kprobes/x86: Fix to set RWX bits correctly before releasing trampoline

Fix kprobes to set(recover) RWX bits correctly on trampoline
buffer before releasing it. Releasing readonly page to
module_memfree() crash the kernel.

Without this fix, if kprobes user register a bunch of kprobes
in function body (since kprobes on function entry usually
use ftrace) and unregister it, kernel hits a BUG and crash.

Link: http://lkml.kernel.org/r/149570868652.3518.14120169373590420503.stgit@devbox

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: d0381c81c2 ("kprobes/x86: Set kprobes pages read-only")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/x86/kernel/kprobes/core.c

@@ -52,6 +52,7 @@
 #include <linux/ftrace.h>
 #include <linux/frame.h>
 #include <linux/kasan.h>
+#include <linux/moduleloader.h>
 
 #include <asm/text-patching.h>
 #include <asm/cacheflush.h>
@@ -417,6 +418,14 @@ static void prepare_boost(struct kprobe *p, struct insn *insn)
 	}
 }
 
+/* Recover page to RW mode before releasing it */
+void free_insn_page(void *page)
+{
+	set_memory_nx((unsigned long)page & PAGE_MASK, 1);
+	set_memory_rw((unsigned long)page & PAGE_MASK, 1);
+	module_memfree(page);
+}
+
 static int arch_copy_kprobe(struct kprobe *p)
 {
 	struct insn insn;

kernel/kprobes.c

@@ -122,7 +122,7 @@ static void *alloc_insn_page(void)
 	return module_alloc(PAGE_SIZE);
 }
 
-static void free_insn_page(void *page)
+void __weak free_insn_page(void *page)
 {
 	module_memfree(page);
 }