Re: [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand

[Jiri Slaby <jirislaby@kernel.org>]

On 11. 07. 24, 1:16, Pawan Gupta wrote:
> On Wed, Jul 10, 2024 at 11:50:50PM +0200, Uros Bizjak wrote:
...
>>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
>>> index d3a814efbff6..d54f6002e5a0 100644
>>> --- a/arch/x86/entry/entry_32.S
>>> +++ b/arch/x86/entry/entry_32.S
>>> @@ -253,6 +253,16 @@
>>>    .Lend_\@:
>>>    .endm
>>> +/*
>>> + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand
>>> + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds.
>>> + */
>>> +.macro CLEAR_CPU_BUFFERS_SAFE
>>> +	ALTERNATIVE "jmp .Lskip_verw\@", "", X86_FEATURE_CLEAR_CPU_BUF
>>> +	verw	%ss:_ASM_RIP(mds_verw_sel)
>>> +.Lskip_verw\@:
>>> +.endm
>>
>> Why not simply:
>>
>> .macro CLEAR_CPU_BUFFERS_SAFE
>> 	ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)),
>> X86_FEATURE_CLEAR_CPU_BUF
>> .endm
> 
> We can do it this way as well. But, there are stable kernels that don't
> support relocations in ALTERNATIVEs. The way it is done in current patch
> can be backported without worrying about which kernels support relocations.

This sounds weird. There are code bases without ALTERNATIVE support at 
all. Will you expand ALTERNATIVE into some cmp & jmp here due to that? No.

Instead, you can send this "backport" to stable for older kernels later, 
once a proper patch is merged.

thanks,
-- 
js
suse labs