Re: [PATCH RFT v8 4/9] fork: Add shadow stack support to clone3()

On Fri, Aug 16, 2024 at 04:38:48PM +0100, Catalin Marinas wrote:

On Fri, Aug 16, 2024 at 02:52:28PM +0000, Edgecombe, Rick P wrote:

quoted

On the x86 side, we don't have a shadow stack access CMPXCHG. We will have to
GUP and do a normal CMPXCHG off of the direct map to handle it fully properly in
any case (CLONE_VM or not).

I guess we could do the same here and for the arm64 gcs_consume_token().
Basically get_user_page_vma_remote() gives us the page together with the
vma that you mentioned needs checking. We can then do a cmpxchg directly
on the page_address(). It's probably faster anyway than doing GUP twice.

There was some complication with get_user_page_vma_remote() while I was
working on an earlier version which meant I didn't use it, though with
adding checking of VMAs perhaps whatever it was isn't such an issue any
more.