Kernel Page Table Isolation (KPTI) is used to mitigate some speculation
based security issues by ensuring that the kernel is not mapped when
userspace is running but this approach is expensive and is incompatible
with SPE.  E0PD, introduced in the ARMv8.5 extensions, provides an
alternative to this which ensures that accesses from userspace to the
kernel's half of the memory map to always fault with constant time,
preventing timing attacks without requiring constant unmapping and
remapping or preventing legitimate accesses.

Currently this feature will only be enabled if all CPUs in the system
support E0PD, if some CPUs do not support the feature at boot time then
the feature will not be enabled and in the unlikely event that a late
CPU is the first CPU to lack the feature then we will reject that CPU.

This initial patch does not yet integrate with KPTI, this will be dealt
with in followup patches.  Ideally we could ensure that by default we
don't use KPTI on CPUs where E0PD is present.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
  arch/arm64/Kconfig                     | 15 ++++++++++++++
  arch/arm64/include/asm/cpucaps.h       |  3 ++-
  arch/arm64/include/asm/pgtable-hwdef.h |  2 ++
  arch/arm64/include/asm/sysreg.h        |  1 +
  arch/arm64/kernel/cpufeature.c         | 27 ++++++++++++++++++++++++++
  5 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 950a56b71ff0..9f881acb7acf 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -1421,6 +1421,21 @@ config ARM64_PTR_AUTH
  
  endmenu
  
+menu "ARMv8.5 architectural features"
+
+config ARM64_E0PD
+	bool "Enable support for E0PD"
+	default y
+	help
+	   E0PD (part of the ARMv8.5 extensions) allows us to ensure
+	   that EL0 accesses made via TTBR1 always fault in constant time,
+	   providing benefits to KPTI with lower overhead and without
+	   disrupting legitimate access to kernel memory such as SPE.
+
+	   This option enables E0PD for TTBR1 where available.
+
+endmenu
+
  config ARM64_SVE
  	bool "ARM Scalable Vector Extension support"
  	default y

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index f19fe4b9acc4..f25388981075 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h

@@ -52,7 +52,8 @@
  #define ARM64_HAS_IRQ_PRIO_MASKING		42
  #define ARM64_HAS_DCPODP			43
  #define ARM64_WORKAROUND_1463225		44
+#define ARM64_HAS_E0PD				45
  
-#define ARM64_NCAPS				45
+#define ARM64_NCAPS				46
  
  #endif /* __ASM_CPUCAPS_H */

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index 3df60f97da1f..685842e52c3d 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h

@@ -292,6 +292,8 @@
  #define TCR_HD			(UL(1) << 40)
  #define TCR_NFD0		(UL(1) << 53)
  #define TCR_NFD1		(UL(1) << 54)
+#define TCR_E0PD0		(UL(1) << 55)
+#define TCR_E0PD1		(UL(1) << 56)
  
  /*
   * TTBR.

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 972d196c7714..36227a5a22ba 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h

@@ -655,6 +655,7 @@
  #define ID_AA64MMFR1_VMIDBITS_16	2
  
  /* id_aa64mmfr2 */
+#define ID_AA64MMFR2_E0PD_SHIFT		60
  #define ID_AA64MMFR2_FWB_SHIFT		40
  #define ID_AA64MMFR2_AT_SHIFT		32
  #define ID_AA64MMFR2_LVA_SHIFT		16

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index cabebf1a7976..2cf2b129ebb4 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -220,6 +220,7 @@ static const struct arm64_ftr_bits ftr_id_aa64mmfr1[] = {
  };
  
  static const struct arm64_ftr_bits ftr_id_aa64mmfr2[] = {
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64MMFR2_E0PD_SHIFT, 4, 0),
  	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR2_FWB_SHIFT, 4, 0),
  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR2_AT_SHIFT, 4, 0),
  	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR2_LVA_SHIFT, 4, 0),

@@ -1245,6 +1246,19 @@ static void cpu_enable_address_auth(struct arm64_cpu_capabilities const *cap)
  }
  #endif /* CONFIG_ARM64_PTR_AUTH */
  
+#ifdef CONFIG_ARM64_E0PD
+static void cpu_enable_e0pd(struct arm64_cpu_capabilities const *cap)
+{
+	/*
+	 * The cpu_enable() callback gets called even on CPUs that
+	 * don't detect the feature so we need to verify if we can
+	 * enable.
+	 */
+	if (this_cpu_has_cap(ARM64_HAS_E0PD))
+		sysreg_clear_set(tcr_el1, 0, TCR_E0PD1);