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 2CCD93B52FD for ; Fri, 24 Apr 2026 11:31:36 +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=1777030297; cv=none; b=qemBu3mCV1x5w2YNI7IxojgWnW7eYhVFAmoQkITCNHaZNd/HuzwwccXq0N8Oz1jy7pXkhgHEGoSMH92R3Qxp+Jp4PWy/FMjuNPMhOdT4psc0KDUiksqP99Q6zKTmYzo+MtPJRrgbCFAO3wBK8TlELT/9uIiEuCdlZ81S7JrWNw8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777030297; c=relaxed/simple; bh=BrR2zw70w1S0+ZbZ2nSR3uE0R1clgnO0h3OBi9HvvMk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=pJMHxlFbuuvtgUa9oHm+5LLJvYu+vA07UpLBHMGVZxxvuBdOl/Xdj6iNtoX3w+GFY6c1afBINTQxJQ6KGIsrMzBAUlNibcsduUWyL9DpRZ93Txkdb41CXbZp4gwMPd9OziqfjIL+26ZOv1O4P4AMN5o762dmTWssHxp3WciT/lg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=YZVOQmjr; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="YZVOQmjr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6888EC19425; Fri, 24 Apr 2026 11:31:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1777030296; bh=BrR2zw70w1S0+ZbZ2nSR3uE0R1clgnO0h3OBi9HvvMk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=YZVOQmjrmmmNC9cm8eQzWxH23mvsbsvoGqgE92Lv81AlZDJt7jIpq42rbk8JA5MbK baU4Em941TZ3Q64/wk15fy6KHLlNNoqsyJuiPdeY5BUcPTr4Ilvtjc/DmUmHhCAcjw zb882hRQRFfSHK/1KQ8zFadOuEhJnUeWP6CwSpfU= Date: Fri, 24 Apr 2026 13:31:34 +0200 From: Greg Kroah-Hartman To: "David Hildenbrand (Arm)" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu Subject: Re: [PATCH v2] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked() Message-ID: <2026042431-charter-ranging-597c@gregkh> References: <2026042303-vendor-outright-b9d2@gregkh> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > 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! greg k-h