Re: [PATCH v10 37/38] compiler-gcc.h: Introduce __nostackp function attribute

[Tom Lendacky <thomas.lendacky@amd.com>]

On 7/18/2017 4:36 AM, Ingo Molnar wrote:
> 
> * Tom Lendacky <thomas.lendacky@amd.com> wrote:
> 
>> Create a new function attribute, __nostackp, that can used to turn off
>> stack protection on a per function basis.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>   include/linux/compiler-gcc.h | 2 ++
>>   include/linux/compiler.h     | 4 ++++
>>   2 files changed, 6 insertions(+)
>>
>> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>> index cd4bbe8..682063b 100644
>> --- a/include/linux/compiler-gcc.h
>> +++ b/include/linux/compiler-gcc.h
>> @@ -166,6 +166,8 @@
>>   
>>   #if GCC_VERSION >= 40100
>>   # define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
>> +
>> +#define __nostackp	__attribute__((__optimize__("no-stack-protector")))
>>   #endif
>>   
>>   #if GCC_VERSION >= 40300
>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> index 219f82f..63cbca1 100644
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -470,6 +470,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>>   #define __visible
>>   #endif
>>   
>> +#ifndef __nostackp
>> +#define __nostackp
>> +#endif
> 
> So I changed this from the hard to read and ambiguous "__nostackp" abbreviation
> (does it mean 'no stack pointer?') to "__nostackprotector", plus added this detail
> to the changelog:
> 
> | ( This is needed by the SME in-place kernel memory encryption feature,
> |   which activates encryption in its sme_enable() function and thus changes the
> |   visible value of the stack protection cookie on function return. )
> 
> Agreed?

Hi Ingo,

I debugged this to needing "__nostackprotector" because sme_enable()
is called very early in the boot process before everything is properly
setup to fully support stack protection when KASLR is enabled. Without
this attribute the call to sme_enable() would fail even if encryption
was disabled with the "mem_encrypt=off" command line option.

If KASLR wasn't enabled, then everything worked fine without the
"__nostackprotector" attribute, encryption enabled or not.

The stack protection support is activated because of the 16-byte
character buffer in the sme_enable() routine.  I think we'll find that
if a character buffer greater than 8 bytes is added to, for example,
__startup_64, then this attribute will need to be added to that routine.

Thanks,
Tom

> 
> Thanks,
> 
> 	Ingo
> 

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