Patch "ftrace/x86: Fix triple fault with graph tracing and suspend-to-ram" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ftrace/x86: Fix triple fault with graph tracing and suspend-to-ram

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ftrace-x86-fix-triple-fault-with-graph-tracing-and-suspend-to-ram.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Josh Poimboeuf <[email protected]>
Date: Thu, 13 Apr 2017 17:53:55 -0500
Subject: ftrace/x86: Fix triple fault with graph tracing and suspend-to-ram

From: Josh Poimboeuf <[email protected]>

commit 34a477e5297cbaa6ecc6e17c042a866e1cbe80d6 upstream.

On x86-32, with CONFIG_FIRMWARE and multiple CPUs, if you enable function graph tracing and then suspend to RAM, it will triple fault and reboot when it resumes.

The first fault happens when booting a secondary CPU:

startup_32_smp()
load_ucode_ap()
prepare_ftrace_return()
ftrace_graph_is_dead()
(accesses 'kill_ftrace_graph')

The early head_32.S code calls into load_ucode_ap(), which has an an ftrace hook, so it calls prepare_ftrace_return(), which calls ftrace_graph_is_dead(), which tries to access the global 'kill_ftrace_graph' variable with a virtual address, causing a fault because the CPU is still in real mode.

The fix is to add a check in prepare_ftrace_return() to make sure it's running in protected mode before continuing. The check makes sure the stack pointer is a virtual kernel address. It's a bit of a hack, but it's not very intrusive and it works well enough.

For reference, here are a few other (more difficult) ways this could have potentially been fixed:

- Move startup_32_smp()'s call to load_ucode_ap() down to *after* paging is enabled. (No idea what that would break.)

- Track down load_ucode_ap()'s entire callee tree and mark all the functions 'notrace'. (Probably not realistic.)

- Pause graph tracing in ftrace_suspend_notifier_call() or bringup_cpu() or __cpu_up(), and ensure that the pause facility can be queried from real mode.

---
 arch/x86/kernel/ftrace.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

@@ -977,6 +977,18 @@ void prepare_ftrace_return(unsigned long
 	unsigned long return_hooker = (unsigned long)
 				&return_to_handler;
 
+	/*
+	 * When resuming from suspend-to-ram, this function can be indirectly
+	 * called from early CPU startup code while the CPU is in real mode,
+	 * which would fail miserably.  Make sure the stack pointer is a
+	 * virtual address.
+	 *
+	 * This check isn't as accurate as virt_addr_valid(), but it should be
+	 * good enough for this purpose, and it's fast.
+	 */
+	if (unlikely((long)__builtin_frame_address(0) >= 0))
+		return;
+
 	if (unlikely(ftrace_graph_is_dead()))
 		return;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/ftrace-x86-fix-triple-fault-with-graph-tracing-and-suspend-to-ram.patch