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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Co09+6WR; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0F671606A884; Sat, 14 Nov 2026 11:11:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Co09+6WR Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1E9A41F000C4; Sat, 14 Nov 2026 11:11:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794654717; bh=9u58OC7k56Oh2FJNigWfAleT6Lj20pexcyKsyjL6XiE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Co09+6WRFlcwqsospnar4EGcys3raTpikkCYP/B/3HB4f81ufpt1wUl9dRZRvIZhq ir9DClZF0v02mvrLjG9ac+WLlrQLJ4v/Ei8y5zMBqTwZyMio8zGaL4DQ+7GCMKbNYP +m5AJzrCkD1hAIKSGungpJeChB2QdIrqVgvAvybVKHuwIYRe19veAKs+sA1zOFUnMD A8233TBM8kmwt1KsvmtXp8Y8SOtsARfV+MqjRICLQpPBKKuXxKXE9MB7e81VPPWguK JnGY8FJwND+KWuRYoChlKmFe4bS+T84Ec4sSLI+RBqKRoojRyIAsbWdwFmHNT7G44M CsbRtU5TnN0wg== Message-ID: <2fc85df3-cffc-4786-a7a8-0f0a6347fd59@kernel.org> Date: Sat, 14 Nov 2026 12:11:49 +0100 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hmm: reject shadow-stack mappings To: "Gregory Price (Meta)" , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, leon@kernel.org, akpm@linux-foundation.org, kas@kernel.org, dave.hansen@linux.intel.com, bp@alien8.de, rppt@kernel.org, stable@vger.kernel.org, "Lorenzo Stoakes (ARM)" References: <20261113195536.2660763-1-gourry@gourry.net> <20261113200303.GA395967@nvidia.com> <179460249376.482102.14669935557901467762.b4-reply@b4> <20261114002027.GE3706007@nvidia.com> <179464112539.482102.8723246686117488051.b4-reply@b4> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/14/26 08:25, Gregory Price (Meta) wrote: > On 2026-11-13 20:20 -0400, Jason Gunthorpe wrote: >> On Fri, Nov 13, 2026 at 03:41:33PM -0500, Gregory Price (Meta) wrote: >>> >>> 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. >> >> Bah, it's not writable at all. The CPU PTE has write disabled and >> someone thought it was a good idea to make pte_write() return true >> because the CPU allows writes through a special instruction on this >> special PTE? So now all the places like this think its writable, when >> it really isn't. >> > > I completely missed this, that will teach me to trust pte_write() I > guess... what a bizarre design choice. I'm sure there's "reasons". > >> You could also fix this by having something like pte_real_write() or >> whatever that doesn't lie about the write bit in the PTE :| >> > > or rename pte_write() to pte_fake_write() instead and restore > pte_write() to its former truthful glory It's writable by the hardware, that's what matters to core-mm, for example, when it comes to things like fork(), mproetect() etc. *nobody* besides selected core-mm code should ever write to random PTEs. They shouldn't ever even care about pte_write(). We have GUP for that, which takes care of that oddity properly when deciding whether we can *actually* write to a mapped page. Well, and we have HMM that reimplements part of GUP in ugly ways (sorry) that we missed. -- Cheers, David