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

[tip-bot for Andy Lutomirski]

Commit-ID:  b64ed19b93c368be0fb6acf05377e8e3a694c92b
Gitweb:     https://git.kernel.org/tip/b64ed19b93c368be0fb6acf05377e8e3a694c92b
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 8 May 2019 03:02:18 -0700
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Sat, 22 Jun 2019 11:38:51 +0200

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>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Ravi Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Link: https://lkml.kernel.org/r/1557309753-24073-4-git-send-email-chang.seok.bae@xxxxxxxxx

---
 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 138f6664b2e2..b0fa5273b0fc 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2857,6 +2857,9 @@
 	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 dad20bc891d5..71defe2d1b7c 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -366,6 +366,22 @@ out:
 	cr4_clear_bits(X86_CR4_UMIP);
 }
 
+/*
+ * 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.
  */
@@ -1370,6 +1386,14 @@ 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."