Re: [PATCH] x86/alternatives: fix .init section reference in _apply_alternatives()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 09, 2024 at 01:56:16PM +0100, Andrew Cooper wrote:
> On 09/04/2024 1:50 pm, Roger Pau Monne wrote:
> > The code in _apply_alternatives() will unconditionally attempt to read
> > __initdata_cf_clobber_{start,end} when called as part of applying 
> > alternatives
> > to a livepatch payload.
> 
> When CET-IBT is active.
> 
> In practice this narrows the impact to ADL/RPL and SPR/EMR.

Oh, yes, forgot to add that.

> 
> > That leads to a page-fault as
> > __initdata_cf_clobber_{start,end} living in .init section will have been
> > unmapped by the time a livepatch gets loaded.
> >
> > Fix by adding a check that limits the clobbering of endbr64 instructions to
> > boot time only.
> >
> > Fixes: 37ed5da851b8 ('x86/altcall: Optimise away endbr64 instruction where 
> > possible')
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> In hindsight I really should have tried livepatching when I was doing
> the IBT work, but that was also done under an embargo deadline.
> 
> I'm happy to tweak the commit message on commit.

Sure, please go ahead.

Thanks, Roger.