linux-kernel - Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall

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Message-ID: <202003251322.180F2536E@keescook>
Date:   Wed, 25 Mar 2020 13:27:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Jann Horn <jannh@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each
 syscall

On Wed, Mar 25, 2020 at 12:15:12PM +0000, Reshetova, Elena wrote:
> > > 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?
> 
> How would you reliably detect that a stack pointer has been leaked
> to userspace while it has been in a syscall? Does not seem to be a trivial
> task to me. 

Well, my expectation is that folks using this defense are also using
panic_on_warn sysctl, etc, so attackers don't get a chance to actually
_use_ register values spilled to dmesg.

-- 
Kees Cook