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 7A1C7369217 for ; Thu, 23 Apr 2026 15:56:01 +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=1776959761; cv=none; b=OgA2/bEBr1P+DuyXhNk/ZZyEzzH9UEHKL0Z9Y0SOZ+6BtYkaLG0ZTAYVrsD9lvCDifoeyd02UwPOFU9kepbETbhf2s1BsnKVYQz+tppS1qb//e6X1WPFpyvF8U6aM9iq9fH3Ba42+xZbEwVl1k1fA6haiTf2+gew00/1p9qNdkM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776959761; c=relaxed/simple; bh=0L5e/EPnegp4JHS0wXmUj5FxsmR4LIX+3gaFnChJO6s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TW3u9QVa4tjdsqpYIno2O1qJGZnPBF7gFqwJ5aNMAEnR6BrLhEikLWs5jldgUPbcluYa5wsOCbdvZwzDiUhiSIvHdbbxgpCNInXGF7jJ2WwxvvviJT56k1Gn6zXyOIWxxIf5eLFwibvCY/TEQhva952CFlX9Pfuy5dbwMGQWLo8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pEhBq3vd; 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="pEhBq3vd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F236BC2BCAF; Thu, 23 Apr 2026 15:55:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776959761; bh=0L5e/EPnegp4JHS0wXmUj5FxsmR4LIX+3gaFnChJO6s=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pEhBq3vdwrjld+IR22eeekTh4+a7rsdskikeEVkX/tZ9JK7SIwOSiFuyezbIQGF/G 4+It6V9BojzewleJ5ErtRo+QcDhD5h6h6tNoxhbHJcLOQlyGIiv7GM5ade5pHMGVdp zt8SoXvHT3ZjeQYk0TYI2Wg11xUfCw2GGzreWVwMTw1oRzXKKQx/sYTxDPWQDIrf7D q2L92KkHygasjzOvRtHqekUD7PJZnTw7/0HHo9v8nJxU6eZ+B5A/8wQ6aRNpL5J82v BodwqsZT80AwLa79kq5nr9f/boblucydGsIpyUbpvx88yO92wNK3swcNny/QtOX59R a1LvbZZBpupJg== Message-ID: Date: Thu, 23 Apr 2026 17:55:56 +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: Greg Kroah-Hartman , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> 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 4/23/26 16:28, 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. > > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Peter Xu > Reported-by: Anthropic > Assisted-by: gkh_clanker_t1000 Assisted-by: David :( (no, I'm not a tool! :) ) > Signed-off-by: Greg Kroah-Hartman > --- > v2: - drop huge comment > - rework error return value based on David's suggestion (heck, > pretty much the full patch was written by him now) > Link to v1: https://lore.kernel.org/r/2026042334-acutely-unadorned-e05c@gregkh > > mm/gup.c | 13 ++++++++++--- > 1 file changed, 10 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index ad9ded39609c..2f6f95a167af 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -1983,6 +1983,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, > struct vm_area_struct *vma; > bool must_unlock = false; > vm_flags_t vm_flags; > + int ret, err = -EFAULT; > long i; > > if (!nr_pages) > @@ -2019,8 +2020,14 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, > > if (pages) { > pages[i] = virt_to_page((void *)start); > - if (pages[i]) > - get_page(pages[i]); > + if (!pages[i]) > + break; Best to mention that change in the patch description. I really think this is the right thing to do (returning NULL in the page array is just very dubious). Acked-by: David Hildenbrand (Arm) -- Cheers, David