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Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
To:     Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Elena Reshetova <elena.reshetova@intel.com>, 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>,
        Jann Horn <jannh@google.com>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200622193146.2985288-1-keescook@chromium.org>
 <20200622193146.2985288-4-keescook@chromium.org>
From:   Alexander Popov <alex.popov@linux.com>
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Message-ID: <f67f3471-699e-06ce-226d-77a2b6ec7ce4@linux.com>
Date:   Tue, 23 Jun 2020 15:38:32 +0300
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Sender: linux-kernel-owner@vger.kernel.org
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On 22.06.2020 22:31, Kees Cook wrote:
> As Linux kernel stack protections have been constantly improving
> (vmap-based stack allocation with guard pages, removal of thread_info,
> STACKLEAK), attackers have had to find new ways for their exploits
> to work. They have done so, continuing to rely on the kernel's stack
> determinism, in situations where VMAP_STACK and THREAD_INFO_IN_TASK_STRUCT
> were not relevant. For example, the following recent attacks would have
> been hampered if the stack offset was non-deterministic between syscalls:
> 
> https://repositorio-aberto.up.pt/bitstream/10216/125357/2/374717.pdf
> (page 70: targeting the pt_regs copy with linear stack overflow)
> 
> https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
> (leaked stack address from one syscall as a target during next syscall)
> 
> The main idea is that since the stack offset is randomized on each system
> call, it is harder for an attack to reliably land in any particular place
> on the thread stack, even with address exposures, as the stack base will
> change on the next syscall. Also, since randomization is performed after
> placing pt_regs, the ptrace-based approach[1] to discover the randomized
> offset during a long-running syscall should not be possible.

Hello Kees!

I would recommend to disable CONFIG_STACKLEAK_METRICS if kernel stack offset
randomization is enabled. It is a debugging feature that provides information
about kernel stack usage. That info can be useful for calculating the random offset.

I would also recommend to check: there might be other kernel features for
debugging or getting statistics that can be used to disclose the random stack
offset.

Best regards,
Alexander