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 BA9453BB9FC for ; Fri, 24 Apr 2026 12:16:24 +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=1777032984; cv=none; b=ler1tyP6rJIkYlFbST/ydyARVSXH7Y+0WMrws5tjC7kA10z04zgnszc7oQUzxcCvpSV8xX0kbStawFktu9DK8dfbZbbLCw8o2qYLJheIRUPlat6iVSlRfl1gcbxOkXWO3KCDUxdvX4VRjZdkXumUoFaIb1hBcmpy2UX6kjhy8aY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777032984; c=relaxed/simple; bh=bpg6Bd10fmGe+BQcNr+6kBcX4Z+i28lK24sYFJnz+ms=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kS6s/RkvC2is64mokDItczRDoB+D1WLz59tQVkBcaJyuVuSbTLwtXkhKtgBH9naxzQ+Bj/EHTYFYPTbgJh7H/wpBnloR8NNLl1hIQKt1tBWnXDZYFMwusdQSQxs4nPwaiHlUW2YHtixG/pHBUndgvoTJj4v1Bn2xWaAMgtoLfbE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=sJ0tb4HG; 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="sJ0tb4HG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B8B1BC19425; Fri, 24 Apr 2026 12:16:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777032984; bh=bpg6Bd10fmGe+BQcNr+6kBcX4Z+i28lK24sYFJnz+ms=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=sJ0tb4HGbbhdYuMn0qIVUVj0alcqc8Q8ueyANCykxd2TCkFMuXGBibV6SUCUQZnJ1 x+mM2INukLxebNBZxFbGvdrmiu1rWCripO9nlSufN7hL3SA5Bf6MLLjOuiXtpramAT BzIARPtuvBTYwe4MUV5F9fJaE/ermR4wDGggGMaOZReQCCdZKK2UXeizHWBFozJrhJ 4LZJp2lg0zfYlwCvahJuCmBRGPLtaQcm8sLCPWaamQG+ulNdEkwiWvtnCKjXzUBES0 /aO6NIr1ZOvk31kOtBG3I+GmMH96jRBKuXO52YeLLL77PvWT/XBw+5WsalZHD2py27 0ejZnETyN8Wiw== Message-ID: Date: Fri, 24 Apr 2026 14:16:21 +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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> <2026042431-charter-ranging-597c@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 4/24/26 13:31, Greg Kroah-Hartman wrote: > On Thu, Apr 23, 2026 at 05:55:56PM +0200, David Hildenbrand (Arm) wrote: >> 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! :) ) > > True, sorry, I guess people can "assist", I should have added that. > > If Andrew's tools automatically pick this up then: > > Assisted-by: David Hildenbrand I think we usually use Suggested-by:, but really I was just joking :) > >>> 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). > > Ick, I see Andrew already grabbed this so I'll just leave it for now, > thanks for the help and review! Andrew, can you add "While at it, don't return NULL pointers in the page array, as this is really not expected for GUP users; instead, just fail and return -EFAULT."? Thanks! -- Cheers, David