Date: Tue, 20 Feb 2018 16:31:42 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: Kees Cook <keescook@...omium.org> Cc: Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>, Ingo Molnar <mingo@...nel.org>, Laura Abbott <labbott@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Al Viro <viro@...iv.linux.org.uk>, Sahara <keun-o.park@...kmatter.ae>, "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>, Michal Hocko <mhocko@...e.com>, Andrea Arcangeli <aarcange@...hat.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH] fork: Allow stack to be wiped on fork On Tue, 16 Jan 2018 21:50:15 -0800 Kees Cook <keescook@...omium.org> wrote: > One of the classes of kernel stack content leaks is exposing the contents > of prior heap or stack contents when a new process stack is allocated. > Normally, those stacks are not zeroed, and the old contents remain in > place. With some types of stack content exposure flaws, those contents > can leak to userspace. Kernels built with CONFIG_CLEAR_STACK_FORK will > no longer be vulnerable to this, as the stack will be wiped each time > a stack is assigned to a new process. There's not a meaningful change > in runtime performance; it almost looks like it provides a benefit. > > Performing back-to-back kernel builds before: > Run times: 157.86 157.09 158.90 160.94 160.80 > Mean: 159.12 > Std Dev: 1.54 > > With CONFIG_CLEAR_STACK_FORK=y: > Run times: 159.31 157.34 156.71 158.15 160.81 > Mean: 158.46 > Std Dev: 1.46 > > ... > > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -904,6 +904,14 @@ config VMAP_STACK > the stack to map directly to the KASAN shadow map using a formula > that is incorrect if the stack is in vmalloc space. > > +config CLEAR_STACK_FORK > + bool "Clear the kernel stack at each fork" > + help > + To resist stack content leak flaws, this clears newly allocated > + kernel stacks to keep previously freed heap or stack contents > + from being present in the new stack. This has almost no > + measurable performance impact. > + It would be much nicer to be able to control this at runtime rather than compile-time. Why not a /proc tunable? We could always use more of those ;)
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