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

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Date:   Fri, 27 Jan 2023 17:12:26 +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

On 26.01.23 21:19, Edgecombe, Rick P wrote:
> On Thu, 2023-01-26 at 09:46 +0100, David Hildenbrand wrote:
>> On 26.01.23 01:59, Edgecombe, Rick P wrote:
>>> On Wed, 2023-01-25 at 10:43 -0800, Rick Edgecombe wrote:
>>>> Thanks for your comments and ideas here, I'll give the:
>>>> pte_t pte_mkwrite(struct vm_area_struct *vma, pte_t pte)
>>>> ...solution a try.
>>>
>>> Well, it turns out there are some pte_mkwrite() callers in other
>>> arch's
>>> that operate on kernel memory and don't have a VMA. So it needed a
>>> new
>>
>> Why not pass in NULL as VMA then and document the semantics? The
>> less
>> similarly named but slightly different functions, the better :)
> 
> Hmm. The x86 and generic versions should probably have the same
> semantics, so then if you pass a NULL, it would do a regular
> pte_mkwrite() I guess?
> 
> I see another benefit of requiring the vma argument, such that raw
> pte_mkwrite()s are less likely to appear in core MM code. But I think
> the NULL is awkward because it's not obvious, to me at least, what the
> implications of that should be.
> 
> So it will be confusing to read in the NULL cases for the other archs.
> We also have some warnings to catch miss cases in the PTE tear down
> code, so the scenario of new code accidentally marking shadow stack
> PTEs as writable is not totally unchecked.
> 
> The three functions that do slightly different things are:
> 
> pte_mkwrite():
> Makes a PTE conventionally writable, only takes a PTE. Very clear that
> it is a low level helper and what it does.
> 
> maybe_mkwrite():
> Might make a PTE writable if the VMA allows it.
> 
> pte_mkwrite_vma():
> Makes a PTE writable in a specific way depending on the VMA
> 
> I wonder if the name pte_mkwrite_vma() is maybe just not clear enough.
> It takes a VMA, yes, but what does it do with it?
> 
> What if it was called pte_mkwrite_type() instead? Some arch's have
> additional types of writable memory and this function creates them. Of
> course they also have the normal type of writable memory, and
> pte_mkwrite() creates that like usual. Doesn't it seem more readable?

The issue is, the more variants we provide the easier it is to make 
mistakes and introduce new buggy code.

It's tempting to simply use pte_mkwrite() and call it a day, where 
people actually should use pte_mkwrite_vma().

Then, they at least have to investigate what to do about the second VMA 
parameter.

-- 
Thanks,

David / dhildenb