[PATCH v6 1/4] pkeys: add API to switch to permissive/zero pkey register

[Dmitry Vyukov <dvyukov@google.com>]

The API allows to switch to permissive pkey register that allows accesses
to all PKEYs, and to a value that allows access to the 0 (default) PKEY.
x86 signal delivery already uses switching to permissive PKEY register
value, and rseq needs to allow access to PKEY 0 while accessing
struct rseq/rseq_cs.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: d7822b1e24f2 ("rseq: Introduce restartable sequences system call")

---
Changes in v5:
 - Removed leftover dead code in enable_zero_pkey_val
 - Clarified commit message

Changes in v4:
 - Added Fixes tag

Changes in v3:
 - Renamed API functions to write_permissive_pkey_val/write_pkey_val
 - Added enable_zero_pkey_val for rseq
 - Added Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Changes in v2:
 - Fixed typo in commit description
---
 arch/x86/Kconfig             |  1 +
 arch/x86/include/asm/pkeys.h | 30 ++++++++++++++++++++++++++++++
 arch/x86/include/asm/pkru.h  | 10 +++++++---
 include/linux/pkeys.h        | 31 +++++++++++++++++++++++++++++++
 mm/Kconfig                   |  2 ++
 5 files changed, 71 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be2c311f5118d..43af2840d098f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1881,6 +1881,7 @@ config X86_INTEL_MEMORY_PROTECTION_KEYS
 	depends on X86_64 && (CPU_SUP_INTEL || CPU_SUP_AMD)
 	select ARCH_USES_HIGH_VMA_FLAGS
 	select ARCH_HAS_PKEYS
+	select ARCH_HAS_PERMISSIVE_PKEY
 	help
 	  Memory Protection Keys provides a mechanism for enforcing
 	  page-based protections, but without requiring modification of the
diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index 2e6c04d8a45b4..614099766d5f2 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_PKEYS_H
 #define _ASM_X86_PKEYS_H
 
+#include "pkru.h"
+
 /*
  * If more than 16 keys are ever supported, a thorough audit
  * will be necessary to ensure that the types that store key
@@ -123,4 +125,32 @@ static inline int vma_pkey(struct vm_area_struct *vma)
 	return (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
 }
 
+typedef u32 pkey_reg_t;
+
+static inline pkey_reg_t write_permissive_pkey_val(void)
+{
+	return write_pkru(0);
+}
+
+static inline pkey_reg_t enable_zero_pkey_val(void)
+{
+	u32 pkru;
+
+	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
+		return 0;
+	/*
+	 * WRPKRU is relatively expensive compared to RDPKRU,
+	 * avoid it if possible.
+	 */
+	pkru = rdpkru();
+	if ((pkru & (PKRU_AD_BIT|PKRU_WD_BIT)) != 0)
+		wrpkru(pkru & ~(PKRU_AD_BIT|PKRU_WD_BIT));
+	return pkru;
+}
+
+static inline void write_pkey_val(pkey_reg_t val)
+{
+	write_pkru(val);
+}
+
 #endif /*_ASM_X86_PKEYS_H */
diff --git a/arch/x86/include/asm/pkru.h b/arch/x86/include/asm/pkru.h
index 74f0a2d34ffdd..b9bf9b7f2753b 100644
--- a/arch/x86/include/asm/pkru.h
+++ b/arch/x86/include/asm/pkru.h
@@ -39,16 +39,20 @@ static inline u32 read_pkru(void)
 	return 0;
 }
 
-static inline void write_pkru(u32 pkru)
+static inline u32 write_pkru(u32 pkru)
 {
+	u32 old_pkru;
+
 	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
-		return;
+		return 0;
 	/*
 	 * WRPKRU is relatively expensive compared to RDPKRU.
 	 * Avoid WRPKRU when it would not change the value.
 	 */
-	if (pkru != rdpkru())
+	old_pkru = rdpkru();
+	if (pkru != old_pkru)
 		wrpkru(pkru);
+	return old_pkru;
 }
 
 static inline void pkru_write_default(void)
diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
index 86be8bf27b41b..262d60f6a15f8 100644
--- a/include/linux/pkeys.h
+++ b/include/linux/pkeys.h
@@ -48,4 +48,35 @@ static inline bool arch_pkeys_enabled(void)
 
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
+#ifndef CONFIG_ARCH_HAS_PERMISSIVE_PKEY
+
+/*
+ * Common name for value of the register that controls access to PKEYs
+ * (called differently on different arches: PKRU, POR, AMR).
+ */
+typedef char pkey_reg_t;
+
+/*
+ * Sets PKEY access register to the most permissive value that allows
+ * accesses to all PKEYs. Returns the current value of PKEY register.
+ * Code should generally arrange switching back to the old value
+ * using write_pkey_val(old_value).
+ */
+static inline pkey_reg_t write_permissive_pkey_val(void)
+{
+	return 0;
+}
+
+/*
+ * Sets PKEY access register to a value that allows access to the 0 (default)
+ * PKEY. Returns the current value of PKEY register.
+ */
+static inline pkey_reg_t enable_zero_pkey_val(void)
+{
+	return 0;
+}
+
+static inline void write_pkey_val(pkey_reg_t val) {}
+#endif /* ! CONFIG_ARCH_HAS_PERMISSIVE_PKEY */
+
 #endif /* _LINUX_PKEYS_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index 1b501db064172..9e874f7713a2b 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1147,6 +1147,8 @@ config ARCH_USES_HIGH_VMA_FLAGS
 	bool
 config ARCH_HAS_PKEYS
 	bool
+config ARCH_HAS_PERMISSIVE_PKEY
+	bool
 
 config ARCH_USES_PG_ARCH_2
 	bool
-- 
2.48.1.658.g4767266eb4-goog