[PATCH v2 09/14] arm64: unwind: strip PAC from kernel addresses

[Amit Daniel Kachhap <amit.kachhap@arm.com>]

From: Mark Rutland <mark.rutland@arm.com>

When we enable pointer authentication in the kernel, LR values saved to
the stack will have a PAC which we must strip in order to retrieve the
real return address.

Strip PACs when unwinding the stack in order to account for this.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
[Amit: Re-position ptrauth_strip_insn_pac, comment]
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
---
Changes since last version:
 * Reposition ptrauth_strip_insn_pac. [James]
 * Added more comments for stripping PAC at EL1. [James]

Link to above discussion: https://www.spinics.net/lists/arm-kernel/msg763624.html

 arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++-----
 arch/arm64/kernel/stacktrace.c        |  3 +++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 599dd09..efd70b5 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -56,15 +56,21 @@ static inline void ptrauth_keys_init_kernel(struct ptrauth_keys_kernel *keys)
 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.
+ * 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(54, vabits_actual)
+#define ptrauth_user_pac_mask()		GENMASK_ULL(54, vabits_actual)
+/* As TBI1 is disabled currently, so bits 63:56 also has PAC */
+#define ptrauth_kernel_pac_mask()	\
+				(GENMASK_ULL(63, 56) | GENMASK_ULL(54, VA_BITS))
 
-/* Only valid for EL0 TTBR0 instruction pointers */
+/* Valid for EL0 TTBR0 and EL1 TTBR1 instruction pointers */
 static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
 {
-	return ptr & ~ptrauth_user_pac_mask();
+	if (ptr & BIT_ULL(55))
+		return ptr | ptrauth_kernel_pac_mask();
+	else
+		return ptr & ~ptrauth_user_pac_mask();
 }
 
 #define ptrauth_thread_init_user(tsk)					\
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index a336cb1..b479df7 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -14,6 +14,7 @@
 #include <linux/stacktrace.h>
 
 #include <asm/irq.h>
+#include <asm/pointer_auth.h>
 #include <asm/stack_pointer.h>
 #include <asm/stacktrace.h>
 
@@ -101,6 +102,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	}
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
+	frame->pc = ptrauth_strip_insn_pac(frame->pc);
+
 	/*
 	 * Frames created upon entry from EL0 have NULL FP and PC values, so
 	 * don't bother reporting these. Frames created by __noreturn functions
-- 
2.7.4


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