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 E69C03EDAD8 for ; Thu, 23 Apr 2026 12:47:27 +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=1776948448; cv=none; b=D9w51d9nMw/LW+xvagXH4qiGUFQc3pp8Em9ZIqO/h/lceHk/TPPAWKFXzO4i2luZlM3TodEaFAReHAnnVXJMr8xu+3e85mjv0wq+GjkMHqVYY7fQm778YXtiN9Qv43nwSu3NGo9cQmSKhEuYQB9UuBSpJI+wZyVsojBB4prsw4k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776948448; c=relaxed/simple; bh=BvepwwNd0J+yd//IBNL4vgYlnVadtAqRJ+ts3qFctMk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=e6i+kgncksJjrM+Ug2PyXyBQSdrA9ym7jLO6jg8dM1Qd9so84/IgoEEeN18ZL8pcwG0VKZ+ACkaNDCLRjnNCdGoaDrn504s0eloATWEyoc7YTPkfSwcPMcmJjyTj+aGINncR+SGqOOlchY36uvXh/Br/qTyZWMHQkoO9XV75ft4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FM34rKm0; 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="FM34rKm0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 94E6BC2BCAF; Thu, 23 Apr 2026 12:47:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776948447; bh=BvepwwNd0J+yd//IBNL4vgYlnVadtAqRJ+ts3qFctMk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=FM34rKm022qsNqEJyh/tZT+Bey1v/VykVSzsV/EbwIuH2x3+upga4IRqgSl+FiX8R 1a0dVDZOslXJNwLpIFxa3SLGWunSUYa7QNnXT96D1VOZHE11Gu5IiVtekw25TkQeQt WRRKRX1b1akRoeKiX0+3KiilHmKc6RH4FCBkYt91sTkx4QERBWCXilfjY2CFqhK7mv zA5Kp0AsOf/JJoVCE8HEWHCY0TWlGYfyXb1XYuMQ209ABgwPIdhCTt/3bUwSSbvU04 Fm041OhduNyQrQ9QUIvsUBqzvJ7VS1oqtml2GNEEezH9iPtwJW4UpxCvAdHZwyiX6L nl8POOqP7EwqA== Message-ID: Date: Thu, 23 Apr 2026 14:47:23 +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] 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: <2026042334-acutely-unadorned-e05c@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 14:31, 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 > Signed-off-by: Greg Kroah-Hartman > --- > My first foray into -mm, eeek! Oh, nommu ... what a great use of our time. I was briefly wondering if we want to add a Fixes: ... but then, this was likely broken for years and nobody cared so far in practice. > > Anyway, this was a crazy report sent to me, and I knocked up this > change, and I have a reproducer if people need/want to see that as well > (it's for nommu systems, so be wary of it.) [...] > - get_page(pages[i]); > + if (pages[i]) { > + /* > + * pin_user_pages*() arrives here with FOLL_PIN > + * set; unpin_user_page() (which is not > + * !CONFIG_MMU-specific) calls > + * gup_put_folio(..., FOLL_PIN) which subtracts > + * GUP_PIN_COUNTING_BIAS (1024). A bare > + * get_page() here adds only 1, so 1023 pins on > + * a fresh page bring refcount to 1024 and a > + * single unpin then frees it out from under the > + * remaining 1022 pins and any live VMA > + * mappings. Use the same grab path as the MMU > + * implementation so pin and unpin are > + * symmetric. > + */ Yeah, drop all that. Especially the hardcoded 1024/1022 is just screaming for trouble longterm. It just follows what we do everywhere else (e.g., follow_page_pte()). > + if (try_grab_folio(page_folio(pages[i]), 1, > + foll_flags)) { > + pages[i] = NULL; > + break; > + } > + } If it fails on the first iteration, we return -EFAULT instead of -ENOMEM. I know, I know, nobody cares. But if we touch it, we might just want to return the error we get from try_grab_folio(). -- Cheers, David