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

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Message-ID: <a947a270-dad6-5486-2e11-feb288aa9210@intel.com>
Date:   Tue, 1 Aug 2023 14:10:20 -0700
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Luck, Tony" <tony.luck@...el.com>
Subject: Re: [PATCH v3 03/12] x86/alternatives: Disable LASS when patching
 kernel alternatives

> 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?

Sohil