Date: Fri, 6 Jul 2018 14:46:10 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Ingo Molnar <mingo@...nel.org> Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Andy Lutomirski <luto@...nel.org>, Tycho Andersen <tycho@...ho.ws>, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Borislav Petkov <bp@...en8.de>, Richard Sandiford <richard.sandiford@....com>, Thomas Gleixner <tglx@...utronix.de>, "H . Peter Anvin" <hpa@...or.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, "Dmitry V . Levin" <ldv@...linux.org>, Emese Revfy <re.emese@...il.com>, Jonathan Corbet <corbet@....net>, Andrey Ryabinin <aryabinin@...tuozzo.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Thomas Garnier <thgarnie@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Alexei Starovoitov <ast@...nel.org>, Josef Bacik <jbacik@...com>, Masami Hiramatsu <mhiramat@...nel.org>, Nicholas Piggin <npiggin@...il.com>, Al Viro <viro@...iv.linux.org.uk>, "David S . Miller" <davem@...emloft.net>, Ding Tianhong <dingtianhong@...wei.com>, David Woodhouse <dwmw@...zon.co.uk>, Josh Poimboeuf <jpoimboe@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, Dominik Brodowski <linux@...inikbrodowski.net>, Juergen Gross <jgross@...e.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Dan Williams <dan.j.williams@...el.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Mathias Krause <minipli@...glemail.com>, Vikas Shivappa <vikas.shivappa@...ux.intel.com>, Kyle Huey <me@...ehuey.com>, Dmitry Safonov <dsafonov@...tuozzo.com>, Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>, Florian Weimer <fweimer@...hat.com>, Boris Lukashev <blukashev@...pervictus.com>, Andrey Konovalov <andreyknvl@...gle.com>, x86@...nel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v13 (resend) 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls On 06.07.2018 01:20, Ingo Molnar wrote: > > * Alexander Popov <alex.popov@...ux.com> wrote: > >> Hello Ingo, >> >> Thanks for your review! I'll fix the style issues you pointed at. >> >> Please also see my answers below. >> >> On 05.07.2018 11:12, Ingo Molnar wrote: >>>> + The tradeoff is the performance impact: on a single CPU system kernel >>>> + compilation sees a 1% slowdown, other systems and workloads may vary >>>> + and you are advised to test this feature on your expected workload >>>> + before deploying it. >>> >>> Is there a way to patch this out runtime? I.e. if a distro enabled it, is there an >>> easy way to disable much of the overhead without rebooting the kernel? >> >> Hm. We can't completely disable STACKLEAK in runtime, since STACKLEAK gcc plugin >> performs compile-time instrumentation of the kernel code. So we can only chop >> off a part of functionality, for example, by introducing some variable and >> checking it before every stack erasing (additional performance impact), but the >> kernel will stay uselessly instrumented. It doesn't look reasonable to me. > > Or we could use what every other performance critical instrumentation feature uses > to reduce overhead (ftrace, perf): kernel patching. I see. It would be a big separate research - how to combine those different kinds of instrumentation. I would propose to postpone it until we have a request for STACKLEAK runtime disabling. >>> If so then please make this: >>> >>> if (WARN_ON(boundary - kstack_ptr >= THREAD_SIZE)) >>> return; >>> >>> or so, to make it non-fatal and to allow users to report it, should it trigger >>> against all expectations. >> >> I've made an experiment. The results: >> 1. BUG_ON() here doesn't freeze the kernel output - I see a full 'PANIC: double >> fault' report; > > Only in text mode - very few users are using text mode. > >> 2. WARN_ON() here gives absolutely same 'PANIC: double fault' here. > > that should only happen if the kernel is otherwise already fatally corrupted, > right? No, I mean WARN_ON() in stackleak_erase_kstack() gives the double fault just like BUG_ON() (without any corruption). In my experiment I've made the following change: - BUG_ON(boundary - kstack_ptr >= THREAD_SIZE); +// BUG_ON(boundary - kstack_ptr >= THREAD_SIZE); + WARN_ON(1); It might be caused by the fact, that stackleak_erase_kstack() is called from the trampoline stack just before returning to the userspace. So I mean 'WARN_ON() + return' here wouldn't give any profit over a single BUG_ON() check. Best regards, Alexander
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