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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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Peter Zijlstra <a.p.zijlstra@...llo.nl>, "Dmitry V . Levin" <ldv@...linux.org>,
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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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