linux-kernel - Re: [PATCH v3 03/12] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <1432878a512dbb361c0fb9f303fda33a7cf68c9b.camel@intel.com>
Date:   Tue, 1 Aug 2023 21:50:14 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Lutomirski, Andy" <luto@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>
Subject: Re: [PATCH v3 03/12] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Tue, 2023-08-01 at 14:10 -0700, Sohil Mehta wrote:
> > Why not do stac/clac in a single place inside __text_poke()?
> 
> It would mostly look something like this:
> > diff --git a/arch/x86/kernel/alternative.c
> > b/arch/x86/kernel/alternative.c
> > index 0fbf8a631306..02ef08e2575d 100644
> > --- a/arch/x86/kernel/alternative.c
> > +++ b/arch/x86/kernel/alternative.c
> > @@ -1781,7 +1781,9 @@ static void *__text_poke(text_poke_f func,
> > void *addr, const void *src, size_t l
> >          prev = use_temporary_mm(poking_mm);
> > 
> >          kasan_disable_current();
> > +       stac();
> >          func((u8 *)poking_addr + offset_in_page(addr), src, len);
> > +       clac();
> >          kasan_enable_current();
> > 
> >          /*
> 
> Since, __text_poke() uses a dynamic function to call into
> text_poke_memcpy() and text_poke_memset(), objtool would still
> complain.
> 
> > arch/x86/kernel/alternative.o: warning: objtool: __text_poke+0x259:
> > call to {dynamic}() with UACCESS enabled
> 
> We could change __text_poke() to not use the dynamic func but it
> might
> be a bit heavy handed to save a couple of lines of stac/clac calls.
> The
> current trade-off seems reasonable to me.
> 
> Did you have something different in mind?

I wondered if it might be something like that. Yes, seems like an ok
tradeoff.