Re: use-after-free in __perf_install_in_context

From: Dmitry Vyukov <dvyukov@google.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: syzkaller <syzkaller@googlegroups.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Kostya Serebryany <kcc@google.com>,
	Alexander Potapenko <glider@google.com>,
	Eric Dumazet <edumazet@google.com>,
	Sasha Levin <sasha.levin@oracle.com>
Subject: Re: use-after-free in __perf_install_in_context
Date: Tue, 8 Dec 2015 19:35:20 +0100	[thread overview]
Message-ID: <CACT4Y+Y-wBH+xfei_q-6FcEoShM8sX4eX3bQyfEppWy522RiYw@mail.gmail.com> (raw)
In-Reply-To: <20151208180528.GA77166@ast-mbp.thefacebook.com>

On Tue, Dec 8, 2015 at 7:05 PM, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
> On Tue, Dec 08, 2015 at 06:56:23PM +0100, Dmitry Vyukov wrote:
>> On Tue, Dec 8, 2015 at 6:54 PM, Alexei Starovoitov
>> <alexei.starovoitov@gmail.com> wrote:
>> > On Tue, Dec 08, 2015 at 05:12:04PM +0100, Dmitry Vyukov wrote:
>> >> On Tue, Dec 8, 2015 at 4:24 AM, Alexei Starovoitov
>> >> <alexei.starovoitov@gmail.com> wrote:
>> >> > On Mon, Dec 07, 2015 at 05:09:21PM +0100, Dmitry Vyukov wrote:
>> >> >> > So it would be _awesome_ if we could somehow extend this callchain to
>> >> >> > include the site that calls call_rcu().
>> >> >>
>> >> >> We have a patch for KASAN in works that adds so-called stack depot
>> >> >> which allows to map a stack trace onto uint32 id. Then we can plumb
>> >> >
>> >> > I was hacking something similar to categorize stack traces with u32 id.
>> >> > How are you planning to limit the number of such stack traces ?
>> >> > and what is the interface for user space to get stack trace from an id?
>> >>
>> >>
>> >> We don't limit number of stack traces. Kernel does not seem to use
>> >> data-driven recursion extensively, so there is limited number of
>> >> stacks. Though, probably we will need to strip non-interrupt part for
>> >> interrupt stacks, otherwise that can produce unbounded number of
>> >> different stacks.
>> >> There is no interface for user-space, it is used only inside of kernel
>> >> to save stacks for memory blocks (rcu callbacks, thread pool items in
>> >> the future).
>> >> The design is based on what we successfully and extensively use in
>> >> user-space sanitizers for years. Current code is here:
>> >> https://github.com/ramosian-glider/kasan/commit/fb0eefd212366401ed5ad244233ef379a27bfb46
>> >
>> > why did you pick approach to never free accumulated stacks?
>> > That limits usability a lot, since once kasan starts using it only
>> > reboot will free the memory. ouch.
>> > what worked for user space doesn't work for kernel.
>>
>>
>> Freeing and reusing will slow down and complicate code significantly.
>> And it is not yet proved to be necessary.
>
> It's a joke, right? allocating kernel pages without ability to free?!

The plan is to smash kernel much earlier than it will run out of memory.

I think this scheme will work pretty well.
I've counted 34403 calls to
kmalloc/kfree/kmem_cache_alloc/kmem_cache_free in kernel. Multiply
this by 2 to account for different stacks leading to the same
malloc/free and assuming that we store 16-byte header and 20 4-byte
frames, this gives us about 6MB. I can live with that. I can live with
60MB as well.