Re: [PATCH 11/16] x86/spec-ctrl: Adjust DO_OVERWRITE_RSB to be shadow stack compatible

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.05.2020 15:25, Andrew Cooper wrote:
> On 07/05/2020 14:22, Jan Beulich wrote:
>> On 02.05.2020 00:58, Andrew Cooper wrote:
>>> @@ -114,6 +114,16 @@
>>>      sub $1, %ecx
>>>      jnz .L\@_fill_rsb_loop
>>>      mov %\tmp, %rsp                 /* Restore old %rsp */
>>> +
>>> +#ifdef CONFIG_XEN_SHSTK
>>> +    mov $1, %ecx
>>> +    rdsspd %ecx
>>> +    cmp $1, %ecx
>>> +    je .L\@_shstk_done
>>> +    mov $64, %ecx                   /* 64 * 4 bytes, given incsspd */
>>> +    incsspd %ecx                    /* Restore old SSP */
>>> +.L\@_shstk_done:
>>> +#endif
>> The latest here I wonder why you don't use alternatives patching.
>> I thought that's what you've introduced the synthetic feature
>> flag for.
> 
> We're already in the middle of an alternative and they don't nest.  More
> importantly, this path gets used on the BSP, after patching and before
> CET gets enabled.

Oh, I should have noticed this. The first point could be dealt with,
but I agree the second pretty much rules out patching.

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan