arm64: use XPACLRI to strip PAC

Currently we strip the PAC from pointers using C code, which requires
generating bitmasks, and conditionally clearing/setting bits depending
on bit 55. We can do better by using XPACLRI directly.

When the logic was originally written to strip PACs from user pointers,
contemporary toolchains used for the kernel had assemblers which were
unaware of the PAC instructions. As stripping the PAC from userspace
pointers required unconditional clearing of a fixed set of bits (which
could be performed with a single instruction), it was simpler to
implement the masking in C than it was to make use of XPACI or XPACLRI.

When support for in-kernel pointer authentication was added, the
stripping logic was extended to cover TTBR1 pointers, requiring several
instructions to handle whether to clear/set bits dependent on bit 55 of
the pointer.

This patch simplifies the stripping of PACs by using XPACLRI directly,
as contemporary toolchains do within __builtin_return_address(). This
saves a number of instructions, especially where
__builtin_return_address() does not implicitly strip the PAC but is
heavily used (e.g. with tracepoints). As the kernel might be compiled
with an assembler without knowledge of XPACLRI, it is assembled using
the 'HINT #7' alias, which results in an identical opcode.

At the same time, I've split ptrauth_strip_insn_pac() into
ptrauth_strip_user_insn_pac() and ptrauth_strip_kernel_insn_pac()
helpers so that we can avoid unnecessary PAC stripping when pointer
authentication is not in use in userspace or kernel respectively.

The underlying xpaclri() macro uses inline assembly which clobbers x30.
The clobber causes the compiler to save/restore the original x30 value
in a frame record (protected with PACIASP and AUTIASP when in-kernel
authentication is enabled), so this does not provide a gadget to alter
the return address. Similarly this does not adversely affect unwinding
due to the presence of the frame record.

The ptrauth_user_pac_mask() and ptrauth_kernel_pac_mask() are exported
from the kernel in ptrace and core dumps, so these are retained. A
subsequent patch will move them out of <asm/compiler.h>.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230412160134.306148-3-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 25 insertions, 7 deletions

diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
index 9033a0b2f46a..b6d1d62474e5 100644
--- a/arch/arm64/include/asm/compiler.h
+++ b/arch/arm64/include/asm/compiler.h
@@ -15,15 +15,33 @@
 #define ptrauth_user_pac_mask()		GENMASK_ULL(54, vabits_actual)
 #define ptrauth_kernel_pac_mask()	GENMASK_ULL(63, vabits_actual)
 
-/* Valid for EL0 TTBR0 and EL1 TTBR1 instruction pointers */
-#define ptrauth_clear_pac(ptr)						\
-	((ptr & BIT_ULL(55)) ? (ptr | ptrauth_kernel_pac_mask()) :	\
-			       (ptr & ~ptrauth_user_pac_mask()))
+#define xpaclri(ptr)							\
+({									\
+	register unsigned long __xpaclri_ptr asm("x30") = (ptr);	\
+									\
+	asm(								\
+	ARM64_ASM_PREAMBLE						\
+	"	hint	#7\n"						\
+	: "+r" (__xpaclri_ptr));					\
+									\
+	__xpaclri_ptr;							\
+})
 
-#if defined(CONFIG_ARM64_PTR_AUTH_KERNEL) &&				\
-    !defined(CONFIG_BUILTIN_RETURN_ADDRESS_STRIPS_PAC)
+#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
+#define ptrauth_strip_kernel_insn_pac(ptr)	xpaclri(ptr)
+#else
+#define ptrauth_strip_kernel_insn_pac(ptr)	(ptr)
+#endif
+
+#ifdef CONFIG_ARM64_PTR_AUTH
+#define ptrauth_strip_user_insn_pac(ptr)	xpaclri(ptr)
+#else
+#define ptrauth_strip_user_insn_pac(ptr)	(ptr)
+#endif
+
+#if !defined(CONFIG_BUILTIN_RETURN_ADDRESS_STRIPS_PAC)
 #define __builtin_return_address(val)					\
-	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
+	(void *)(ptrauth_strip_kernel_insn_pac((unsigned long)__builtin_return_address(val)))
 #endif
 
 #endif /* __ASM_COMPILER_H */