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

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Date:   Mon, 27 Mar 2017 17:01:53 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] ftrace/x86: fix x86-32 triple fault with graph tracing
 and suspend-to-ram

Dear Josh,


On 03/27/17 16:54, Josh Poimboeuf 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 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@...gen.mpg.de>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/x86/kernel/ftrace.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)

Thank you for debugging this. It’s great that you were able to reproduce 
this in QEMU. Hopefully, that’ll make for an easy test case. ;-)

> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 8f3d9cf..1c5c4e2 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -983,6 +983,17 @@ 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;

The coding style requires the `return;` to be on a separate line.

> +
>  	if (unlikely(ftrace_graph_is_dead()))
>  		return;

I’ll test your change this evening.


Kind regards,

Paul