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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 13 Aug 2019 15:55:07 -0700
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Subject: Re: [PATCH v8 15/27] mm: Handle shadow stack page fault
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
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On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> When a task does fork(), its shadow stack (SHSTK) must be duplicated
> for the child.  This patch implements a flow similar to copy-on-write
> of an anonymous page, but for SHSTK.
>
> A SHSTK PTE must be RO and dirty.  This dirty bit requirement is used
> to effect the copying.  In copy_one_pte(), clear the dirty bit from a
> SHSTK PTE to cause a page fault upon the next SHSTK access.  At that
> time, fix the PTE and copy/re-use the page.

Is using VM_SHSTK and special-casing all of this really better than
using a special mapping or other pseudo-file-backed VMA and putting
all the magic in the vm_operations?

--Andy