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

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