linux-kernel - [PATCH] x86/pkeys: make protection keys an "eager" feature

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Date:   Fri, 07 Oct 2016 09:23:42 -0700
From:   Dave Hansen <dave@...1.net>
To:     linux-kernel@...r.kernel.org
Cc:     Dave Hansen <dave@...1.net>, dave.hansen@...el.com,
        stable@...r.kernel.org, x86@...nel.org
Subject: [PATCH] x86/pkeys: make protection keys an "eager" feature

From: Dave Hansen <dave.hansen@...el.com>

Our XSAVE features are divided into two categories: those that
generate FPU exceptions, and those that do not.  MPX and pkeys do
not generate FPU exceptions and thus can not be used lazily.  We
disable them when lazy mode is forced on.

We have a pair of masks to collect these two sets of features, but
XFEATURE_MASK_PKRU was added to the wrong mask: XFEATURE_MASK_LAZY.
Fix it by moving the feature to XFEATURE_MASK_EAGER.

Note: this only causes problem if you boot with lazy FPU mode
(eagerfpu=off) which is *not* the default.  It also only affects
hardware which is not currently publicly available.  It looks like
eager mode is going away, but we still need this patch applied
to any kernel that has protection keys and lazy mode, which is 4.6
through 4.8 at this point, and 4.9 if the lazy removal isn't sent
to Linus for 4.9.

Signed-off-by: Dave Hansen <dave.hansen@...el.com>
Fixes: c8df40098451 ("x86/fpu, x86/mm/pkeys: Add PKRU xsave fields and data structures")
Cc: <stable@...r.kernel.org> # v4.6+
Cc: x86@...nel.org
---

 b/arch/x86/include/asm/fpu/xstate.h |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff -puN arch/x86/include/asm/fpu/xstate.h~x86-pkeys-add-as-eager-feature arch/x86/include/asm/fpu/xstate.h
--- a/arch/x86/include/asm/fpu/xstate.h~x86-pkeys-add-as-eager-feature	2016-10-07 08:54:17.363165294 -0700
+++ b/arch/x86/include/asm/fpu/xstate.h	2016-10-07 08:54:17.366165429 -0700
@@ -27,11 +27,12 @@
 				 XFEATURE_MASK_YMM | \
 				 XFEATURE_MASK_OPMASK | \
 				 XFEATURE_MASK_ZMM_Hi256 | \
-				 XFEATURE_MASK_Hi16_ZMM	 | \
-				 XFEATURE_MASK_PKRU)
+				 XFEATURE_MASK_Hi16_ZMM)

 /* Supported features which require eager state saving */
-#define XFEATURE_MASK_EAGER	(XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR)
+#define XFEATURE_MASK_EAGER	(XFEATURE_MASK_BNDREGS | \
+				 XFEATURE_MASK_BNDCSR | \
+				 XFEATURE_MASK_PKRU)

 /* All currently supported features */
 #define XCNTXT_MASK	(XFEATURE_MASK_LAZY | XFEATURE_MASK_EAGER)
_