Re: [PATCH] net: 6pack: fix slab-out-of-bounds in decode_data

[Pavel Skripkin]

On 8/13/21 5:58 PM, Dan Carpenter wrote:
> On Fri, Aug 13, 2021 at 02:28:55PM +0300, Pavel Skripkin wrote:
> > Syzbot reported slab-out-of bounds write in decode_data().
> > The problem was in missing validation checks.
> >
> > Syzbot's reproducer generated malicious input, which caused
> > decode_data() to be called a lot in sixpack_decode(). Since
> > rx_count_cooked is only 400 bytes and noone reported before,
> > that 400 bytes is not enough, let's just check if input is malicious
> > and complain about buffer overrun.
> >
> > Fail log:
> > ==================================================================
> > BUG: KASAN: slab-out-of-bounds in drivers/net/hamradio/6pack.c:843
> > Write of size 1 at addr ffff888087c5544e by task kworker/u4:0/7
> >
> > CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.6.0-rc3-syzkaller #0
> > ...
> > Workqueue: events_unbound flush_to_ldisc
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x197/0x210 lib/dump_stack.c:118
> >  print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
> >  __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
> >  kasan_report+0x12/0x20 mm/kasan/common.c:641
> >  __asan_report_store1_noabort+0x17/0x20 mm/kasan/generic_report.c:137
> >  decode_data.part.0+0x23b/0x270 drivers/net/hamradio/6pack.c:843
> >  decode_data drivers/net/hamradio/6pack.c:965 [inline]
> >  sixpack_decode drivers/net/hamradio/6pack.c:968 [inline]
> >
> > Reported-and-tested-by: syzbot+fc8cd9a673d4577fb2e4@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
> > ---
> >  drivers/net/hamradio/6pack.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
> > index fcf3af76b6d7..f4ffc2a80ab7 100644
> > --- a/drivers/net/hamradio/6pack.c
> > +++ b/drivers/net/hamradio/6pack.c
> > @@ -827,6 +827,12 @@ static void decode_data(struct sixpack *sp, unsigned char inbyte)
> >  		return;
> >  	}
> >  
> > +	if (sp->rx_count_cooked + 3 >= sizeof(sp->cooked_buf)) {
>
> It should be + 2 instead of + 3.
>
> We write three bytes.  idx, idx + 1, idx + 2.  Otherwise, good fix!

Indeed. Will fix in v2, thank you for pointing it out!


With regards,
Pavel Skripkin