Date: Thu, 26 Jul 2018 13:18:34 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Kees Cook <keescook@...gle.com> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Ingo Molnar <mingo@...nel.org>, 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 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v14 7/7] stackleak, sysctl: Allow runtime disabling of kernel stack erasing On 25.07.2018 02:59, Kees Cook wrote: > On Tue, Jul 24, 2018 at 4:41 PM, Alexander Popov <alex.popov@...ux.com> wrote: >> On 25.07.2018 01:56, Kees Cook wrote: >>> On Thu, Jul 19, 2018 at 4:31 AM, Alexander Popov <alex.popov@...ux.com> wrote: >>>> @@ -78,6 +113,11 @@ void __used stackleak_track_stack(void) >>>> */ >>>> unsigned long sp = (unsigned long)&sp; >>>> >>>> +#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE >>>> + if (static_branch_unlikely(&stack_erasing_bypass)) >>>> + return; >>>> +#endif >>> >>> I would expect stackleak_erase() to be the expensive part, not the >>> tracking part? Shouldn't timings be unchanged by leaving this in >>> unconditionally, which would mean the sysctl could be re-enabled? >> >> Dropping the bypass in stackleak_track_stack() will not help against the >> troubles with re-enabling stack erasing (tracking and erasing depend on each > > Isn't the tracking checking "sp < current->lowest_stack", so if > erasure was off, lowest_stack would only ever get further into the > stack? And when erasure was turned back on, it would start getting > reset correctly again. Or is the concern the poison searching could > break? It seems like it would still work right? I must be missing > something. :) Umm.. You are right, that would be a solution. Let's assume that we: - allow stackleak_track_stack() to work, - skip stackleak_erase() giving most of performance penalty. When we enable the 'stack_erasing_bypass', the 'lowest_stack' is not reset at the end of syscall, it just continues to go down at next syscalls (because of enabled tracking). In some sense it is similar to having a very long syscall. Now if we re-enable erasing, the poison search in stackleak_erase() starts from the _valid_ 'lowest_stack', which should work fine. I'll send the improved version of the patch soon. Thanks! Best regards, Alexander
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