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From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Elena Reshetova <elena.reshetova@intel.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Potapenko <glider@google.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-arm-kernel@lists.infradead.org,
	Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall
Date: Tue, 24 Mar 2020 16:07:53 -0700
Message-ID: <202003241604.7269C810B@keescook> (raw)
In-Reply-To: <CAG48ez3yYkMdxEEW6sJzBC5BZSbzEZKnpWzco32p-TJx7y_srg@mail.gmail.com>

[-enrico, who is bouncing]

On Tue, Mar 24, 2020 at 10:28:35PM +0100, Jann Horn wrote:
> On Tue, Mar 24, 2020 at 9:32 PM Kees Cook <keescook@chromium.org> wrote:
> > This is a continuation and refactoring of Elena's earlier effort to add
> > kernel stack base offset randomization. In the time since the previous
> > discussions, two attacks[1][2] were made public that depended on stack
> > determinism, so we're no longer in the position of "this is a good idea
> > but we have no examples of attacks". :)
> [...]
> > [1] https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
> 
> This one only starts using the stack's location after having parsed
> it out of dmesg (which in any environment that wants to provide a
> reasonable level of security really ought to be restricted to root),
> right? If you give people read access to dmesg, they can leak all
> sorts of pointers; not just the stack pointer, but also whatever else
> happens to be in the registers at that point - which is likely to give
> the attacker more ways to place controlled data at a known location.
> See e.g. <https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html>,
> which leaks the pointer to a BPF map out of dmesg.

It was mentioned that it would re-use the base across syscalls, so this
defense would have frustrated it.

More to my point was that there still are attacks using a deterministic
stack as part of the exploit chain. We have a low-cost way to make that
go away.

> Also, are you sure that it isn't possible to make the syscall that
> leaked its stack pointer never return to userspace (via ptrace or
> SIGSTOP or something like that), and therefore never realign its
> stack, while keeping some controlled data present on the syscall's
> stack?
> 
> > [2] https://repositorio-aberto.up.pt/bitstream/10216/125357/2/374717.pdf
> 
> That's a moderately large document; which specific part are you referencing?

IIRC, section 3.3 discusses using the stack for CFI bypass, though
thinking about it again, it may have been targeting pt_regs. I'll
double check and remove this reference if that's the case.

But, as I mention, this is proactive and I'd like to stop yet more
things from being able to depend on the stack location.

-- 
Kees Cook

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-24 20:32 Kees Cook
2020-03-24 20:32 ` [PATCH v2 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-03-24 22:06   ` Peter Zijlstra
2020-03-24 20:32 ` [PATCH v2 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-03-26 15:48   ` Alexander Potapenko
2020-03-24 20:32 ` [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-30 11:25   ` Mark Rutland
2020-03-30 18:18     ` Kees Cook
2020-03-30 18:27     ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-03-28 22:26   ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 5/5] arm64: entry: " Kees Cook
2020-03-25 13:21   ` Mark Rutland
2020-03-25 20:22     ` Kees Cook
2020-03-26 11:15       ` Mark Rutland
2020-03-26 16:31         ` Kees Cook
2020-03-30 11:26           ` Mark Rutland
2020-04-20 20:54   ` Will Deacon
2020-04-20 22:34     ` Kees Cook
2020-04-21  7:02       ` Will Deacon
2020-03-24 21:28 ` [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Jann Horn
2020-03-24 23:07   ` Kees Cook [this message]
2020-03-25 12:15     ` Reshetova, Elena
2020-03-25 20:27       ` Kees Cook
2020-03-25 23:20         ` Jann Horn
2020-03-26 17:18           ` Kees Cook

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