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
