Re: [Xen-devel] [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Mon, Mar 14, 2016 at 5:02 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Sat, Mar 12, 2016 at 10:08:49AM -0800, Andy Lutomirski wrote:
>> This demotes an OOPS and likely panic due to a failed non-"safe" MSR
>> access to a WARN_ONCE and, for RDMSR, a return value of zero.  If
>> panic_on_oops is set, then failed unsafe MSR accesses will still
>> oops and panic.
>>
>> To be clear, this type of failure should *not* happen.  This patch
>> exists to minimize the chance of nasty undebuggable failures due on
>> systems that used to work due to a now-fixed CONFIG_PARAVIRT=y bug.
>>
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/msr.h | 10 ++++++++--
>>  arch/x86/mm/extable.c      | 33 +++++++++++++++++++++++++++++++++
>>  2 files changed, 41 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
>> index 93fb7c1cffda..1487054a1a70 100644
>> --- a/arch/x86/include/asm/msr.h
>> +++ b/arch/x86/include/asm/msr.h
>> @@ -92,7 +92,10 @@ static inline unsigned long long native_read_msr(unsigned 
>> int msr)
>>  {
>>       DECLARE_ARGS(val, low, high);
>>
>> -     asm volatile("rdmsr" : EAX_EDX_RET(val, low, high) : "c" (msr));
>> +     asm volatile("1: rdmsr\n"
>> +                  "2:\n"
>> +                  _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_rdmsr_unsafe)
>> +                  : EAX_EDX_RET(val, low, high) : "c" (msr));
>>       if (msr_tracepoint_active(__tracepoint_read_msr))
>>               do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), 0);
>>       return EAX_EDX_VAL(val, low, high);
>> @@ -119,7 +122,10 @@ static inline unsigned long long 
>> native_read_msr_safe(unsigned int msr,
>>  static inline void native_write_msr(unsigned int msr,
>>                                   unsigned low, unsigned high)
>>  {
>> -     asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
>> +     asm volatile("1: wrmsr\n"
>> +                  "2:\n"
>> +                  _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
>
> This might be a good idea:
>
> [    0.220066] cpuidle: using governor menu
> [    0.224000] ------------[ cut here ]------------
> [    0.224000] WARNING: CPU: 0 PID: 1 at arch/x86/mm/extable.c:74 
> ex_handler_wrmsr_unsafe+0x73/0x80()
> [    0.224000] unchecked MSR access error: WRMSR to 0xdeadbeef (tried to 
> write 0x000000000000caca)
> [    0.224000] Modules linked in:
> [    0.224000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc7+ #7
> [    0.224000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
> 1.7.5-20140531_083030-gandalf 04/01/2014
> [    0.224000]  0000000000000000 ffff88007c0d7c08 ffffffff812f13a3 
> ffff88007c0d7c50
> [    0.224000]  ffffffff81a40ffe ffff88007c0d7c40 ffffffff8105c3b1 
> ffffffff81717710
> [    0.224000]  ffff88007c0d7d18 0000000000000000 ffffffff816207d0 
> 0000000000000000
> [    0.224000] Call Trace:
> [    0.224000]  [<ffffffff812f13a3>] dump_stack+0x67/0x94
> [    0.224000]  [<ffffffff8105c3b1>] warn_slowpath_common+0x91/0xd0
> [    0.224000]  [<ffffffff816207d0>] ? amd_cpu_notify+0x40/0x40
> [    0.224000]  [<ffffffff8105c43c>] warn_slowpath_fmt+0x4c/0x50
> [    0.224000]  [<ffffffff816207d0>] ? amd_cpu_notify+0x40/0x40
> [    0.224000]  [<ffffffff8131de53>] ? __this_cpu_preempt_check+0x13/0x20
> [    0.224000]  [<ffffffff8104efe3>] ex_handler_wrmsr_unsafe+0x73/0x80
>
> and it looks helpful and all but when you do it pretty early - for
> example I added a
>
>          wrmsrl(0xdeadbeef, 0xcafe);
>
> at the end of pat_bsp_init() and the machine explodes with an early
> panic. I'm wondering what is better - early panic or an early #GP from a
> missing MSR.

You're hitting:

    /* special handling not supported during early boot */
    if (handler != ex_handler_default)
        return 0;


which means that the behavior with and without my series applied is
identical, for better or for worse.

>
> And more specifically, can we do better to handle the early case
> gracefully too?

We could probably remove that check and let custom fixups run early.
I don't see any compelling reason to keep them disabled.  That should
probably be a separate change, though.

--Andy

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