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Preserving old buffer and tolerating outdated vc members until set_origin()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accessing outdated vc members.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=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>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index ccb533f..a4f08c0 100644 (file)
--- 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;