linux-kernel - [PATCH v13 12/16] x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit

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Date:   Thu, 28 May 2020 16:13:58 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     tglx@...utronix.de, luto@...nel.org, ak@...ux.intel.com
Cc:     corbet@....net, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        shuah@...nel.org, gregkh@...uxfoundation.org, tony.luck@...el.com,
        chang.seok.bae@...el.com, dave.hansen@...ux.intel.com,
        peterz@...radead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, jarkko.sakkinen@...ux.intel.com,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH v13 12/16] x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit

From: Andy Lutomirski <luto@...nel.org>

Now that FSGSBASE is fully supported, remove unsafe_fsgsbase, enable
FSGSBASE by default, and add nofsgsbase to disable it.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Andi Kleen <ak@...ux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@...el.com>
Cc: H. Peter Anvin <hpa@...or.com>
Link: https://lkml.kernel.org/r/1557309753-24073-17-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 .../admin-guide/kernel-parameters.txt         |  3 +-
 arch/x86/kernel/cpu/common.c                  | 32 ++++++++-----------
 2 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index af3aaade195b..1924845c879c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3033,8 +3033,7 @@
 	no5lvl		[X86-64] Disable 5-level paging mode. Forces
 			kernel to use 4-level paging instead.
 
-	unsafe_fsgsbase	[X86] Allow FSGSBASE instructions.  This will be
-			replaced with a nofsgsbase flag.
+	nofsgsbase	[X86] Disables FSGSBASE instructions.
 
 	no_console_suspend
 			[HW] Never suspend the console
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4224760c74e2..0d480cbadc7d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -418,21 +418,21 @@ static void __init setup_cr_pinning(void)
 	static_key_enable(&cr_pinning.key);
 }
 
-/*
- * Temporary hack: FSGSBASE is unsafe until a few kernel code paths are
- * updated. This allows us to get the kernel ready incrementally.
- *
- * Once all the pieces are in place, these will go away and be replaced with
- * a nofsgsbase chicken flag.
- */
-static bool unsafe_fsgsbase;
-
-static __init int setup_unsafe_fsgsbase(char *arg)
+static __init int x86_nofsgsbase_setup(char *arg)
 {
-	unsafe_fsgsbase = true;
+	/* Require an exact match without trailing characters. */
+	if (strlen(arg))
+		return 0;
+
+	/* Do not emit a message if the feature is not present. */
+	if (!boot_cpu_has(X86_FEATURE_FSGSBASE))
+		return 1;
+
+	setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
+	pr_info("FSGSBASE disabled via kernel command line\n");
 	return 1;
 }
-__setup("unsafe_fsgsbase", setup_unsafe_fsgsbase);
+__setup("nofsgsbase", x86_nofsgsbase_setup);
 
 /*
  * Protection Keys are not available in 32-bit mode.
@@ -1495,12 +1495,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_umip(c);
 
 	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
-		if (unsafe_fsgsbase)
-			cr4_set_bits(X86_CR4_FSGSBASE);
-		else
-			clear_cpu_cap(c, X86_FEATURE_FSGSBASE);
-	}
+	if (cpu_has(c, X86_FEATURE_FSGSBASE))
+		cr4_set_bits(X86_CR4_FSGSBASE);
 
 	/*
 	 * The vendor-specific functions might have changed features.
-- 
2.25.1