This is a note to let you know that I've just added the patch titled

vt: defer kfree() of vc_screenbuf in vc_do_resize()

to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From f8d1653daec02315e06d30246cff4af72e76e54e Mon Sep 17 00:00:00 2001

From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Date: Wed, 29 Jul 2020 23:57:01 +0900 Subject: vt: defer kfree() of vc_screenbuf in vc_do_resize()

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Preserving old buffer and tolerating outdated vc members until set_origin() completes would be easier than preventing vc-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con_set_origin() from accessing outdated vc members.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=6649da2081e2ebdc65c0642c214b27fe91099db...

Reported-by: syzbot syzbot+9116ecc1978ca3a12f43@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/1596034621-4714-1-git-send-email-penguin-kernel@I-... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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 ccb533fd00a2..a4f08c0da84b 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -1201,7 +1201,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc, unsigned int old_rows, old_row_size, first_copied_row; unsigned int new_cols, new_rows, new_row_size, new_screen_size; unsigned int user; - unsigned short *newscreen; + unsigned short *oldscreen, *newscreen; struct uni_screen *new_uniscr = NULL;

WARN_CONSOLE_UNLOCKED(); @@ -1299,10 +1299,11 @@ 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;