From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 F2A7652CCEF for ; Tue, 22 Sep 2026 10:09:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790071766; cv=none; b=qcxJAnggnD8TkkjhJACdJcne/zLoLMODvwDPDEO+3Rb2ZT4+cFv/s37ougckn6llpgWjMt9zuH1NrEyDm1bGbPruvJkyavq2JERTlumrdmMEgWMQ2fz2cAJkSya/516M/XkEL4l6T6ksYY4PRQ6+Ye2YGVpV59Q6U7EgprFMwpA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790071766; c=relaxed/simple; bh=uxFw/qBNbycNG898Jw8zf/MBPorwy3XrK1Zb/NPC0Fc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ew9cQ+Q75xuIKKytKsrEDTzGMPs2Ptn36GnnNqoCmMa7fW2OUKroz7KaxPYc26qadqpeVPbxdscmA5vnr7umSOa4LU0dYBrtU8SnAgIW2RPvO82MzTq9K5Mj0c2Izl5OWMayl0IgC80Iid225QV3G2z2QpybNB/ldXgpKuKK5mI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fcPxk3XB; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fcPxk3XB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C75A1F00893; Tue, 22 Sep 2026 10:09:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790071757; bh=M515vgULO6sESHPjvy0cqdT6MwRgWhcOocOQQdC2Amo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fcPxk3XBeyCU+TYL6/ABeyCZ5xmnCeJqQF+1hREGVIBwOYty7TEWMIygJ0G7kWc4y 7JrSJ1qi8Voe0d6ovVhPYaMGwduBb3gV6fUZCL0nA4EIhdFUPjS93z4lc0Ap4jE6oY kkMmg1PMzj20JBAPU3B+2mON1xz6NbYId4b3461xBDcaz3xd+vQqRdQne7Vv3FzYyf 5+j6OL6nzzsxPiuUBFvk+4fX7SxYoMOL9J0McXKrtiA7Pkl3ksDoo5ZnXqiHZIrLyV SUdGTJo+r2njBjeQRmLUmaXHqmf0FQQylExGMR2326IWVhiac8E5vvcn7Jbhi2AeKD hgO5m81MjA0Ug== Message-ID: Date: Tue, 22 Sep 2026 12:09:12 +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 v4] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked() To: Andrew Morton , Jason Gunthorpe , John Hubbard , Greg Kroah-Hartman , Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang References: <20260921-nommu_gup_pin-v4-1-c410041cc479@kernel.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 9/21/26 11:57, David Hildenbrand (Arm) wrote: > From: Greg Kroah-Hartman > > 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. Keep supporting the traditional behavior where users specify > a pages array but don't set FOLL_GET. > > 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. > > [ david: support traditional behavior with no FOLL_GET, extend > description ] > > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Peter Xu > Cc: Lance Yang > Reported-by: Anthropic > Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages") > Assisted-by: gkh_clanker_t1000 > Signed-off-by: Greg Kroah-Hartman > Signed-off-by: David Hildenbrand (Arm) > --- > v3 -> v4: > * Put FOLL_GET fixup into the NOMMU __get_user_pages_locked() > > I'll queue the updated version after some more review. > --- > mm/gup.c | 17 ++++++++++++++--- > 1 file changed, 14 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index eb898ea1ee22e..4e9b5ac2cb62e 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) > @@ -1999,6 +2000,10 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, > *locked = 1; > } > > + /* See the MMU variant: support the traditional behavior. */ > + if (pages && !(foll_flags & FOLL_PIN)) > + foll_flags |= FOLL_GET; > + > /* calculate required read or write permissions. > * If FOLL_FORCE is set, we only require the "MAY" flags. > */ > @@ -2019,8 +2024,14 @@ 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; > + ret = try_grab_folio(page_folio(pages[i]), 1, foll_flags); BTW Sashiko reports: "Does this code leak the folio if it is a slab page?" ... "When the operation completes, the caller releases the page via put_page(). However, put_page() explicitly checks folio_test_slab() and returns without decrementing the refcount." get_page() contains if (WARN_ON_ONCE(folio_test_slab(folio))) for a good reason. It's not supposed to happen. So if we would get slab pages here we'd be in "preexisting problem" territory. As slab pages have a frozen refcount, we'd similarly trigger if (WARN_ON_ONCE(folio_ref_count(folio) <= 0)) Now in try_grab_folio and refuse the operation. So I'll ignore this one. -- Cheers, David