From: "Gregory Price (Meta)" <gourry@gourry.net>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "Gregory Price (Meta)" <gourry@gourry.net>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@meta.com, leon@kernel.org,
akpm@linux-foundation.org, david@kernel.org, kas@kernel.org,
dave.hansen@linux.intel.com, bp@alien8.de, rppt@kernel.org,
stable@vger.kernel.org, "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Subject: Re: [PATCH] mm/hmm: reject shadow-stack mappings
Date: Fri, 13 Nov 2026 15:41:33 -0500 [thread overview]
Message-ID: <179460249376.482102.14669935557901467762.b4-reply@b4> (raw)
In-Reply-To: <20261113200303.GA395967@nvidia.com>
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@kernel.org/
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
next prev parent reply other threads:[~2026-11-13 20:41 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-13 19:55 Gregory Price (Meta)
2026-11-13 19:57 ` Lorenzo Stoakes (ARM)
2026-11-13 20:15 ` Gregory Price (Meta)
2026-11-13 20:03 ` Jason Gunthorpe
2026-11-13 20:41 ` Gregory Price (Meta) [this message]
2026-11-13 20:03 ` David Hildenbrand (Arm)
2026-11-13 20:42 ` Gregory Price (Meta)
2026-11-13 21:19 ` David Hildenbrand (Arm)
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