linux-kernel - [RFC PATCH 1/8] arm64: Kconfig: force ARM64

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Message-ID: <20241030101803.2037606-11-ardb+git@google.com>
Date: Wed, 30 Oct 2024 11:18:05 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Ryan Roberts <ryan.roberts@....com>, 
	Anshuman Khandual <anshuman.khandual@....com>, Kees Cook <keescook@...omium.org>
Subject: [RFC PATCH 1/8] arm64: Kconfig: force ARM64_PAN=y when enabling TTBR0
 sw PAN

From: Ard Biesheuvel <ardb@...nel.org>

We have a couple of instances of Kconfig constraints where PAN must be
enabled too if TTBR0 sw PAN is enabled, primarily to avoid dealing with
the modified TTBR0_EL1 sysreg format that is used when 52-bit physical
addressing and/or CnP are enabled (support for either implies support
for hardware PAN as well, which will supersede PAN emulation if both are
available)

Let's simplify this, and always enable ARM64_PAN when enabling TTBR0 sw
PAN. This decouples the PAN configuration from the VA size selection,
permitting us to simplify the latter in subsequent patches. (Note that
PAN and TTBR0 sw PAN can still be disabled after this patch, but not
independently)

To avoid a convoluted circular Kconfig dependency involving KCSAN, make
ARM64_MTE select ARM64_PAN too, instead of depending on it.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/Kconfig | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 232dcade2783..ec218ef22f2b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1368,7 +1368,6 @@ config ARM64_VA_BITS_48
 
 config ARM64_VA_BITS_52
 	bool "52-bit"
-	depends on ARM64_PAN || !ARM64_SW_TTBR0_PAN
 	help
 	  Enable 52-bit virtual addressing for userspace when explicitly
 	  requested via a hint to mmap(). The kernel will also use 52-bit
@@ -1420,7 +1419,6 @@ config ARM64_PA_BITS_48
 config ARM64_PA_BITS_52
 	bool "52-bit"
 	depends on ARM64_64K_PAGES || ARM64_VA_BITS_52
-	depends on ARM64_PAN || !ARM64_SW_TTBR0_PAN
 	help
 	  Enable support for a 52-bit physical address space, introduced as
 	  part of the ARMv8.2-LPA extension.
@@ -1667,6 +1665,7 @@ config RODATA_FULL_DEFAULT_ENABLED
 config ARM64_SW_TTBR0_PAN
 	bool "Emulate Privileged Access Never using TTBR0_EL1 switching"
 	depends on !KCSAN
+	select ARM64_PAN
 	help
 	  Enabling this option prevents the kernel from accessing
 	  user-space memory directly by pointing TTBR0_EL1 to a reserved
@@ -1923,7 +1922,6 @@ config ARM64_RAS_EXTN
 config ARM64_CNP
 	bool "Enable support for Common Not Private (CNP) translations"
 	default y
-	depends on ARM64_PAN || !ARM64_SW_TTBR0_PAN
 	help
 	  Common Not Private (CNP) allows translation table entries to
 	  be shared between different PEs in the same inner shareable
@@ -2118,7 +2116,7 @@ config ARM64_MTE
 	depends on AS_HAS_ARMV8_5
 	depends on AS_HAS_LSE_ATOMICS
 	# Required for tag checking in the uaccess routines
-	depends on ARM64_PAN
+	select ARM64_PAN
 	select ARCH_HAS_SUBPAGE_FAULTS
 	select ARCH_USES_HIGH_VMA_FLAGS
 	select ARCH_USES_PG_ARCH_2
-- 
2.47.0.163.g1226f6d8fa-goog