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

[Pawan Gupta]

On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
> 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.

You were right, with KASLR enabled, instructions with fixed addressing
in alternatives don't get relocated. I guess we will have to keep
rip-relative as is. Thanks for catching that.