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Date: Tue, 24 Jul 2018 15:56:57 -0700
From: Kees Cook <keescook@...gle.com>
To: Alexander Popov <alex.popov@...ux.com>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>, 
	Brad Spengler <spender@...ecurity.net>, Ingo Molnar <mingo@...nel.org>, 
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	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>, 
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	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 Thu, Jul 19, 2018 at 4:31 AM, Alexander Popov <alex.popov@...ux.com> wrote:
> Introduce CONFIG_STACKLEAK_RUNTIME_DISABLE option, which provides
> 'stack_erasing_bypass' sysctl. It can be used in runtime to disable
> kernel stack erasing for kernels built with CONFIG_GCC_PLUGIN_STACKLEAK.
> Stack erasing will then remain disabled and STACKLEAK_METRICS will not
> be updated until the next boot.
>
> Signed-off-by: Alexander Popov <alex.popov@...ux.com>
> [...]
> +That erasing reduces the information which kernel stack leak bugs
> +can reveal and blocks some uninitialized stack variable attacks.
> +The tradeoff is the performance impact: on a single CPU system kernel
> +compilation sees a 1% slowdown, other systems and workloads may vary.

I continue to have a hard time measuring even the 1% impact. Clearly I
need some better workloads. :)

> [...]
>  asmlinkage void stackleak_erase(void)
>  {
>         /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
> @@ -22,6 +52,11 @@ asmlinkage void stackleak_erase(void)
>         unsigned int poison_count = 0;
>         const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
>
> +#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
> +       if (static_branch_unlikely(&stack_erasing_bypass))
> +               return;
> +#endif

I collapsed this into a macro (and took your other fix) and will push
this to my -next tree:

+#define skip_erasing() static_branch_unlikely(&stack_erasing_bypass)
+#else
+#define skip_erasing() false
+#endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
...
+       if (skip_erasing())
+               return;
+

> +
>         /* Search for the poison value in the kernel stack */
>         while (kstack_ptr > boundary && poison_count <= depth) {
>                 if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
> @@ -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?

-Kees

-- 
Kees Cook
Pixel Security

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