From 830e87324d3530d017222ceae643dad6beac6650 Mon Sep 17 00:00:00 2001 From: Tetsuo Handa Date: Wed, 29 Jul 2020 23:57:01 +0900 Subject: [PATCH] vt: defer kfree() of vc_screenbuf in vc_do_resize() commit f8d1653daec02315e06d30246cff4af72e76e54e upstream. syzbot is reporting UAF bug in set_origin() from vc_do_resize() [1], for vc_do_resize() calls kfree(vc->vc_screenbuf) before calling set_origin(). Unfortunately, in set_origin(), vc->vc_sw->con_set_origin() might access vc->vc_pos when scroll is involved in order to manipulate cursor, but vc->vc_pos refers already released vc->vc_screenbuf until vc->vc_pos gets updated based on the result of vc->vc_sw->con_set_origin(). Preserving old buffer and tolerating outdated vc members until set_origin() completes would be easier than preventing vc->vc_sw->con_set_origin() from accessing outdated vc members. [1] https://syzkaller.appspot.com/bug?id=6649da2081e2ebdc65c0642c214b27fe91099db3 Reported-by: syzbot Signed-off-by: Tetsuo Handa Cc: stable Link: https://lore.kernel.org/r/1596034621-4714-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp Signed-off-by: Greg Kroah-Hartman --- drivers/tty/vt/vt.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 3e5a361..1f00f42 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -868,7 +868,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc, unsigned int old_rows, old_row_size; unsigned int new_cols, new_rows, new_row_size, new_screen_size; unsigned int user; - unsigned short *newscreen; + unsigned short *oldscreen, *newscreen; WARN_CONSOLE_UNLOCKED(); @@ -950,10 +950,11 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc, if (new_scr_end > new_origin) scr_memsetw((void *)new_origin, vc->vc_video_erase_char, new_scr_end - new_origin); - kfree(vc->vc_screenbuf); + oldscreen = vc->vc_screenbuf; vc->vc_screenbuf = newscreen; vc->vc_screenbuf_size = new_screen_size; set_origin(vc); + kfree(oldscreen); /* do part of a reset_terminal() */ vc->vc_top = 0; -- 2.7.4