Re: general protection fault in watchdog

[Michal Hocko]

On Fri 14-12-18 14:42:33, Dmitry Vyukov wrote:
> On Fri, Dec 14, 2018 at 2:28 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > On Fri 14-12-18 14:11:05, Dmitry Vyukov wrote:
> > > On Fri, Dec 14, 2018 at 1:51 PM syzbot
> > > <syzbot+7713f3aa67be76b1552c@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > Hello,
> > > >
> > > > syzbot found the following crash on:
> > > >
> > > > HEAD commit:    f5d582777bcb Merge branch 'for-linus' of git://git.kernel...
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=16aca143400000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=c8970c89a0efbb23
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=7713f3aa67be76b1552c
> > > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1131381b400000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13bae593400000
> > > >
> > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > Reported-by: syzbot+7713f3aa67be76b1552c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > +linux-mm for memcg question
> > >
> > > What the repro does is effectively just
> > > setsockopt(EBT_SO_SET_ENTRIES). This eats all machine memory and
> > > causes OOMs. Somehow it also caused the GPF in watchdog when it
> > > iterates over task list, perhaps some scheduler code leaves a dangling
> > > pointer on OOM failures.
> > >
> > > But what bothers me is a different thing. syzkaller test processes are
> > > sandboxed with a restrictive memcg which should prevent them from
> > > eating all memory. do_replace_finish calls vmalloc, which uses
> > > GFP_KERNEL, which does not include GFP_ACCOUNT (GFP_KERNEL_ACCOUNT
> > > does). And page alloc seems to change memory against memcg iff
> > > GFP_ACCOUNT is provided.
> > > Am I missing something or vmalloc is indeed not accounted (DoS)? I see
> > > some explicit uses of GFP_KERNEL_ACCOUNT, e.g. the one below, but they
> > > seem to be very sparse.
> > >
> > > static void *seq_buf_alloc(unsigned long size)
> > > {
> > >      return kvmalloc(size, GFP_KERNEL_ACCOUNT);
> > > }
> > >
> > > Now looking at the code I also don't see how kmalloc(GFP_KERNEL) is
> > > accounted... Which makes me think I am still missing something.
> >
> > You are not missing anything. We do not account all allocations and you
> > have to explicitly opt-in by __GFP_ACCOUNT. This is a deliberate
> > decision. If the allocation is directly controlable by an untrusted user
> > and the memory is associated with a process life time then this looks
> > like a good usecase for __GFP_ACCOUNT. If an allocation outlives a
> > process then there the flag should be considered with a great care
> > because oom killer is not able to resolve the memcg pressure and so the
> > limit enforcement is not effective.
> 
> Interesting.
> I understand that namespaces, memcg's and processes (maybe even
> threads) can have arbitrary overlapping. But I naively thought that in
> canonical hierarchical cases it should all somehow work.
> Question 1: is there some other, stricter sandboxing mechanism?

I do not think so

> We try
> to sandbox syzkaller processes with everything available , because
> these OOMs usually leads either to dead machines or hang/stall false
> positives, which are nasty.

Which is a useful test on its own. If you are able to trigger the global
OOM from a restricted environment then you have a good candidate to
consider a new __GFP_ACCOUNT user.

> Question 2: this is a simple DoS vector, right? If I put a container
> into a 1MB memcg, it can still eat arbitrary amount of non-pagable
> kernel memory?

As I've said. If there is a direct vector to allocated an unbounded
amount of memory from the userspace (trusted users aside) then yes this
sounds like a DoS to me.

-- 
Michal Hocko
SUSE Labs