Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

[Koenig, Christian]

Am 10.09.19 um 21:26 schrieb Thomas HellstrÃm (VMware):
> On 9/10/19 6:11 PM, Andy Lutomirski wrote:
>>
>>> On Sep 5, 2019, at 8:24 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> 
>>> wrote:
>>>
>>>> On Thu, Sep 05, 2019 at 05:21:24PM +0200, Thomas HellstrÃm (VMware) 
>>>> wrote:
>>>>> On 9/5/19 4:15 PM, Dave Hansen wrote:
>>>>> Hi Thomas,
>>>>>
>>>>> Thanks for the second batch of patches! These look much improved 
>>>>> on all
>>>>> fronts.
>>>> Yes, although the TTM functionality isn't in yet. Hopefully we 
>>>> won't have to
>>>> bother you with those though, since this assumes TTM will be using 
>>>> the dma
>>>> API.
>>> Please take a look at dma_mmap_prepare and dma_mmap_fault in this
>>> branch:
>>>
>>> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-mmap-improvements
>>>
>>> they should allow to fault dma api pages in the page fault handler. 
>>> But
>>> this is totally hot off the press and not actually tested for the last
>>> few patches. Note that I've also included your two patches from this
>>> series to handle SEV.
>> I read that patch, and it seems like youâve built in the assumption 
>> that all pages in the mapping use identical protection or, if not, 
>> that the same fake vma hack that TTM already has is used to fudge 
>> around it. Could it be reworked slightly to avoid this?
>>
>> I wonder if itâs a mistake to put the encryption bits in vm_page_prot 
>> at all.
>
> From my POW, the encryption bits behave quite similar in behaviour to 
> the caching mode bits, and they're also in vm_page_prot. They're the 
> reason TTM needs to modify the page protection in the fault handler in 
> the first place.
>
> The problem seen in TTM is that we want to be able to change the 
> vm_page_prot from the fault handler, but it's problematic since we 
> have the mmap_sem typically only in read mode. Hence the fake vma 
> hack. From what I can tell it's reasonably well-behaved, since 
> pte_modify() skips the bits TTM updates, so mprotect() and mremap() 
> works OK. I think split_huge_pmd may run into trouble, but we don't 
> support it (yet) with TTM.

Ah! I actually ran into this while implementing huge page support for 
TTM and never figured out why that doesn't work. Dropped CPU huge page 
support because of this.

>
> We could probably get away with a WRITE_ONCE() update of the 
> vm_page_prot before taking the page table lock since
>
> a) We're locking out all other writers.
> b) We can't race with another fault to the same vma since we hold an 
> address space lock ("buffer object reservation")
> c) When we need to update there are no valid page table entries in the 
> vma, since it only happens directly after mmap(), or after an 
> unmap_mapping_range() with the same address space lock. When another 
> reader (for example split_huge_pmd()) sees a valid page table entry, 
> it also sees the new page protection and things are fine.

Yeah, that's exactly why I always wondered why we need this hack with 
the vma copy on the stack.

>
> But that would really be a special case. To solve this properly we'd 
> probably need an additional lock to protect the vm_flags and 
> vm_page_prot, taken after mmap_sem and i_mmap_lock.

Well we already have a special lock for this: The reservation object. So 
memory barriers etc should be in place and I also think we can just 
update the vm_page_prot on the fly.

Christian.

>
> /Thomas
>
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>
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