8647(https://nvd.nist.gov/vuln/detail/CVE-2020-8647), detail description
> about this CVE is in bugzilla "https://bugzilla.kernel.org/show_bug.cgi?id=206359".

this is much better. So this is one of the bugzilla reports...

> error information:
> BUG: KASan: use after free in vc_do_resize+0x49e/0xb30 at addr ffff88000016b9c0
> Read of size 2 by task syz-executor.3/24164
> page:ffffea0000005ac0 count:0 mapcount:0 mapping:          (null) index:0x0
> page flags: 0xfffff00000000()
> page dumped because: kasan: bad access detected
> CPU: 0 PID: 24164 Comm: syz-executor.3 Not tainted 3.10.0-862.14.2.1.x86_64+ #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
> Call Trace:
>  [<ffffffffb059f309>] dump_stack+0x1e/0x20
>  [<ffffffffaf8af957>] kasan_report+0x577/0x950
>  [<ffffffffaf8ae652>] __asan_load2+0x62/0x80
>  [<ffffffffafe3728e>] vc_do_resize+0x49e/0xb30
>  [<ffffffffafe3795c>] vc_resize+0x3c/0x60
>  [<ffffffffafe1d80d>] vt_ioctl+0x16ed/0x2670
>  [<ffffffffafe0089a>] tty_ioctl+0x46a/0x1a10
>  [<ffffffffaf92db3d>] do_vfs_ioctl+0x5bd/0xc40
>  [<ffffffffaf92e2f2>] SyS_ioctl+0x132/0x170
>  [<ffffffffb05c9b1b>] system_call_fastpath+0x22/0x27
> 
> In function vc_do_resize:
> ......
> if (vc->vc_y > new_rows) {
> 	.......
> 	old_origin += first_copied_row * old_row_size;
> } else
> 	first_copied_row = 0;
> end = old_origin + old_row_size * min(old_rows, new_rows);
> ......
> while (old_origin < end) {
> 	scr_memcpyw((unsigned short *) new_origin,
> 		    (unsigned short *) old_origin, rlth);
> 	if (rrem)
> 		scr_memsetw((void *)(new_origin + rlth),
> 			    vc->vc_video_erase_char, rrem);
> 	old_origin += old_row_size;
> 	new_origin += new_row_size;
> }
> ......
> 
> We can see that before calculate variable "end" may update variable "old_origin"
> with "old_origin += first_copied_row * old_row_size", variable "end" is equal to
> "old_origin + (first_copied_row + min(old_rows, new_rows))* old_row_size", it's
> possible that "first_copied_row + min(old_rows, new_rows)" large than "old_rows".
> So when call scr_memcpyw function cpoy data from origin buffer to new buffer in
> "while" loop, which "old_origin" may large than real old buffer end. Now, we
> calculate origin buffer end before update "old_origin" to avoid illegal memory
> access.

I need to look into the code first. Note to myself, the line was added in:
commit 9fc2b2d0cf743008d8f6be6293278f4ef61f09f3
Author: Francisco Jerez <currojerez@...eup.net>
Date:   Sun Aug 22 17:37:24 2010 +0200

    vt: Fix console corruption on driver hand-over.

> Reported-by: Jiri Slaby <jslaby@...e.com>

You misunderstood me. syzbot, while reporting a bug, amends this to the
reports:
"""
IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+SOMETHING@...kaller.appspotmail.com
"""

It is not the case in this case, as this went through bugzilla, not
using syzbot.

So instead of the Reported-by, we should have:
References: https://bugzilla.kernel.org/show_bug.cgi?id=206359
here.

> Signed-off-by: Ye Bin <yebin10@...wei.com>
> ---
>  drivers/tty/vt/vt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 8fa059ec6cc8..1d7217bef678 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1231,6 +1231,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
>  	old_origin = vc->vc_origin;
>  	new_origin = (long) newscreen;
>  	new_scr_end = new_origin + new_screen_size;
> +	end = old_origin + old_row_size * min(old_rows, new_rows);
>  
>  	if (vc->vc_y > new_rows) {
>  		if (old_rows - vc->vc_y < new_rows) {
> @@ -1249,7 +1250,6 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
>  		old_origin += first_copied_row * old_row_size;
>  	} else
>  		first_copied_row = 0;
> -	end = old_origin + old_row_size * min(old_rows, new_rows);
>  
>  	vc_uniscr_copy_area(new_uniscr, new_cols, new_rows,
>  			    get_vc_uniscr(vc), rlth/2, first_copied_row,
> 

thanks,
-- 
js
suse labs