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

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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

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@xxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

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.


David / dhildenb

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