[PATCH 4.14 62/91] vt: defer kfree() of vc_screenbuf in vc_do_resize()

Greg Kroah-Hartman
2020-09-01T16:57:46Z

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

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Preserving old buffer and tolerating outdated vc members until set_origin()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accessing outdated vc members.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@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-love.SAKURA.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
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;