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

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Date:   Fri, 27 Jan 2023 17:19:59 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
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Cc:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

> 
> Now shadow stack memory creation is tightly controlled. Either created
> via special syscall or automatically with a new thread.

Good, it would be valuable to document that somewhere ("Neve rapplies to 
VM_SHARED|VM_MAYSHARE VMAs").

[...]

>>
>> The other thing I had in mind was that we have to make sure that
>> we're
>> not accidentally setting "Write=0,Dirty=1" in mk_pte() /
>> pte_modify().
>>
>> Assume we had a "Write=1,Dirty=1" PTE, and we effectively wrprotect
>> using pte_modify(), we have to make sure to move the dirty bit to
>> the
>> saved_dirty bit.
> 
> For the mk_pte() case, I don't think a Write=0,Dirty=1 prot could come
> from anywhere. I guess the MAP_SHARED case is a little less bounded. We
> could maybe add a warning for this case.

Right, Write=0,Dirty=1  shouldn't apply at that point if shstk are 
always wrprotected as default.

> 
> For the pte_modify() case, this does happen. There are two scenarios
> considered:
> 1. A Write=0,Dirty=0 PTE is made dirty. This can't happen today as
> Dirty is filtered via _PAGE_CHG_MASK. Basically pte_modify() doesn't
> support it.

It should simply set the saved_dirty bit I guess. But I don't think 
pte_modify() is actually supposed to set PTEs dirty (primary goal is to 
change protection IIRC).

> 2. A Write=1,Dirty=1 PTE gets write protected. This does happen because
> the Write=0 prot comes from protection_map, and pte_modify() would
> leave the Dirty=1 bit alone. The main case I know of is mprotect(). It
> is handled by changes to pte_modify() by doing the Dirty->SoftDirty
> fixup if needed.

Right, we'd have to move the dirty bit to the saved_dirty bit. (we have 
to handle soft-dirty, too, whenever setting the PTE dirty -- either via 
the dirty bit or via the saved_dirty bit)

> 
> So pte_modify()s job should not be too tricky. What you can't do with
> it though, is create shadow stack PTEs. But it is ok for our uses
> because of the explicit mkwrite().

I think you are correct.

-- 
Thanks,

David / dhildenb