The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ce684552a266cb1c7cc2f7e623f38567adec6653 Mon Sep 17 00:00:00 2001

From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Date: Sun, 12 Jul 2020 20:10:12 +0900 Subject: [PATCH] vt: Reject zero-sized screen buffer size.

syzbot is reporting general protection fault in do_con_write() [1] caused by vc->vc_screenbuf == ZERO_SIZE_PTR caused by vc->vc_screenbuf_size == 0 caused by vc->vc_cols == vc->vc_rows == vc->vc_size_row == 0 caused by fb_set_var() from ioctl(FBIOPUT_VSCREENINFO) on /dev/fb0 , for gotoxy(vc, 0, 0) from reset_terminal() from vc_init() from vc_allocate() from con_install() from tty_init_dev() from tty_open() on such console causes vc->vc_pos == 0x10000000e due to ((unsigned long) ZERO_SIZE_PTR) + -1U * 0 + (-1U << 1).

I don't think that a console with 0 column or 0 row makes sense. And it seems that vc_do_resize() does not intend to allow resizing a console to 0 column or 0 row due to

new_cols = (cols ? cols : vc->vc_cols); new_rows = (lines ? lines : vc->vc_rows);

exception.

Theoretically, cols and rows can be any range as long as 0 < cols * rows * 2 <= KMALLOC_MAX_SIZE is satisfied (e.g. cols == 1048576 && rows == 2 is possible) because of

vc->vc_size_row = vc->vc_cols << 1; vc->vc_screenbuf_size = vc->vc_rows * vc->vc_size_row;

in visual_init() and kzalloc(vc->vc_screenbuf_size) in vc_allocate().

Since we can detect cols == 0 or rows == 0 via screenbuf_size = 0 in visual_init(), we can reject kzalloc(0). Then, vc_allocate() will return an error, and con_write() will not be called on a console with 0 column or 0 row.

We need to make sure that integer overflow in visual_init() won't happen. Since vc_do_resize() restricts cols <= 32767 and rows <= 32767, applying 1 <= cols <= 32767 and 1 <= rows <= 32767 restrictions to vc_allocate() will be practically fine.

This patch does not touch con_init(), for returning -EINVAL there does not help when we are not returning -ENOMEM.

[1] https://syzkaller.appspot.com/bug?extid=017265e8553724e514e8

Reported-and-tested-by: syzbot syzbot+017265e8553724e514e8@syzkaller.appspotmail.com Signed-off-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Cc: stable stable@vger.kernel.org Link: https://lore.kernel.org/r/20200712111013.11881-1-penguin-kernel@I-love.SAKUR... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 48a8199f7845..42d8c67a481f 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -1092,10 +1092,19 @@ static const struct tty_port_operations vc_port_ops = { .destruct = vc_port_destruct, };

+/* + * Change # of rows and columns (0 means unchanged/the size of fg_console) + * [this is to be used together with some user program + * like resize that changes the hardware videomode] + */ +#define VC_MAXCOL (32767) +#define VC_MAXROW (32767) + int vc_allocate(unsigned int currcons) /* return 0 on success */ { struct vt_notifier_param param; struct vc_data *vc; + int err;

WARN_CONSOLE_UNLOCKED();

@@ -1125,6 +1134,11 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */ if (!*vc->vc_uni_pagedir_loc) con_set_default_unimap(vc);

+ err = -EINVAL; + if (vc->vc_cols > VC_MAXCOL || vc->vc_rows > VC_MAXROW || + vc->vc_screenbuf_size > KMALLOC_MAX_SIZE || !vc->vc_screenbuf_size) + goto err_free; + err = -ENOMEM; vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_KERNEL); if (!vc->vc_screenbuf) goto err_free; @@ -1143,7 +1157,7 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */ visual_deinit(vc); kfree(vc); vc_cons[currcons].d = NULL; - return -ENOMEM; + return err; }

static inline int resize_screen(struct vc_data *vc, int width, int height, @@ -1158,14 +1172,6 @@ static inline int resize_screen(struct vc_data *vc, int width, int height, return err; }

-/* - * Change # of rows and columns (0 means unchanged/the size of fg_console) - * [this is to be used together with some user program - * like resize that changes the hardware videomode] - */ -#define VC_RESIZE_MAXCOL (32767) -#define VC_RESIZE_MAXROW (32767) - /** * vc_do_resize - resizing method for the tty * @tty: tty being resized @@ -1201,7 +1207,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc, user = vc->vc_resize_user; vc->vc_resize_user = 0;

- if (cols > VC_RESIZE_MAXCOL || lines > VC_RESIZE_MAXROW) + if (cols > VC_MAXCOL || lines > VC_MAXROW) return -EINVAL;

new_cols = (cols ? cols : vc->vc_cols); @@ -1212,7 +1218,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc, if (new_cols == vc->vc_cols && new_rows == vc->vc_rows) return 0;

- if (new_screen_size > KMALLOC_MAX_SIZE) + if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size) return -EINVAL; newscreen = kzalloc(new_screen_size, GFP_USER); if (!newscreen) @@ -3393,6 +3399,7 @@ static int __init con_init(void) INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK); tty_port_init(&vc->port); visual_init(vc, currcons, 1); + /* Assuming vc->vc_{cols,rows,screenbuf_size} are sane here. */ vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT); vc_init(vc, vc->vc_rows, vc->vc_cols, currcons || !vc->vc_sw->con_save_screen);