Re: Avoid speculative indirect calls in kernel

[Tom Lendacky]

On 1/4/2018 5:47 PM, Tom Lendacky wrote:
> On 1/4/2018 2:05 PM, David Woodhouse wrote:
>> On Thu, 2018-01-04 at 14:00 -0600, Tom Lendacky wrote:
>>> Yes, lfence is sufficient. As long as the target is in the register
>>> before the lfence and we jump through the register all is good, i.e.:
>>
>> Thanks. Can I have a Reviewed-by: for this then please:
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> While this works, a more efficient way to do the lfence support would be
> to not use the retpoline in this case.  Changing the indirect jumps to
> do the "mov [rax], rax; lfence; jmp *rax" sequence would be quicker. I'm
> not sure if this is feasible given the need to do a retpoline if you can't
> use lfence, though.
> 
> Thanks,
> Tom
> 

I do need to send the patches that make lfence a serializing instruction
for AMD.  I'll get those out as soon as I can.

Thanks,
Tom

>>
>> http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/08d9eda03
>>
>> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> Date: Thu, 4 Jan 2018 20:01:53 +0000
>> Subject: [PATCH] x86/retpoline: Simplify AMD variant of retpoline thunk
>>
>> On AMD (which is X86_FEATURE_K8), just the lfence is sufficient.
>>
>> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> ---
>> Âarch/x86/lib/retpoline.S | 2 +-
>> Â1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
>> index bbdda5cc136e..26070976bff0 100644
>> --- a/arch/x86/lib/retpoline.S
>> +++ b/arch/x86/lib/retpoline.S
>> @@ -11,7 +11,7 @@
>> Â
>> ÂENTRY(__x86.indirect_thunk.\reg)
>> Â	CFI_STARTPROC
>> -	ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
>> +	ALTERNATIVE_2 "call 2f", __stringify(lfence;jmp *%\reg), X86_FEATURE_K8, __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
>> Â1:
>> Â	lfence
>> Â	jmp	1b
>> --Â
>> 2.14.3
>>