Re: [RFC][PATCH] x86/ftrace: Have ftrace trampolines turn read-only at the end of system boot up

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Apr 30, 2020 at 08:21:47PM -0400, Steven Rostedt wrote:
> The cause is the "ftrace=function" would register the function tracer
> and create a trampoline, and it will set it as executable and
> read-only. Then the "trace_options=func_stack_trace" would then update
> the same trampoline to include the stack tracer version of the function
> tracer. But since the trampoline already exists, it updates it with
> text_poke_bp(). The problem is that text_poke_bp() called while
> system_state == SYSTEM_BOOTING, it will simply do a memcpy() and not
> the page mapping, as it would think that the text is still read-write.
> But in this case it is not, and we take a fault and crash.
> 
> Instead, lets keep the ftrace trampolines read-write during boot up,
> and then when the kernel executable text is set to read-only, the
> ftrace trampolines get set to read-only as well.

Would it be easier to just call a new __text_poke_bp() which skips the
SYSTEM_BOOTING check, since you know the trampoline will always be
read-only?

Like:

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index 67315fa3956a..710106256916 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -45,6 +45,7 @@ extern void *text_poke(void *addr, const void *opcode, size_t len);
 extern void text_poke_sync(void);
 extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
 extern int poke_int3_handler(struct pt_regs *regs);
+extern void __text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
 extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
 
 extern void text_poke_queue(void *addr, const void *opcode, size_t len, const void *emulate);
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 7867dfb3963e..9cc983cc9291 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1265,6 +1265,14 @@ void __ref text_poke_queue(void *addr, const void *opcode, size_t len, const voi
 	text_poke_loc_init(tp, addr, opcode, len, emulate);
 }
 
+void __ref __text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
+{
+	struct text_poke_loc tp;
+
+	text_poke_loc_init(&tp, addr, opcode, len, emulate);
+	text_poke_bp_batch(&tp, 1);
+}
+
 /**
  * text_poke_bp() -- update instructions on live kernel on SMP
  * @addr:	address to patch
@@ -1278,13 +1286,10 @@ void __ref text_poke_queue(void *addr, const void *opcode, size_t len, const voi
  */
 void __ref text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
 {
-	struct text_poke_loc tp;
-
 	if (unlikely(system_state == SYSTEM_BOOTING)) {
 		text_poke_early(addr, opcode, len);
 		return;
 	}
 
-	text_poke_loc_init(&tp, addr, opcode, len, emulate);
-	text_poke_bp_batch(&tp, 1);
+	__text_poke_bp(addr, opcode, len, emulate);
 }
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 867c126ddabe..c36f51f01f6e 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -469,7 +469,7 @@ void arch_ftrace_update_trampoline(struct ftrace_ops *ops)
 	mutex_lock(&text_mutex);
 	/* Do a safe modify in case the trampoline is executing */
 	new = ftrace_call_replace(ip, (unsigned long)func);
-	text_poke_bp((void *)ip, new, MCOUNT_INSN_SIZE, NULL);
+	__text_poke_bp((void *)ip, new, MCOUNT_INSN_SIZE, NULL);
 	mutex_unlock(&text_mutex);
 }