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

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Date:   Mon, 23 Jan 2023 20:47:05 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "david@...hat.com" <david@...hat.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 Mon, 2023-01-23 at 10:50 +0100, David Hildenbrand wrote:
> 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@...hat.com>
> > Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> > Tested-by: John Allen <john.allen@....com>
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.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".

I don't think that idea came up. So vma->vm_page_prot would have the SW
shadow stack flag for VM_SHADOW_STACK, and pte_mkwrite() could do
Write=0,Dirty=1 part. It seems like it should work.

> 
> 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.

I guess the downside is that it uses an extra software bit. But the
other positive is that it's less error prone, so that someone writing
core-mm code won't introduce a change that makes shadow stack VMAs
Write=1 if they don't know to also check for VM_SHADOW_STACK.