Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

David Hildenbrand
2023-01-23T09:51:12Z

On 19.01.23 22:22, Rick Edgecombe wrote:
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which requires some core mm changes to function
> properly.
>
> Since shadow stack memory can be changed from userspace, is both
> VM_SHADOW_STACK and VM_WRITE. But it should not be made conventionally
> writable (i.e. pte_mkwrite()). So some code that calls pte_mkwrite() needs
> to be adjusted.
>
> One such case is when memory is made writable without an actual write
> fault. This happens in some mprotect operations, and also prot_numa faults.
> In both cases code checks whether it should be made (conventionally)
> writable by calling vma_wants_manual_pte_write_upgrade().
>
> One way to fix this would be have code actually check if memory is also
> VM_SHADOW_STACK and in that case call pte_mkwrite_shstk(). But since
> most memory won't be shadow stack, just have simpler logic and skip this
> optimization by changing vma_wants_manual_pte_write_upgrade() to not
> return true for VM_SHADOW_STACK_MEMORY. This will simply handle all
> cases of this type.
>
> Cc: David Hildenbrand <david@redhat.com>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---

Instead of having these x86-shadow stack details all over the MM space,
was the option explored to handle this more in arch specific code?

IIUC, one way to get it working would be

1) Have a SW "shadowstack" PTE flag.
2) Have an "SW-dirty" PTE flag, to store "dirty=1" when "write=0".

pte_mkwrite(), pte_write(), pte_dirty ... can then make decisions based
on the "shadowstack" PTE flag and hide all these details from core-mm.

When mapping a shadowstack page (new page, migration, swapin, ...),
which can be obtained by looking at the VMA flags, the first thing you'd
do is set the "shadowstack" PTE flag.

--
Thanks,

David / dhildenb