From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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 Preserving old buffer and tolerating outdated vc members until set_origin() completes would be easier than preventing vc-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con_set_origin() from accessing outdated vc members.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 <syzbot+9116ecc1978ca3a12f43@xxxxxxxxxxxxxxxxxxxxxxxxx> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: stable <stable@xxxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/1596034621-4714-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/tty/vt/vt.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -865,7 +865,7 @@ static int vc_do_resize(struct tty_struc 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(); @@ -947,10 +947,11 @@ static int vc_do_resize(struct tty_struc 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;
