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[71.191.145.202]) by smtp.gmail.com with ESMTPSA id af79cd13be357-9494c5fd154sm262112585a.2.2026.11.13.12.41.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 12:41:34 -0800 (PST) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: Re: [PATCH] mm/hmm: reject shadow-stack mappings From: "Gregory Price (Meta)" To: Jason Gunthorpe Cc: "Gregory Price (Meta)" , 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)" In-Reply-To: <20261113200303.GA395967@nvidia.com> References: <20261113195536.2660763-1-gourry@gourry.net> <20261113200303.GA395967@nvidia.com> Date: Fri, 13 Nov 2026 15:41:33 -0500 Message-Id: <179460249376.482102.14669935557901467762.b4-reply@b4> X-Mailer: b4 0.17-dev 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