Re: [PATCH] ARM: Mark the FDT_FIXED sections as shareable

[Leizhen (ThunderTown)]

On 2022/6/7 1:12, Ard Biesheuvel wrote:
> On Mon, 6 Jun 2022 at 17:52, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>>
>> Hello Zhen Lei,
>>
>> On Mon, 6 Jun 2022 at 14:49, Zhen Lei <thunder.leizhen@xxxxxxxxxx> wrote:
>>>
>>> commit 7a1be318f579 ("ARM: 9012/1: move device tree mapping out of linear
>>> region") use FDT_FIXED_BASE to map the whole FDT_FIXED_SIZE memory area
>>> which contains fdt. But it only reserves the exact physical memory that
>>> fdt occupied. Unfortunately, this mapping is non-shareable. An illegal or
>>> speculative read access can bring the RAM content from non-fdt zone into
>>> cache, PIPT makes it to be hit by subsequently read access through
>>> shareable mapping(such as linear mapping), and the cache consistency
>>> between cores is lost due to non-shareable property.
>>>
>>> |<---------FDT_FIXED_SIZE------>|
>>> |                               |
>>>  -------------------------------
>>> | <non-fdt> | <fdt> | <non-fdt> |
>>>  -------------------------------
>>>
>>> 1. CoreA read <non-fdt> through MT_ROM mapping, the old data is loaded
>>>    into the cache.
>>> 2. CoreB write <non-fdt> to update data through linear mapping. CoreA
>>>    received the notification to invalid the corresponding cachelines, but
>>>    the property non-shareable makes it to be ignored.
>>> 3. CoreA read <non-fdt> through linear mapping, cache hit, the old data
>>>    is read.
>>>
>>
>> Thanks for the excellent write-up, and for what must have been a lot
>> of work to narrow down and diagnose!

Yes, it took a lot of time, a lot of boards.

>>
>>> To eliminate this risk, mark the MT_ROM sections as shareable.
>>>
>>> The other user of MT_ROM is XIP_KERNEL. XIP allows the kernel to run from
>>> flash to save RAM space. Not sure if anyone is still using XIP in order to
>>> save a little memory and not care about performance degradation. Add a new
>>> memory type MT_ROM_XIP to be compatible with it.
>>>
>>> BTW: Another solution is to memblock_reserve() all the sections that fdt
>>> spans, but this will waste 2-4MiB memory.
>>>
>>
>> I agree that we should not add shareable attributes to the memory type
>> used by XIP kernels for code regions: NOR flash is not usually
>> integrated in a way that allows it to participate in the coherency
>> protocol, so that will likely break things.
>>
>> I think, though, that it would be better to leave MT_ROM alone, and
>> introduce a new type MT_MEMORY_RO instead, which is wired up in the
>> right way (see below), so that we get NX attributes, and can use it to
>> create non-section mappings as well.

Right, NX should also be set. I will try MT_MEMORY_RO.

>>
>> Then, as a followup which does not need to go into -stable, we can
>> reduce the size of the mapping: there is really no need for the
>> permanent mapping to be section granular - this is only for the early
>> asm code that is not able to create 2 levels of page tables.
>>
> 
> Actually, on second thought, I think reducing the size of the FDT
> mapping is also needed for correctness, as the non-fdt regions could
> potentially be covered by a no-map memory reservation, or get mapped
> non-cacheable for things like non-coherent DMA.

I'll keep the section mapping first, because the fix for adding the
shareable attribute is explicit.

> .
> 

-- 
Regards,
  Zhen Lei