vt: defer kfree() of vc_screenbuf in vc_do_resize()
authorTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Wed, 29 Jul 2020 14:57:01 +0000 (23:57 +0900)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 3 Sep 2020 09:21:20 +0000 (11:21 +0200)
commit830e87324d3530d017222ceae643dad6beac6650
tree07d156caadb7c2d32aa2d7c8d770f4077a82bbad
parent8e9edff8ddc917eaa58112f1a305b4c1273582af
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Preserving old buffer and tolerating outdated vc members until set_origin()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accessing outdated vc members.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 <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