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

[Paul Menzel <pmenzel@molgen.mpg.de>]

Dear Josh,


On 03/27/17 17:01, Paul Menzel wrote:

> 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.

With both patches applied `./analyze_suspend.py -config 
suspend-callgraph.cfg -filter i915` succeeds on a Lenovo X60t, so 
suspend and resume work perfectly, when tracing is enabled.

Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>

It’d be awesome, if you could tag both patches for inclusion into the 
stable Linux Kernel series.


Kind regards,

Paul