Re: [PATCH v3 2/2] efi: efi_mem_reserve(): don't reserve through memblock after mm_init()

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 6 January 2017 at 17:46, Nicolai Stange <nicstange@gmail.com> wrote:
> Ard Biesheuvel <ard.biesheuvel@linaro.org> writes:
>
>> On 6 January 2017 at 13:02, Nicolai Stange <nicstange@gmail.com> wrote:
>>> Ard Biesheuvel <ard.biesheuvel@linaro.org> writes:
>>>
>>>> On 5 January 2017 at 12:51, Nicolai Stange <nicstange@gmail.com> wrote:
>>>>> Before invoking the arch specific handler, efi_mem_reserve() reserves
>>>>> the given memory region through memblock.
>>>>>
>>>>> efi_mem_reserve() can get called after mm_init() though -- through
>>>>> efi_bgrt_init(), for example. After mm_init(), memblock is dead and should
>>>>> not be used anymore.
>>>>>
>>>>> Let efi_mem_reserve() check whether memblock is dead and not do the
>>>>> reservation if so. Emit a warning from the generic efi_arch mem_reserve()
>>>>> in this case: if the architecture doesn't provide any other means of
>>>>> registering the region as reserved, the operation would be a nop.
>>>>>
>>>>> Fixes: 4bc9f92e64c8 ("x86/efi-bgrt: Use efi_mem_reserve() to avoid copying image data")
>>>>> Signed-off-by: Nicolai Stange <nicstange@gmail.com>
>>>>> ---
>>>>> Applicable to next-20170105.
>>>>> No changes to v2.
>>>>> Boot-tested on x86_64.
>>>>>
>>>>>  drivers/firmware/efi/efi.c | 7 +++++--
>>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
>>>>> index 92914801e388..158a8df2f4af 100644
>>>>> --- a/drivers/firmware/efi/efi.c
>>>>> +++ b/drivers/firmware/efi/efi.c
>>>>> @@ -403,7 +403,10 @@ u64 __init efi_mem_desc_end(efi_memory_desc_t *md)
>>>>>         return end;
>>>>>  }
>>>>>
>>>>> -void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size) {}
>>>>> +void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size)
>>>>> +{
>>>>> +       WARN(slab_is_available(), "efi_mem_reserve() has no effect");
>>>>> +}
>>>>>
>>>>>  /**
>>>>>   * efi_mem_reserve - Reserve an EFI memory region
>>>>> @@ -419,7 +422,7 @@ void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size) {}
>>>>>   */
>>>>>  void __init efi_mem_reserve(phys_addr_t addr, u64 size)
>>>>>  {
>>>>> -       if (!memblock_is_region_reserved(addr, size))
>>>>> +       if (!slab_is_available() && !memblock_is_region_reserved(addr, size))
>>>>>                 memblock_reserve(addr, size);
>>>>>
>>>
>>> More context:
>>>
>>>             /*
>>>              * Some architectures (x86) reserve all boot services ranges
>>>              * until efi_free_boot_services() because of buggy firmware
>>>              * implementations. This means the above memblock_reserve() is
>>>              * superfluous on x86 and instead what it needs to do is
>>>              * ensure the @start, @size is not freed.
>>>              */
>>>             efi_arch_mem_reserve(addr, size);
>>>     }
>>>
>>>
>>>> I share Dave's concern: on x86, this will silently ignore the
>>>> reservation if slab_is_available() returns true,
>>>
>>> AFAICS, x86 has got an efi_arch_mem_reserve() which doesn't ignore the
>>> reservation at any stage.
>>>
>>
>> Thanks for the clarification. But my concern is whether changing the
>> EFI memory map is going to have any effect at this stage, i.e., after
>> slab_is_available() returns true: haven't we already communicated to
>> the kernel which RAM regions it may allocate from? How does it know
>> the memory map has changed, and how do we ensure that it has not
>> already allocated from the region we are reserving here?
>
> Ah, I see what you mean. I think it works like this on x86:
>
> All EFI_BOOT_SERVICES_* regions as reported by the firmware are marked
> as reserved at memblock unconditionally through the early setup_arch()
> => efi_reserve_boot_services(). This prevents these from getting handed
> over to the "normal" kernel MM until efi_free_boot_services()
> gets called later on. The latter frees these EFI_BOOT_SERVICES_* regions,
> but only if their EFI_MEMORY_RUNTIME flag is not set.
>
> Now, efi_arch_mem_reserve() basically just sets the EFI_MEMORY_RUNTIME
> flag, allowing the given region to survive beyond efi_free_boot_services().
>
> Corrolary 1: any efi_mem_reserve() after efi_free_boot_services wouldn't
> have any effect.
>

This is my point exactly. But it appears efi_free_boot_services()
occurs much later than I thought, and so there is a sizabe time window
where SLAB is up but reservations can still be made. But we don't
check whether efi_free_boot_services() has been called. Another
problem is that we never check that the reservation is covered by a
BootServicesData region, which are the only ones that are guaranteed
to be retained up to this point.

> Corollary 2: anything handed to efi_arch_mem_reserve() must live within
> some memory region which had been reported by firmware already.
>

Yes, but the EFI memory map describes all of RAM, so this is not an
issue by itself. The issue is that the region must have been covered
by a BootServicesCode or BootServicesData region

> Indeed, at its very top, there is
>
>   if (efi_mem_desc_lookup(addr, &md)) {
>     pr_err("Failed to lookup EFI memory descriptor for %pa\n", &addr);
>     return;
>   }
>
>   if (addr + size > md.phys_addr + (md.num_pages << EFI_PAGE_SHIFT)) {
>         pr_err("Region spans EFI memory descriptors, %pa\n", &addr);
>         return;
>   }
>
> For further information, the comment at the x86's efi_arch_mem_reserve()
> might be helpful.
>
>
> I hope this is correct and helps.
>
> Thanks,
>
> Nicolai