linux-kernel - Re: [PATCH 1/5] x86/cpufeatures: Define feature bits to support mitigation of PSF

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Date:   Fri, 9 Apr 2021 21:39:29 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Saripalli, RK" <rsaripal@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/5] x86/cpufeatures: Define feature bits to support
 mitigation of PSF

On Fri, Apr 09, 2021 at 01:22:49PM -0500, Saripalli, RK wrote:
> > And I think you don't need this one either if we do a "light" controls
> > thing but lemme look at the rest first.

Ok, and what I mean with "lite" version is something like this below
which needs finishing and testing.

Initially, it could support the cmdline params:

predict_store_fwd={on,off,auto}

to give people the opportunity to experiment with the feature.

If it turns out that prctl and seccomp per-task toggling is needed then
sure, we can extend but I don't see the reason for a whole separate set
of options yet. Especially is ssbd already controls this.

AFAICT, of course and if I'm not missing some other aspect here.

Thx.

---
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 2d11384dc9ab..226b73700f88 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1165,3 +1165,22 @@ void set_dr_addr_mask(unsigned long mask, int dr)
 		break;
 	}
 }
+
+static int __init psf_cmdline(char *str)
+{
+	if (!boot_cpu_has(X86_FEATURE_PSFD))
+		return 0;
+
+	if (!str)
+		return -EINVAL;
+
+	if (!strcmp(str, "off")) {
+		x86_spec_ctrl_base |= SPEC_CTRL_PSFD;
+		setup_clear_cpu_cap(X86_FEATURE_PSFD);
+	}
+
+	return 0;
+}
+early_param("predict_store_fwd", psf_cmdline);
+
+

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette