From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3D3233DA7C2 for ; Fri, 24 Apr 2026 14:19:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777040376; cv=none; b=X3D3b8I5754ePXUFxkyye1aOnFZ6cFAgLBdb/mDRD26l59YZj3eYqKcRQoQzqKyWrYc015HaEmZBPZdOmRk/VM8DgLK8504E4tjZ4UNFSTvK0mor8U9lKC+7BbPizKcpUSGIHXSXstfKxtRUqpAXzeBSBz1VIFVNEjA9jEAT8kA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777040376; c=relaxed/simple; bh=CxHM58NpmQrbLpCcUPXfUThwYQpzZAqUhNDe2CK5d54=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JCNdJh8OscZJpZlgFDwqS/pN1WiW5gAFoAGwsKZHEjx684ZobCvJzFXO0SM7X3FgfBE4fwNP0oxKn5+49TgFC1lahDwWSJLT4vA8klFAxI4DhVuvJ7b8mqmCcBcVLJ6n9cLNfzcFoFvgh/qciacq7ySNsZFQuMYs1l2go9GfwZA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GrdIEGT5; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GrdIEGT5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D526FC2BCB2; Fri, 24 Apr 2026 14:19:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777040376; bh=CxHM58NpmQrbLpCcUPXfUThwYQpzZAqUhNDe2CK5d54=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=GrdIEGT5dd29aeRSp9p9Xj/eqqtbMHDNGU3SqHHSNemLYtVxmK/iokYP3Xk+RTCcC OyrtHWW9npnZScQ958cViCG6tx7M/zz/bj5LwaXeUzhs5rMmAUC6fKinkMLeZbxbv4 zs19Cpf6pvfYqFQQ79DJyG0mP1YE5VpAPtB/DSb5CTWQ05zVVyJvUoCp3C+L5zYyN2 PoMfFRXHPJWFBjRMVEPacWU1d3vaB2MxyQY1Pv2jdbW+90z7A/2nwWt/764WOA7MIN 9x1MZCjjyx8JxlWTh3K49J7IUP9EcHx0eJALHygs/tizIuycfm9LB3KUbXCm9fMH7z qmJIyTgr/IQqw== Message-ID: Date: Fri, 24 Apr 2026 16:19:32 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked() To: Andrew Morton , Greg Kroah-Hartman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> <20260424063817.552a894d4b5bfa4de40792fa@linux-foundation.org> 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 4/24/26 15:38, Andrew Morton wrote: > On Thu, 23 Apr 2026 16:28:04 +0200 Greg Kroah-Hartman wrote: > >> The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare >> get_page() reference for each page regardless of foll_flags: >> if (pages[i]) >> get_page(pages[i]); >> >> This is reached from pin_user_pages*() with FOLL_PIN set. >> unpin_user_page() is shared between MMU and NOMMU configurations and >> unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts >> GUP_PIN_COUNTING_BIAS (1024) from the folio refcount. >> >> This means that pin adds 1, and then unpin will subtract 1024. >> >> If a user maps a page (refcount 1), registers it 1023 times as an >> io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then >> unregisters: the first unpin_user_page subtracts 1024, refcount hits 0, >> the page is freed and returned to the buddy allocator. The remaining >> 1022 unpins write into whatever was reallocated, and the user's VMA >> still maps the freed page (NOMMU has no MMU to invalidate it). >> Reallocating the page for an io_uring pbuf_ring then lets userspace >> corrupt the new owner's data through the stale mapping. >> >> Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1 >> for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are >> symmetric. > > Battle of the bots? > https://sashiko.dev/#/patchset/2026042303-vendor-outright-b9d2@gregkh It references the if (pages && !(flags & FOLL_PIN)) flags |= FOLL_GET; I'm not sure if there is actual NOMMU code that triggers it. For example, uprobes uses that pattern, but I suspect that that's not a thing on NOMMU. Probably best to just squash: diff --git a/mm/gup.c b/mm/gup.c index ad9ded39609c..44bd28cf6e00 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1988,6 +1988,9 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, if (!nr_pages) return 0; + if (pages && !(foll_flags & FOLL_PIN)) + foll_flags |= FOLL_GET; + /* * The internal caller expects GUP to manage the lock internally and the * lock must be released when this returns. -- Cheers, David