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Subject[tip: x86/fsgsbase] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE
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@kernel.org>
AuthorDate: Thu, 28 May 2020 16:13:48 -04:00
Committer: Thomas Gleixner <tglx@linutronix.de>
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@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/1557309753-24073-4-git-send-email-chang.seok.bae@intel.com
Link: https://lkml.kernel.org/r/20200528201402.1708239-3-sashal@kernel.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 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."
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