linux-kernel - Re: [PATCH v3 20/35] x86/bugs: Define attack vectors

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Message-ID: <20250227034813.booxbhxnff66dnqx@desk>
Date: Wed, 26 Feb 2025 19:50:38 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: "Kaplan, David" <David.Kaplan@....com>, Borislav Petkov <bp@...en8.de>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 20/35] x86/bugs: Define attack vectors

On Wed, Feb 26, 2025 at 05:23:29PM -0800, Josh Poimboeuf wrote:
> On Wed, Feb 26, 2025 at 04:35:28PM -0800, Pawan Gupta wrote:
> > On Wed, Feb 26, 2025 at 03:44:40PM -0800, Josh Poimboeuf wrote:
> > > On Wed, Feb 26, 2025 at 02:13:24PM -0800, Pawan Gupta wrote:
> > > > On Wed, Feb 26, 2025 at 09:03:58PM +0000, Kaplan, David wrote:
> > > > > > Extending =auto to take attack vectors is going to be tricky, because it already
> > > > > > takes ",nosmt" which would conflict with ",no_cross_thread".
> > > > > >
> > > > > > How about we keep =off, and =auto as is, and add:
> > > > > >
> > > > > >   mitigations=selective,no_user_kernel,no_cross_thread,...
> > > > > >
> > > > > > Requiring the user to explicitly select attack vectors to disable (rather than to
> > > > > > enable). This would be more verbose, but it would be clear that the user is explicitly
> > > > > > selecting attack vectors to disable. Also, if a new attack vector gets added in future,
> > > > > > it would be mitigated by default, without requiring the world to change their cmdline.
> > > > > 
> > > > > I kind of like that.
> > > 
> > > While it might be true that we don't necessarily need both opt-in and
> > > opt-out options...
> > > 
> > > I'm missing the point of the "selective" thing vs just
> > > "auto,no_whatever"?
> > 
> > That was my first thought, but then I realized that in "auto,nosmt" nosmt
> > is the opposite of disabling the mitigation. It would be cleaner to have
> > "=selective,no_whatever" which is self-explanatory.
> 
> The "auto,nosmt,no_whatever" is indeed a bit confusing because of the
> opposite meanings of the word "no", but least it sort of makes some
> kind of sense if you consider the existing "auto,nosmt" option to be the
> starting point.
> 
> And we could document it from that perspective: start with "auto" or
> "auto,smt" and then optionally append the ",no_*" options for the vectors
> you want to disable.
>
> IMO "selective" doesn't seem very self-explanatory, it says nothing to
> indicate "opting out of defaults", in fact it sounds to me more like
> opting in.  At least with "auto,no_whatever" it's more clear that it
> starts with the defaults and subtracts from there.

Thats fair. I certainly don't want to be adding new option if we are
willing to live with some minor quirks with auto,nosmt.

Like, should the order in which nosmt appears after =auto matter? IOW,
"=auto,no_foo,nosmt" would be equivalent to "=auto,nosmt,no_foo"? I believe
they should be treated the same.

Arches that don't support attack vectors, but support smt, should treat
"=auto,no_foo,nosmt" as "=auto,nosmt".

So as to treat nosmt as any other attack vector, CPU_MITIGATIONS_AUTO_NOSMT
should go away. I am thinking we can modify cpu_mitigations_auto_nosmt() to
check for smt attack vector:

---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index b605334f8ee6..6ddbee6a0b6b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3193,22 +3193,27 @@ void __init boot_cpu_hotplug_init(void)
 enum cpu_mitigations {
 	CPU_MITIGATIONS_OFF,
 	CPU_MITIGATIONS_AUTO,
-	CPU_MITIGATIONS_AUTO_NOSMT,
 };
 
+#define MITIGATE_SMT            BIT(0)
+#define MITIGATE_USER_KERNEL    BIT(1)
+#define MITIGATE_USER_USER      BIT(2)
+#define MITIGATE_GUEST_HOST     BIT(3)
+
 static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
+static unsigned int cpu_attack_vectors __ro_after_init = ~0;
 
 static int __init mitigations_parse_cmdline(char *arg)
 {
-	if (!strcmp(arg, "off"))
+	if (!strcmp(arg, "off")) {
 		cpu_mitigations = CPU_MITIGATIONS_OFF;
-	else if (!strcmp(arg, "auto"))
+	} else if (strstr(arg, "auto")) {
 		cpu_mitigations = CPU_MITIGATIONS_AUTO;
-	else if (!strcmp(arg, "auto,nosmt"))
-		cpu_mitigations = CPU_MITIGATIONS_AUTO_NOSMT;
-	else
+		cpu_attack_vectors = parse_cpu_attack_vectors(arg);
+	} else {
 		pr_crit("Unsupported mitigations=%s, system may still be vulnerable\n",
 			arg);
+	}
 
 	return 0;
 }
@@ -3223,7 +3228,7 @@ EXPORT_SYMBOL_GPL(cpu_mitigations_off);
 /* mitigations=auto,nosmt */
 bool cpu_mitigations_auto_nosmt(void)
 {
-	return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
+	return (cpu_mitigations == CPU_MITIGATIONS_AUTO) && (cpu_attack_vectors & MITIGATE_SMT);
 }
 EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt);
 #else