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 77A4331715C; Wed, 15 Apr 2026 05:44:30 +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=1776231870; cv=none; b=HdJsdOxDhvkWho6AQoqHfQuShaRj/xIpqWYkkKUAvWmtY4VVv8Ah4LTQHKzKeq9VAdAiXhxNInfPVf3DVzSSVSbJFQVx2BQFZcauBw/JVoVw8+oO7UjS4Aop5SvTVyK6wJ2h/dx3XTkpR74rzcfckcAaNBI8mVP3XeB0VAMqf10= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776231870; c=relaxed/simple; bh=ULEdZKOlyHUFAHTvKEpw6ZXT98VHr7FG4P/ABfzjD5U=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=JfxxgdGNlGmVcMFB0LXydMNdAX0w6hl7TfVsNdwZ4eDWMBfSYynbA73UmLBGsjEWRPi+Rcv2HfdMdMkOQCxiswXHolr4BCxAvWXyImXm9wSqBTCko+eC0HJadU9Q2iCsY2sGznp5k0BT0akIiS+gCr4K5Wrb0HbiZsmQD8J/7zI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=OK21Xv7f; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="OK21Xv7f" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D201AC2BCB0; Wed, 15 Apr 2026 05:44:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1776231870; bh=ULEdZKOlyHUFAHTvKEpw6ZXT98VHr7FG4P/ABfzjD5U=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=OK21Xv7fgD18lW8YMItHZM5UiVgKPJ8r66Ohfg321bvYzgZHvjqvQSxNpDtzUwTkX jo1owznB7qT7j668xhhQ6UBAdR2PcKQ0sfqmvB1s0Ik4998BzvI3yJ9MLMFPXzCld1 PrWud/iFUoL+ds6JxwJXiX02KVDDhQ96eZ+Nd0js= Date: Tue, 14 Apr 2026 22:44:21 -0700 From: Andrew Morton To: Muchun Song Cc: David Hildenbrand , Oscar Salvador , Charan Teja Kalla , Muchun Song , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org Subject: Re: [PATCH] mm/sparse: Fix race on mem_section->usage in pfn walkers Message-Id: <20260414224421.c030868f5960ad0115ac1668@linux-foundation.org> In-Reply-To: <20260415022326.53218-1-songmuchun@bytedance.com> References: <20260415022326.53218-1-songmuchun@bytedance.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 15 Apr 2026 10:23:26 +0800 Muchun Song wrote: > When memory is hot-removed, section_deactivate() can tear down > mem_section->usage while concurrent pfn walkers still inspect the > subsection map via pfn_section_valid() or pfn_section_first_valid(). > > After commit 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing > memory_section->usage") converted the teardown to an RCU-based > scheme, the code still relies on SECTION_HAS_MEM_MAP becoming visible > to readers before ms->usage is cleared and queued for freeing. > > That ordering is not guaranteed. section_deactivate() can clear > ms->usage and queue kfree_rcu() before another CPU observes the > SECTION_HAS_MEM_MAP clear. A concurrent pfn walker can therefore see > valid_section() return true, enter its sched-RCU read-side critical > section after kfree_rcu() has already been queued, and then dereference > a stale ms->usage pointer. Then what happens? Can it oops? > And pfn_to_online_page() can call pfn_section_valid() without its > own sched-RCU read-side critical section, which has similar problem. > > The race looks like this: > > compact_zone() memunmap_pages > ============== ============== > __remove_pages()-> > sparse_remove_section()-> > section_deactivate(): > a) [ Clear SECTION_HAS_MEM_MAP > is reordered to b) ] > kfree_rcu(ms->usage) > __pageblock_pfn_to_page > ...... > pfn_valid(): > rcu_read_lock_sched() > valid_section() // return true > pfn_section_valid() > [Access ms->usage which is UAF] > WRITE_ONCE(ms->usage, NULL) > rcu_read_unlock_sched() b) Clear SECTION_HAS_MEM_MAP > > Fix this by using rcu_replace_pointer() when clearing ms->usage in > section_deactivate(), then it does not rely on the order of clearing > of SECTION_HAS_MEM_MAP. > > Fixes: 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing memory_section->usage") December 2023. > Signed-off-by: Muchun Song > --- > This patch is focused on the ms->usage lifetime race only. > > ... > > I am not fully sure whether that reasoning is correct, or whether current > callers are expected to rely on additional hotplug serialization instead. > Comments on whether this is a real issue, and how the vmemmap lifetime is > expected to be handled here, would be very helpful. Thanks. Quite a bit for consideration. > --- a/mm/sparse-vmemmap.c > +++ b/mm/sparse-vmemmap.c > @@ -601,8 +601,10 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages, > * was allocated during boot. > */ > if (!PageReserved(virt_to_page(ms->usage))) { > - kfree_rcu(ms->usage, rcu); > - WRITE_ONCE(ms->usage, NULL); > + struct mem_section_usage *usage; > + > + usage = rcu_replace_pointer(ms->usage, NULL, true); > + kfree_rcu(usage, rcu); > } > memmap = pfn_to_page(SECTION_ALIGN_DOWN(pfn)); > } This part isn't applicable to 7.0 - it depends on material I've sent to Linus for 7.1-rc1. So for now I'll drop this into mm-unstable to get it some runtime testing. If people like this patch and we decide to proceed with it then I can make it a hotfix for 7.1-rcX. But the -stable people will be wanting a backportable version of it, if we decide to backport,