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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Preserving old buffer and tolerating outdated vc members until set_origin()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accessing outdated vc members.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>
This commit is contained in:
Tetsuo Handa 2020-07-29 23:57:01 +09:00 committed by Greg Kroah-Hartman
parent 9123e3a74e
commit f8d1653dae
1 changed files with 3 additions and 2 deletions
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5
drivers/tty/vt/vt.c
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@ -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;