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

From: Thomas HellstrÃm (VMware)
Date: Tue Sep 10 2019 - 15:26:57 EST

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
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
Please take a look at dma_mmap_prepare and dma_mmap_fault in this

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.

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.

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.