Re: KASAN: stack-out-of-bounds Read in xfrm_selector_match (2)

[Paul Moore]

On Thu, Sep 24, 2020 at 11:08 PM Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Sep 21, 2020 at 07:56:20AM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    eb5f95f1 Merge tag 's390-5.9-6' of git://git.kernel.org/pu..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13996ad5900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=ffe85b197a57c180
> > dashboard link: https://syzkaller.appspot.com/bug?extid=577fbac3145a6eb2e7a5
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+577fbac3145a6eb2e7a5@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KASAN: stack-out-of-bounds in xfrm_flowi_dport include/net/xfrm.h:877 [inline]
> > BUG: KASAN: stack-out-of-bounds in __xfrm6_selector_match net/xfrm/xfrm_policy.c:216 [inline]
> > BUG: KASAN: stack-out-of-bounds in xfrm_selector_match+0xf36/0xf60 net/xfrm/xfrm_policy.c:229
> > Read of size 2 at addr ffffc9001914f55c by task syz-executor.4/15633
> >
> > CPU: 0 PID: 15633 Comm: syz-executor.4 Not tainted 5.9.0-rc5-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x198/0x1fd lib/dump_stack.c:118
> >  print_address_description.constprop.0.cold+0x5/0x497 mm/kasan/report.c:383
> >  __kasan_report mm/kasan/report.c:513 [inline]
> >  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
> >  xfrm_flowi_dport include/net/xfrm.h:877 [inline]
>
> This one goes back more than ten years.  This patch should fix
> it.
>
> ---8<---
> The struct flowi must never be interpreted by itself as its size
> depends on the address family.  Therefore it must always be grouped
> with its original family value.
>
> In this particular instance, the original family value is lost in
> the function xfrm_state_find.  Therefore we get a bogus read when
> it's coupled with the wrong family which would occur with inter-
> family xfrm states.
>
> This patch fixes it by keeping the original family value.
>
> Note that the same bug could potentially occur in LSM through
> the xfrm_state_pol_flow_match hook.  I checked the current code
> there and it seems to be safe for now as only secid is used which
> is part of struct flowi_common.  But that API should be changed
> so that so that we don't get new bugs in the future.  We could
> do that by replacing fl with just secid or adding a family field.

I'm thinking it might be better to pass the family along with the flow
instead of passing just the secid (less worry of passing an incorrect
secid that way).  Let me see if I can cobble together a quick patch
for testing before bed ...

-- 
paul moore
www.paul-moore.com