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
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
