[tip: x86/fsgsbase] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE

[tip-bot2 for Andy Lutomirski]

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

Commit-ID:     dd649bd0b3aa012740059b1ba31ecad28a408f7f
Gitweb:        https://git.kernel.org/tip/dd649bd0b3aa012740059b1ba31ecad28a408f7f
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Thu, 28 May 2020 16:13:48 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 18 Jun 2020 15:46:59 +02:00

x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE

This is temporary.  It will allow the next few patches to be tested
incrementally.

Setting unsafe_fsgsbase is a root hole.  Don't do 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>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/1557309753-24073-4-git-send-email-chang.seok.bae@xxxxxxxxx
Link: https://lkml.kernel.org/r/20200528201402.1708239-3-sashal@xxxxxxxxxx


---
 Documentation/admin-guide/kernel-parameters.txt |  3 ++-
 arch/x86/kernel/cpu/common.c                    | 24 ++++++++++++++++-
 2 files changed, 27 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad..7308db7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3079,6 +3079,9 @@
 	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.
+
 	no_console_suspend
 			[HW] Never suspend the console
 			Disable suspending of consoles during suspend and
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 043d93c..7438a31 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -442,6 +442,22 @@ static void __init setup_cr_pinning(void)
 }
 
 /*
+ * 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)
+{
+	unsafe_fsgsbase = true;
+	return 1;
+}
+__setup("unsafe_fsgsbase", setup_unsafe_fsgsbase);
+
+/*
  * Protection Keys are not available in 32-bit mode.
  */
 static bool pku_disabled;
@@ -1495,6 +1511,14 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_smap(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);
+	}
+
 	/*
 	 * The vendor-specific functions might have changed features.
 	 * Now we do "generic changes."