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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 13 Apr 2017 17:53:55 -0500
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> 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>

This is pretty much the same thing we were talking about before, right?

If so, then:

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

if not, I need to spend more time reviewing it ;-)

-- Steve

> ---
> 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;
>