[RFC][PATCH 1/8] x86/pkeys: add PKRU storage outside of task XSAVE buffer

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

PKRU has space in the task XSAVE buffer, but is not context-switched by
XSAVE/XRSTOR.  It is switched more eagerly than other FPU state because
PKRU affects things like copy_to/from_user().  This is because PKRU
affects user *PERMISSION* accesses, not just accesses made from user
*MODE* itself.

Prepare to move PKRU away from being XSAVE-managed.  Allocate space in
the thread_struct for it and save/restore it in the context-switch path
separately from the XSAVE-managed features.

Leave the XSAVE storage in place for now to ensure bisectability.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: Andy Lutomirski <luto@kernel.org>
---

 b/arch/x86/include/asm/pkru.h  |    5 +++++
 b/arch/x86/kernel/cpu/common.c |    3 +++
 b/arch/x86/kernel/process_64.c |    9 ++++-----
 b/arch/x86/mm/pkeys.c          |    2 ++
 4 files changed, 14 insertions(+), 5 deletions(-)

diff -puN arch/x86/include/asm/pkru.h~pkru-stash-thread-value arch/x86/include/asm/pkru.h
--- a/arch/x86/include/asm/pkru.h~pkru-stash-thread-value	2021-06-22 14:49:06.594051763 -0700
+++ b/arch/x86/include/asm/pkru.h	2021-06-22 14:49:06.607051763 -0700
@@ -44,11 +44,16 @@ static inline void write_pkru(u32 pkru)
 	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
 		return;
 	/*
+	 * Update the actual register.
+	 *
 	 * WRPKRU is relatively expensive compared to RDPKRU.
 	 * Avoid WRPKRU when it would not change the value.
 	 */
 	if (pkru != rdpkru())
 		wrpkru(pkru);
+
+	/* Update the thread-local, context-switched value: */
+	current->thread.pkru = pkru;
 }
 
 static inline void pkru_write_default(void)
diff -puN arch/x86/kernel/cpu/common.c~pkru-stash-thread-value arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~pkru-stash-thread-value	2021-06-22 14:49:06.596051763 -0700
+++ b/arch/x86/kernel/cpu/common.c	2021-06-22 14:49:06.608051763 -0700
@@ -482,6 +482,9 @@ static __always_inline void setup_pku(st
 	cr4_set_bits(X86_CR4_PKE);
 	/* Load the default PKRU value */
 	pkru_write_default();
+
+	/* Establish the default value for future tasks: */
+	init_task.thread.pkru = init_pkru_value;
 }
 
 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
diff -puN arch/x86/kernel/process_64.c~pkru-stash-thread-value arch/x86/kernel/process_64.c
--- a/arch/x86/kernel/process_64.c~pkru-stash-thread-value	2021-06-22 14:49:06.599051763 -0700
+++ b/arch/x86/kernel/process_64.c	2021-06-22 14:49:06.608051763 -0700
@@ -349,15 +349,14 @@ static __always_inline void load_seg_leg
 static __always_inline void x86_pkru_load(struct thread_struct *prev,
 					  struct thread_struct *next)
 {
-	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
-		return;
+	u32 pkru = read_pkru();
 
 	/* Stash the prev task's value: */
-	prev->pkru = rdpkru();
+	prev->pkru = pkru;
 
 	/*
-	 * PKRU writes are slightly expensive.  Avoid them when not
-	 * strictly necessary:
+	 * PKRU writes are slightly expensive.  Avoid
+	 * them when not strictly necessary:
 	 */
 	if (prev->pkru != next->pkru)
 		wrpkru(next->pkru);
diff -puN arch/x86/mm/pkeys.c~pkru-stash-thread-value arch/x86/mm/pkeys.c
--- a/arch/x86/mm/pkeys.c~pkru-stash-thread-value	2021-06-22 14:49:06.604051763 -0700
+++ b/arch/x86/mm/pkeys.c	2021-06-22 14:49:06.609051763 -0700
@@ -159,6 +159,8 @@ static ssize_t init_pkru_write_file(stru
 		return -EINVAL;
 
 	WRITE_ONCE(init_pkru_value, new_init_pkru);
+	WRITE_ONCE(init_task.thread.pkru, new_init_pkru);
+
 	return count;
 }
 
_