Am 11.12.20 um 08:50 schrieb Thomas Hellström (Intel):
Hi, Christian
Thanks for the reply.
On 12/10/20 11:53 AM, Christian König wrote:
Am 09.12.20 um 17:46 schrieb Thomas Hellström (Intel):
On 12/9/20 5:37 PM, Jason Gunthorpe wrote:
On Wed, Dec 09, 2020 at 05:36:16PM +0100, Thomas Hellström (Intel)
wrote:
Jason, Christian
In most implementations of the callback mentioned in the subject
there's a
fence wait.
What exactly is it needed for?
Invalidate must stop DMA before returning, so presumably drivers using
a dma fence are relying on a dma fence mechanism to stop DMA.
Yes, so far I follow, but what's the reason drivers need to stop DMA?
Well in general an invalidation means that the specified part of the
page tables are updated, either with new addresses or new access flags.
In both cases you need to stop the DMA because you could otherwise
work with stale data, e.g. read/write with the wrong addresses or
write to a read only region etc...
Yes. That's clear. I'm just trying to understand the complete
implications of doing that.
Is it for invlidation before breaking COW after fork or something
related?
This is just one of many use cases which could invalidate a range.
But there are many more, both from the kernel as well as userspace.
Just imaging that userspace first mmaps() some anonymous memory r/w,
starts a DMA to it and while the DMA is ongoing does a readonly
mmap() of libc to the same location.
My understanding of this particular case is that hardware would
continue to DMA to orphaned pages that are pinned until the driver is
done with DMA, unless hardware would somehow in-flight pick up the new
PTE addresses pointing to libc but not the protection?
Exactly that is not guaranteed under all circumstances. Especially since
HMM tries to avoid grabbing a reference to the underlying pages. And it
depends when the destination addresses of the DMA are read and when the
access flags are evaluated.
But even when it causes no security problem the requirement we have to
fulfill here is that the DMA is coherent. In other words we either have
to delay updates to the page tables until the DMA operation is completed
or apply both address and access flag changes in a way the DMA operation
immediately sees it as well.
Regards,
Christian.
Thanks,
Thomas
Since most hardware doesn't have recoverable page faults guess what
would happen if we don't wait for the DMA to finish? That would be a
security hole you can push an elephant through :)
Cheers,
Christian.
Thanks,
Thomas
Jason
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