4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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. Reported-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Tested-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Cc: "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx> Cc: linux-acpi@xxxxxxxxxxxxxxx Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Len Brown <lenb@xxxxxxxxxx> Link: http://lkml.kernel.org/r/5c1272269a580660703ed2eccf44308e790c7a98.1492123841.git.jpoimboe@xxxxxxxxxx Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kernel/ftrace.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) --- a/arch/x86/kernel/ftrace.c +++ b/arch/x86/kernel/ftrace.c @@ -983,6 +983,18 @@ 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;