Re: [PATCH] mm/hmm: reject shadow-stack mappings

[Gregory Price (Meta)]

On 2026-11-13 16:03 -0400, Jason Gunthorpe wrote:
> On Fri, Nov 13, 2026 at 02:55:36PM -0500, Gregory Price (Meta) wrote:
> > HMM can grant ordinary device write access to a userspace shadow stack.
> > On x86, pte_write() is true for a shadow-stack PTE, so a populated page
> > can be returned with HMM_PFN_WRITE without taking a CPU write fault.
> 
> I'm not sure I understand this? The pte is writable but you don't want
> only DMA to write to it?
>
> Is there something special on x86 that makes the shadow stack pte
> non-writable by the CPU?
>

Some of the context is here:
https://lore.kernel.org/all/20261111-mason-migrate-et-al-fixes-v1-10-3d2454362e8d@xxxxxxxxxx/

We had a similar issue with migration, but this should handle the DMA
case as well.

It's "technically writable" but the kernel is supposed to prevent that.

I will say my understanding of the larger HMM picture is limited, so
there might be a better way than special-casing it like this.

-- 
~Gregory