linux-kernel - [PATCH v7 03/18] x86/fsgsbase/64: Add 'unsafe

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Date:   Wed,  8 May 2019 03:02:18 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>
Cc:     Ravi Shankar <ravi.v.shankar@...el.com>,
        "Chang S . Bae" <chang.seok.bae@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH v7 03/18] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE

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

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@...nel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Reviewed-by: Andi Kleen <ak@...ux.intel.com>
Reviewed-by: Andy Lutomirski <luto@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Randy Dunlap <rdunlap@...radead.org>
---
 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 fd03e2b..7fe1da0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2827,6 +2827,9 @@
 	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 8739bdf..5743cb9 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -366,6 +366,22 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
 }
 
 /*
+ * 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;
@@ -1344,6 +1360,14 @@ 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."
-- 
2.7.4