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[PATCH v3] ftrace/x86/32: fix triple fault with graph tracing and suspend-to-ram

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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.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Cc: stable@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
v3: Added Tested-by: and Cc:stable tags

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

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 9dd546b..a3adf1f 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -995,6 +995,18 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 	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;
 
-- 
2.7.4

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[PATCH v3] ftrace/x86/32: fix triple fault with graph tracing and suspend-to-ram Josh Poimboeuf <jpoimboe@redhat.com> - 2017-04-14 01:00 +0200
  Re: [PATCH v3] ftrace/x86/32: fix triple fault with graph tracing  and suspend-to-ram Steven Rostedt <rostedt@goodmis.org> - 2017-04-14 01:10 +0200
    Re: [PATCH v3] ftrace/x86/32: fix triple fault with graph tracing  and suspend-to-ram Josh Poimboeuf <jpoimboe@redhat.com> - 2017-04-14 04:20 +0200
  [tip:x86/urgent] ftrace/x86: Fix triple fault with graph tracing  and suspend-to-ram tip-bot for Josh Poimboeuf <tipbot@zytor.com> - 2017-04-14 12:00 +0200

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