linux-kernel - Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL

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Message-ID: <sb7p6quwxkn4w4etgsxlqd6fcsia4xobf73d3fnybxafxrmvwi@ajg5lkdxtnfy>
Date: Tue, 7 Oct 2025 17:14:29 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>, "Kaplan, David" <David.Kaplan@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"x86@...nel.org" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, 
	Klaus Kusche <klaus.kusche@...puterix.info>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL_MSG

On Tue, Oct 07, 2025 at 04:08:21PM -0700, Pawan Gupta wrote:
> On Wed, Oct 08, 2025 at 12:12:29AM +0200, Borislav Petkov wrote:
> > On Tue, Oct 07, 2025 at 11:22:57AM -0700, Pawan Gupta wrote:
> > > IMO, rather than targeting the mitigation enabling code it might make more
> > > sense to compile out the actual mitigations scattered accross the kernel.
> > > This may also improve performance by reducing the code footprint, and can
> > > also help getting a cleaner disassembly.

Isn't that what CONFIG_CPU_MITIGATIONS=n already does today?

> > Probably... however, it needs to be done smartly because sprinkling ifdeffery
> > and turning what is an already unreadable mess into a bigger abomination,
> > won't fly. Perhaps split out the mitigations glue into separate compilation
> > units and build-disable them... we'll see.
> 
> Ya, that would be better.

On a sort of related note, it's confusing that there are two completely
different classes of MITIGATION options which get conflated:

1) compile the kernel with support for certain mitigations:

    MITIGATION_RETPOLINE
    MITIGATION_RETHUNK
    MITIGATION_PAGE_TABLE_ISOLATION
    MITIGATION_UNRET_ENTRY
    MITIGATION_CALL_DEPTH_TRACKING
    MITIGATION_IBPB_ENTRY
    MITIGATION_IBRS_ENTRY
    MITIGATION_SRSO
    MITIGATION_SLS

2) enable bug-specific runtime defaults:

    MITIGATION_GDS
    MITIGATION_RFDS
    MITIGATION_SPECTRE_BHI
    MITIGATION_MDS
    MITIGATION_TAA
    MITIGATION_MMIO_STALE_DATA
    MITIGATION_L1TF
    MITIGATION_RETBLEED
    MITIGATION_SPECTRE_V1
    MITIGATION_SPECTRE_V2
    MITIGATION_SRBDS
    MITIGATION_SSB
    MITIGATION_ITS
    MITIGATION_TSA
    MITIGATION_VMSCAPE

In general, #1 uglify the kernel.  And some #2 depend on #1.

IMO, we should rename the #1 options.  For example:

  MITIGATION_RETPOLINE -> X86_UGLY_INDIRECT_THUNKS
  MITIGATION_RETHUNK   -> X86_UGLY_RETURN_THUNKS

  etc...

Then one only needs to grep their .config file for UGLY to understand
why their disassembly is so inscrutable ;-)

-- 
Josh