[tip: x86/fsgsbase] x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID:     b745cfba44c152c34363eea9e052367b6b1d652b
Gitweb:        https://git.kernel.org/tip/b745cfba44c152c34363eea9e052367b6b1d652b
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Thu, 28 May 2020 16:13:58 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 18 Jun 2020 15:47:05 +02:00

x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit

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@xxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/1557309753-24073-17-git-send-email-chang.seok.bae@xxxxxxxxx
Link: https://lkml.kernel.org/r/20200528201402.1708239-13-sashal@xxxxxxxxxx


---
 Documentation/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 7308db7..8c0d045 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3079,8 +3079,7 @@
 	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 7438a31..18857ce 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -441,21 +441,21 @@ 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.
@@ -1512,12 +1512,8 @@ 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.