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Date: Thu, 5 Apr 2018 12:20:24 -0700
From: Kees Cook <>
To: Alexander Popov <>
Cc: Kurt Seifried <>,, 
	James Morris <>, "Serge E. Hallyn" <>, 
	Brad Spengler <>, PaX Team <>, 
	"Reshetova, Elena" <>
Subject: Re: Linux Kernel Defence Map

On Thu, Apr 5, 2018 at 5:32 AM, Alexander Popov <> wrote:
> On 05.04.2018 01:17, Kees Cook wrote:
>> (I think "info leaks" and "finding kernel objects" may need some kind
>> of clarifying language for how they're different)
> Info Exposure is a vulnerability (red node). STACKLEAK, PAGE_POISONING, etc
> mitigate this kind of bugs.
> Finding Kernel Objects is an exploitation technique (orange node). KASLR,
> RANDSTRUCT are statistical defences which make it harder for an adversary.
> Kees, Kurt, does it sound reasonable?

Yeah, that makes sense.

>> Upstream's /proc/sys/net/core/bpf_jit_harden (see commit 4f3446bb809f)
> Thanks, added.
>> and other JIT features (RO-setting, randomized offset, etc) are
>> designed to defend against JIT Abuse.
> Didn't manage to find config for them. Are they always enabled?

Yes. Per-arch inplementations of bpf_int_jit_compile() make calls to
bpf_jit_binary_alloc() which does the randomized page offset with trap
instructions, and calls bpf_jit_binary_lock_ro() to make the memory
read-only at the end.

>> UDEREF and SMAP pointing at ret2usr+ROP is fine, but seems
>> "incomplete". Is there a good name for "reading user memory and
>> operating on a malicious structure"? It's a more narrow exploit
>> technique than ROP or executing userspace memory, but it's important
>> to cover.
> Yes, agree. That's what I did exploiting CVE-2017-2636: allocating struct
> skb_shared_info in the userspace memory with the destructor callback pointing to
> native_write_cr4() to disable SMEP. Is it what you mean?

Yup. Function pointers are the traditional target.

> I've added "ret2usr + type confusion". Do you like it?
> Kurt, that is CWE-843: Access of Resource Using Incompatible Type ('Type
> Confusion').

"type confusion" seems weird to me, but I haven't spent a lot of time
weighing the options of the naming of these things. "Overwriting a
function pointer" is the method, and the bug is "unexpectedly
accessing userspace memory from the kernel" (which is usually
"something overwrite a pointer").

> Kees, thanks again for such a cool feedback. The map is updated.

Very cool! Maybe also add an out-of-tree bubble for "Clang CFI", which
gives forward-edge protection for code-reuse...


Kees Cook
Pixel Security

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