Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Jiri Slaby]

On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
> > Macro used for MDS mitigation executes VERW with relative addressing for
> > the operand. This is unnecessary and creates a problem for backports on
> > older kernels that don't support relocations in alternatives. Relocation
> > support was added by commit 270a69c4485d ("x86/alternative: Support
> > relocations in alternatives"). Also asm for fixed addressing is much
> > more cleaner than relative RIP addressing.
> >
> > Simplify the asm by using fixed addressing for VERW operand.
> >
> > Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> > Reported-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> > Closes: 
> > https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@xxxxxxxx/
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/include/asm/nospec-branch.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/nospec-branch.h 
> > b/arch/x86/include/asm/nospec-branch.h
> > index 2aa52cab1e46..ab19c7f1167b 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -323,7 +323,7 @@
> >    * Note: Only the memory operand variant of VERW clears the CPU 
> > buffers.
> >    */
> >   .macro CLEAR_CPU_BUFFERS
> > -    ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), 
> > X86_FEATURE_CLEAR_CPU_BUF
> > +    ALTERNATIVE "", __stringify(verw mds_verw_sel), 
> > X86_FEATURE_CLEAR_CPU_BUF
>
> Actually thinking about it more and discussing with Jiri (cc'ed), will 
> this work with KASLR enabled?

I might of course be wrong. We appear to rely on the asm+linker here. 
Please see also:
https://lore.kernel.org/r/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@xxxxxxxxxx

thanks,
--
js
suse labs