arm64: mm: Handle PAN faults on uaccess CPY* instructions

A subsequent patch will use CPY* instructions to copy between user and
kernel memory. Add handling for PAN faults caused by an intended kernel
memory access erroneously accessing user memory, in order to make it
easier to debug kernel bugs and to keep the same behavior as with
regular loads/stores.

Signed-off-by: Kristina Martšenko <kristina.martsenko@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20250228170006.390100-3-kristina.martsenko@arm.com
[catalin.marinas@arm.com: Folded the extable search into insn_may_access_user()]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/extable.h

@@ -33,6 +33,8 @@ do {							\
 	(b)->data = (tmp).data;				\
 } while (0)
 
+bool insn_may_access_user(unsigned long addr, unsigned long esr);
+
 #ifdef CONFIG_BPF_JIT
 bool ex_handler_bpf(const struct exception_table_entry *ex,
 		    struct pt_regs *regs);

arch/arm64/mm/extable.c

@@ -20,6 +20,21 @@ static bool cpy_faulted_on_uaccess(const struct exception_table_entry *ex,
 	return uaccess_is_write == fault_on_write;
 }
 
+bool insn_may_access_user(unsigned long addr, unsigned long esr)
+{
+	const struct exception_table_entry *ex = search_exception_tables(addr);
+
+	if (!ex)
+		return false;
+
+	switch (ex->type) {
+	case EX_TYPE_UACCESS_CPY:
+		return cpy_faulted_on_uaccess(ex, esr);
+	default:
+		return true;
+	}
+}
+
 static inline unsigned long
 get_ex_fixup(const struct exception_table_entry *ex)
 {

arch/arm64/mm/fault.c

@@ -606,7 +606,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
 			die_kernel_fault("execution of user memory",
 					 addr, esr, regs);
 
-		if (!search_exception_tables(regs->pc))
+		if (!insn_may_access_user(regs->pc, esr))
 			die_kernel_fault("access to user memory outside uaccess routines",
 					 addr, esr, regs);
 	}