Re: 6pack: stack-out-of-bounds in sixpack_receive_buf

[One Thousand Gnomes]

On Sat, 3 Sep 2016 15:38:08 +0200
Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:

> Hello,
> 
> While running syzkaller fuzzer I've got the following report:
> 
> BUG: KASAN: stack-out-of-bounds in sixpack_receive_buf+0xf8a/0x1450 at
> addr ffff880037fbf850
> Read of size 1 by task syz-executor/6759
> page:ffffea0000dfefc0 count:0 mapcount:0 mapping:          (null) index:0x0
> flags: 0x1fffc0000000000()
> page dumped because: kasan: bad access detected
> CPU: 3 PID: 6759 Comm: syz-executor Not tainted 4.8.0-rc3-next-20160825+ #8
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>  ffffffff886b6fe0 ffff880037fbf520 ffffffff82db38d9 ffffffff37fbf5b0
>  fffffbfff10d6dfc ffff880037fbf5b0 ffff880037fbf850 ffff880037fbf850
>  ffff880037d3f180 dffffc0000000000 ffff880037fbf5a0 ffffffff8180a383
> Call Trace:
>  [<ffffffff8180a3ee>] __asan_report_load1_noabort+0x3e/0x40
> mm/kasan/report.c:319
>  [<     inline     >] sixpack_decode drivers/net/hamradio/6pack.c:1001
>  [<ffffffff8425f96a>] sixpack_receive_buf+0xf8a/0x1450
> drivers/net/hamradio/6pack.c:462
>  [<ffffffff8323b368>] tty_ldisc_receive_buf+0x168/0x1b0
> drivers/tty/tty_buffer.c:433
>  [<ffffffff832616de>] paste_selection+0x27e/0x3e0 drivers/tty/vt/selection.c:363
>  [<ffffffff8327f286>] tioclinux+0x126/0x410 drivers/tty/vt/vt.c:2683
>  [<ffffffff8325c1ef>] vt_ioctl+0x13ef/0x2910 drivers/tty/vt/vt_ioctl.c:365
>  [<ffffffff832245cd>] tty_ioctl+0x69d/0x21e0 drivers/tty/tty_io.c:2983
>  [<     inline     >] vfs_ioctl fs/ioctl.c:43
>  [<ffffffff818a1dfc>] do_vfs_ioctl+0x18c/0x1080 fs/ioctl.c:675
>  [<     inline     >] SYSC_ioctl fs/ioctl.c:690
>  [<ffffffff818a2d7f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:681
>  [<ffffffff86e10700>] entry_SYSCALL_64_fastpath+0x23/0xc1
> Memory state around the buggy address:
>  ffff880037fbf700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>  ffff880037fbf780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >ffff880037fbf800: 00 00 00 00 00 00 00 00 00 00 f3 f3 f3 f3 00 00  
>                                                  ^
>  ffff880037fbf880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>  ffff880037fbf900: 00 00 00 00 f1 f1 f1 f1 00 f4 f4 f4 f2 f2 f2 f2
> ==================================================================
> 
> 
> It is then followed by similar reports that access subsequent stack bytes.
> Unfortunately I can't reproduce it (though, I got 6 similar crashes in
> different runs). Looking at code, the following looks suspicious -- we
> limit copy by 512 bytes, but use the original count which can be
> larger than 512:
> 
> static void sixpack_receive_buf(struct tty_struct *tty,
>     const unsigned char *cp, char *fp, int count)
> {
>     unsigned char buf[512];
>     ....
>     memcpy(buf, cp, count < sizeof(buf) ? count : sizeof(buf));
>     ....
>     sixpack_decode(sp, buf, count1);
> 

Your suspicion is correct.

Alan