[v2] vt: fix use after free in function "vc_do_resize"

[Ye Bin]

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";.

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.

Reported-by: Jiri Slaby <jslaby@xxxxxxxx>
Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
---
 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,
-- 
2.17.2