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From: Paul Menzel <pmenzel@molgen.mpg.de>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>
Cc: Len Brown <lenb@kernel.org>,
	linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
	x86@kernel.org, Borislav Petkov <bp@alien8.de>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH] ftrace/x86: fix x86-32 triple fault with graph tracing and suspend-to-ram
Date: Mon, 27 Mar 2017 17:01:53 +0200	[thread overview]
Message-ID: <d8fd5cfd-e273-ad29-9281-2436cb0c3723@molgen.mpg.de> (raw)
In-Reply-To: <20170327145441.aybim6rmc6nxelij@treble>

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@molgen.mpg.de>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.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

  reply	other threads:[~2017-03-27 15:02 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-16 13:56 [PATCH] acpi: fix incompatibility with mcount-based function graph tracing Josh Poimboeuf
2017-03-16 14:41 ` Steven Rostedt
2017-03-21 20:44 ` Paul Menzel
2017-03-22  0:04   ` Paul Menzel
2017-03-24 18:12   ` Josh Poimboeuf
2017-03-24 18:41     ` Steven Rostedt
2017-03-25 13:20       ` Rafael J. Wysocki
2017-03-27 14:08         ` Josh Poimboeuf
2017-03-27 14:54           ` [PATCH] ftrace/x86: fix x86-32 triple fault with graph tracing and suspend-to-ram Josh Poimboeuf
2017-03-27 15:01             ` Paul Menzel [this message]
2017-03-27 15:20               ` [PATCH v2] " Josh Poimboeuf
2017-03-27 15:24               ` [PATCH] " Steven Rostedt
2017-03-28  9:51               ` Paul Menzel
2017-03-28 15:39                 ` Steven Rostedt
2017-03-28 15:55                   ` Josh Poimboeuf
2017-03-28 21:12                     ` Rafael J. Wysocki
2017-03-28 21:42                       ` Josh Poimboeuf
2017-03-28 21:47                         ` Rafael J. Wysocki
2017-03-27 16:59           ` [PATCH] acpi: fix incompatibility with mcount-based function graph tracing Rafael J. Wysocki
2017-03-26 20:57 [PATCH] trace: Make trace_hwlat timestamp y2038 safe Deepa Dinamani
2017-03-27  9:25 ` kbuild test robot
2017-03-27  9:55   ` Arnd Bergmann
2017-03-27 14:28     ` Steven Rostedt
2017-03-27 14:53       ` Arnd Bergmann
2017-03-27 15:30         ` Steven Rostedt
2017-03-27 15:35           ` Arnd Bergmann
2017-03-27 16:11             ` Steven Rostedt
2017-03-27 21:02         ` Deepa Dinamani
2017-03-28  7:26           ` Arnd Bergmann
2017-03-27 10:04 ` kbuild test robot

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