arm64: perf: strip PAC when unwinding userspace

When the kernel is unwinding userspace callchains, we can't expect that
the userspace consumer of these callchains has the data necessary to
strip the PAC from the stored LR.

This patch has the kernel strip the PAC from user stackframes when the
in-kernel unwinder is used. This only affects the LR value, and not the
FP.

This only affects the in-kernel unwinder. When userspace performs
unwinding, it is up to userspace to strip PACs as necessary (which can
be determined from DWARF information).
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pointer_auth.h

@@ -69,6 +69,12 @@ static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
  */
 #define ptrauth_user_pac_mask()	GENMASK(54, vabits_user)
 
+/* Only valid for EL0 TTBR0 instruction pointers */
+static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
+{
+	return ptr & ~ptrauth_user_pac_mask();
+}
+
 #define ptrauth_thread_init_user(tsk)					\
 do {									\
 	struct task_struct *__ptiu_tsk = (tsk);				\
@@ -80,6 +86,7 @@ do {									\
 	ptrauth_keys_switch(&(tsk)->thread_info.keys_user)
 
 #else /* CONFIG_ARM64_PTR_AUTH */
+#define ptrauth_strip_insn_pac(lr)	(lr)
 #define ptrauth_thread_init_user(tsk)
 #define ptrauth_thread_switch(tsk)
 #endif /* CONFIG_ARM64_PTR_AUTH */

arch/arm64/kernel/perf_callchain.c

@@ -18,6 +18,7 @@
 #include <linux/perf_event.h>
 #include <linux/uaccess.h>
 
+#include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
 
 struct frame_tail {
@@ -35,6 +36,7 @@ user_backtrace(struct frame_tail __user *tail,
 {
 	struct frame_tail buftail;
 	unsigned long err;
+	unsigned long lr;
 
 	/* Also check accessibility of one struct frame_tail beyond */
 	if (!access_ok(VERIFY_READ, tail, sizeof(buftail)))
@@ -47,7 +49,9 @@ user_backtrace(struct frame_tail __user *tail,
 	if (err)
 		return NULL;
 
-	perf_callchain_store(entry, buftail.lr);
+	lr = ptrauth_strip_insn_pac(buftail.lr);
+
+	perf_callchain_store(entry, lr);
 
 	/*
 	 * Frame pointers should strictly progress back up the stack