[PATCH 08/15] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE

From: Chang S. Bae
Date: Mon Mar 19 2018 - 16:57:00 EST


From: Andy Lutomirski <luto@xxxxxxxxxx>

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>
[chang: Fix the deactivated flag]
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/x86/kernel/cpu/common.c | 25 +++++++++++++++++++++++++
2 files changed, 28 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1d1d53f..87c2260 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2567,6 +2567,9 @@
emulation library even if a 387 maths coprocessor
is present.

+ 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 348cf48..019bdc3 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -356,6 +356,23 @@ 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. Setting
+ * unsafe_fsgsbase will allow the series to be bisected if necessary.
+ *
+ * Once all the pieces are in place, this 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;
@@ -1238,6 +1255,14 @@ 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