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

Edgecombe, Rick P
2023-01-28T00:51:12Z

On Fri, 2023-01-27 at 17:12 +0100, David Hildenbrand wrote:
> 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.

Ok, I'll give it a spin. So far it looks ok. The downside is the giant
tree-wide pte_mkwrite() signature change, but once that is over with
there are other advantages. Like getting rid of maybe_mkwrite()'s
awareness of shadow stack so the logic is more centralized. Please let
me know if you don't feel comfortable with a suggested-by credit tag.

Thanks,
Rick