[PATCH v3] efi: implement mandatory locking for UEFI Runtime Services

[Ard Biesheuvel]

On 4 August 2014 15:00, Matt Fleming <matt at console-pimps.org> wrote:
> On Fri, 11 Jul, at 09:09:16AM, Ard Biesheuvel wrote:
>> According to section 7.1 of the UEFI spec, Runtime Services are not fully
>> reentrant, and there are particular combinations of calls that need to be
>> serialized. Use a spinlock to serialize all Runtime Services with respect
>> to all others, even if this is more than strictly needed.
>
> It'd be good to include a point about why we're only using a spinlock,
> namely that anything else introduces complication to code that is
> unlikely to be performance critical.
>

OK

> [...]
>
>> + *
>> + * In order to prevent deadlocks under NMI, the wrappers for these functions
>> + * only grab the efi_runtime_lock or rtc_lock spinlocks if !efi_in_nmi().
>> + */
>> +#ifndef efi_in_nmi
>> +#define efi_in_nmi() (0)
>> +#endif
>
> It shouldn't be necessary to do the NMI checking for *all* runtime
> services, unless you use, for instance, the timer functions on ARM64.
>

No, I only added the checks to functions that are listed in the UEFI
spec as requiring the special treatment.
Table 32 is copied right from the spec. (And if that is not obvious, I
should definitely add a mention there)


>> +/*
>>   * As per commit ef68c8f87ed1 ("x86: Serialize EFI time accesses on rtc_lock"),
>>   * the EFI specification requires that callers of the time related runtime
>>   * functions serialize with other CMOS accesses in the kernel, as the EFI time
>> @@ -30,10 +95,17 @@ static efi_status_t virt_efi_get_time(efi_time_t *tm, efi_time_cap_t *tc)
>>  {
>>       unsigned long flags;
>>       efi_status_t status;
>> +     bool __in_nmi = efi_in_nmi();
>>
>> -     spin_lock_irqsave(&rtc_lock, flags);
>> +     if (!__in_nmi) {
>> +             spin_lock_irqsave(&rtc_lock, flags);
>> +             spin_lock(&efi_runtime_lock);
>> +     }
>>       status = efi_call_virt(get_time, tm, tc);
>> -     spin_unlock_irqrestore(&rtc_lock, flags);
>> +     if (!__in_nmi) {
>> +             spin_unlock(&efi_runtime_lock);
>> +             spin_unlock_irqrestore(&rtc_lock, flags);
>> +     }
>>       return status;
>>  }
>>
>> @@ -42,8 +114,11 @@ static efi_status_t virt_efi_set_time(efi_time_t *tm)
>>       unsigned long flags;
>>       efi_status_t status;
>>
>> +     BUG_ON(efi_in_nmi());
>
> I think we can safely drop these BUG_ON()s. I would expect things to
> explode pretty spectacularly anyway, even without them if we're in NMI
> context. BUG_ON()s are usually reserved for "this is a fatal condition
> and we cannot make any forward progress at all, so stop".
>
> But also see my earlier point about how most of these functions aren't
> called from NMI context.
>

I added the BUG_ON() to functions not listed in the table, and added
the treatment to functions that are listed in the table.
Happy to drop the BUG_ON()'s, though.

>>  static efi_status_t virt_efi_get_next_variable(unsigned long *name_size,
>>                                              efi_char16_t *name,
>>                                              efi_guid_t *vendor)
>>  {
>> -     return efi_call_virt(get_next_variable, name_size, name, vendor);
>> +     unsigned long flags;
>> +     efi_status_t status;
>> +     bool __in_nmi = efi_in_nmi();
>> +
>> +     if (!__in_nmi)
>> +             spin_lock_irqsave(&efi_runtime_lock, flags);
>> +     status = efi_call_virt(get_next_variable, name_size, name, vendor);
>> +     if (!__in_nmi)
>> +             spin_unlock_irqrestore(&efi_runtime_lock, flags);
>> +     return status;
>
> We shouldn't ever be in NMI context when calling get_next_variable(),
> right?
>

Spec allows it ...

>> @@ -119,17 +245,33 @@ static void virt_efi_reset_system(int reset_type,
>>                                 unsigned long data_size,
>>                                 efi_char16_t *data)
>>  {
>> +     unsigned long flags;
>> +     bool __in_nmi = efi_in_nmi();
>> +
>> +     if (!__in_nmi)
>> +             spin_lock_irqsave(&efi_runtime_lock, flags);
>>       __efi_call_virt(reset_system, reset_type, status, data_size, data);
>> +     if (!__in_nmi)
>> +             spin_unlock_irqrestore(&efi_runtime_lock, flags);
>>  }
>
> Is it even possible to perform a reset through the usual machinery in
> NMI context? I don't think this is possible for x86. What about for
> arm64?
>

We don't have NMI so all the in_nmi() macros evaluate to false anyway.

>>  static efi_status_t virt_efi_update_capsule(efi_capsule_header_t **capsules,
>>                                           unsigned long count,
>>                                           unsigned long sg_list)
>>  {
>> +     unsigned long flags;
>> +     efi_status_t status;
>> +     bool __in_nmi = efi_in_nmi();
>> +
>>       if (efi.runtime_version < EFI_2_00_SYSTEM_TABLE_REVISION)
>>               return EFI_UNSUPPORTED;
>>
>> -     return efi_call_virt(update_capsule, capsules, count, sg_list);
>> +     if (!__in_nmi)
>> +             spin_lock_irqsave(&efi_runtime_lock, flags);
>> +     status = efi_call_virt(update_capsule, capsules, count, sg_list);
>> +     if (!__in_nmi)
>> +             spin_unlock_irqrestore(&efi_runtime_lock, flags);
>> +     return status;
>>  }
>
> I don't think we need to check for being in NMI context here.
>

Likewise, the spec says different ...

>>  static efi_status_t virt_efi_query_capsule_caps(efi_capsule_header_t **capsules,
>> @@ -137,11 +279,20 @@ static efi_status_t virt_efi_query_capsule_caps(efi_capsule_header_t **capsules,
>>                                               u64 *max_size,
>>                                               int *reset_type)
>>  {
>> +     unsigned long flags;
>> +     efi_status_t status;
>> +     bool __in_nmi = efi_in_nmi();
>> +
>>       if (efi.runtime_version < EFI_2_00_SYSTEM_TABLE_REVISION)
>>               return EFI_UNSUPPORTED;
>>
>> -     return efi_call_virt(query_capsule_caps, capsules, count, max_size,
>> -                          reset_type);
>> +     if (!__in_nmi)
>> +             spin_lock_irqsave(&efi_runtime_lock, flags);
>> +     status = efi_call_virt(query_capsule_caps, capsules, count, max_size,
>> +                            reset_type);
>> +     if (!__in_nmi)
>> +             spin_unlock_irqrestore(&efi_runtime_lock, flags);
>> +     return status;
>>  }
>
> Definitely should not be perfoming QueryCapsuleCapabilities() in NMI
> context.
>

Well, again, the spec allows it. But I am happy to remove it as it
does not affect ARM anyway


Regards,
Ard.