arm64: mask PAC bits of __builtin_return_address

Functions like vmap() record how much memory has been allocated by their
callers, and callers are identified using __builtin_return_address(). Once
the kernel is using pointer-auth the return address will be signed. This
means it will not match any kernel symbol, and will vary between threads
even for the same caller.

The output of /proc/vmallocinfo in this case may look like,
0x(____ptrval____)-0x(____ptrval____)   20480 0x86e28000100e7c60 pages=4 vmalloc N0=4
0x(____ptrval____)-0x(____ptrval____)   20480 0x86e28000100e7c60 pages=4 vmalloc N0=4
0x(____ptrval____)-0x(____ptrval____)   20480 0xc5c78000100e7c60 pages=4 vmalloc N0=4

The above three 64bit values should be the same symbol name and not
different LR values.

Use the pre-processor to add logic to clear the PAC to
__builtin_return_address() callers. This patch adds a new file
asm/compiler.h and is transitively included via include/compiler_types.h on
the compiler command line so it is guaranteed to be loaded and the users of
this macro will not find a wrong version.

Helper macros ptrauth_kernel_pac_mask/ptrauth_clear_pac are created for
this purpose and added in this file. Existing macro ptrauth_user_pac_mask
moved from asm/pointer_auth.h.

Signed-off-by: Amit Daniel Kachhap <[email protected]>
Reviewed-by: James Morse <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>

Author: amitdanielkachhap

arch/arm64/Kconfig

@@ -118,6 +118,7 @@ config ARM64
 	select HAVE_ALIGNED_STRUCT_PAGE if SLUB
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_BITREVERSE
+	select HAVE_ARCH_COMPILER_H
 	select HAVE_ARCH_HUGE_VMAP
 	select HAVE_ARCH_JUMP_LABEL
 	select HAVE_ARCH_JUMP_LABEL_RELATIVE

arch/arm64/include/asm/compiler.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_COMPILER_H
+#define __ASM_COMPILER_H
+
+#if defined(CONFIG_ARM64_PTR_AUTH)
+
+/*
+ * The EL0/EL1 pointer bits used by a pointer authentication code.
+ * This is dependent on TBI0/TBI1 being enabled, or bits 63:56 would also apply.
+ */
+#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 __builtin_return_address(val)					\
+	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
+
+#endif /* CONFIG_ARM64_PTR_AUTH */
+
+#endif /* __ASM_COMPILER_H */

arch/arm64/include/asm/pointer_auth.h

@@ -68,16 +68,9 @@ static __always_inline void ptrauth_keys_switch_kernel(struct ptrauth_keys_kerne
 
 extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);
 
-/*
- * The EL0 pointer bits used by a pointer authentication code.
- * This is dependent on TBI0 being enabled, or bits 63:56 would also apply.
- */
-#define ptrauth_user_pac_mask()	GENMASK(54, vabits_actual)
-
-/* Only valid for EL0 TTBR0 instruction pointers */
 static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
 {
-	return ptr & ~ptrauth_user_pac_mask();
+	return ptrauth_clear_pac(ptr);
 }
 
 #define ptrauth_thread_init_user(tsk)					\