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Date: Wed, 25 Jul 2018 02:41:04 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Kees Cook <keescook@...gle.com>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
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 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 01:56, Kees Cook wrote:
> 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;
> +

That's nice! Thank you, I'll test it tomorrow.

>> +
>>         /* 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?

Dropping the bypass in stackleak_track_stack() will not help against the
troubles with re-enabling stack erasing (tracking and erasing depend on each
other). Moreover, it will also make the STACKLEAK_METRICS show insane values. So
I think we should have the bypass in both functions.

Best regards,
Alexander

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