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 53F263E0C65 for ; Thu, 23 Apr 2026 13:00:47 +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=1776949247; cv=none; b=BNIsqBC5AgDGYwfQhisFeoR4dwYXFimA/SFOrQeCr9JocbsVYA36S1U0pL9AP24SABms7/a/faQfJjFb0rDYNok61Y8vFvLf7M3qH2NAwvAMcJT8JvzI81lxQaV4nNutwpbOb1Jm2wZyJ/FqgcNoBxcPO+You0twN3l9Dt4iSww= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776949247; c=relaxed/simple; bh=jMWsIhaVRBlToE55+8Oeeq5aDuLS3n8Yq9bOM/6UQeU=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=U8/vLfYZu9pGoLU4/rcmSA05V2HOhPBh0+9LdmjGdV5hnZHMAVPY0qZhobZI+Z/ZLQvlLvrCkYxDvOiSP7deKhfdXXK+fRnebKsDznUc38INxZ0XX/NEOuN56HxPoTZb6CgG3oINudKVvAMzskgAhQMJDBxriZIA2USlOk7fYIU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HkwlU+VT; 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="HkwlU+VT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 18D20C2BCAF; Thu, 23 Apr 2026 13:00:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776949247; bh=jMWsIhaVRBlToE55+8Oeeq5aDuLS3n8Yq9bOM/6UQeU=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=HkwlU+VTF2GZq0Mo3BmGUtRyD+ABo6lL5nb6wpNZBoDZBz9pyOjQtrVKVYc15+UTY uc/EMl/z3olOl/7n+KiI2RseEXpEuTzl9aF04Gz5Zfp83spptVD3udHETvzYr/1uBN SivqnzQX6MS4luWWHjYEGa+E7j7oTplqX1pO/akiEunEjuI5+SwF6oUotuRJzYHlZf 2xR65jFxPKCyoVr2H6/ZY1bZir0b3wBC8/f3fDarzrIJxoU4sl6JwN3my7LV/1WJNl 4pvTJV9JDEAdAlNswrfWD/bSgXQLVcaY6kyOidERdS+ijzzhMhNRFF+kXbh4g/VUMw mR4kRS4ReR+Pw== Message-ID: Date: Thu, 23 Apr 2026 15:00:42 +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() From: "David Hildenbrand (Arm)" 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> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/23/26 14:47, David Hildenbrand (Arm) wrote: > 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(). > BTW, looking into this, I am not sure if continuing on !pages[i] is a sane thing to do. IIRC, we are not supposed to return NULL-page pointer from this function. We support it in unpin_user_pages() only for internal purposes. unpin_user_pages_dirty_lock() does not support/expect that. That should likely be an -EFAULT (if the first page). -- Cheers, David