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

[Jiri Slaby]

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