From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 2A55428CF41 for ; Mon, 7 Jul 2025 09:13:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751879640; cv=none; b=k6zf8UhcVdTfyxQVHm7B7NXueVafgirc4pSMCeI3vKMfLS3EqpZyWL01XVSTO9JcVhjN9UBOVsKFwoKHPfn0LNjB+2pgn1wUkUoBXLzeEiVCdZHWxhVr8a1wLTCR8sA825Fe9KcOGWFICRuY7Rfn8KRSv4c3/mIDmAoG/KlNL6M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751879640; c=relaxed/simple; bh=1QLjQcEr8pINe9brC2KlEBgSw99MZ6AV9to49+sZsrU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=A7S6NFPUUm8YoJWob4AV9LypHffQLhv5m6o+W3tE4BpufvB43lkIAAWoeLd74kFjimIxraquOR1VWJl6Q3tK1xz2+3ivr6AauzDXPHpArfMow9gLX0Mp7WGiECpkOpmDPe8j1voiJhxSmH2lO+TpkZ++7TsfuV6uprsr43iNyfg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ZfLbEP/v; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ZfLbEP/v" Message-ID: <072268ae-3dea-46f8-8c9e-203d062eab82@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1751879626; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CgyqmMgmj4RoHL2MEMeBZbWgZV9nUOOsFAtiV0PoOpY=; b=ZfLbEP/vZq2o4+akgW+lc4Afrvl/00HiSK0NwhRm0A7WUXvct0BrON8rtglHmdzeq5KYSC Xv/RTbWP+ZfT7gszBHH2OFBdN1XVATLskmoyuwGiwM4ktvtQnzZuftIU1kgvxkqx71qwzm jpzJevRaDNfnRPuYyjDh/EXfDo5znqo= Date: Mon, 7 Jul 2025 17:13:24 +0800 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v4 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap Content-Language: en-US To: Harry Yoo Cc: akpm@linux-foundation.org, david@redhat.com, 21cnbao@gmail.com, baolin.wang@linux.alibaba.com, chrisl@kernel.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, v-songbaohua@oppo.com, x86@kernel.org, huang.ying.caritas@gmail.com, zhengtangquan@oppo.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, mingzhe.yang@ly.com, stable@vger.kernel.org, Barry Song , Lance Yang References: <20250701143100.6970-1-lance.yang@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2025/7/7 13:40, Harry Yoo wrote: > On Tue, Jul 01, 2025 at 10:31:00PM +0800, Lance Yang wrote: >> From: Lance Yang >> >> As pointed out by David[1], the batched unmap logic in try_to_unmap_one() >> may read past the end of a PTE table when a large folio's PTE mappings >> are not fully contained within a single page table. >> >> While this scenario might be rare, an issue triggerable from userspace must >> be fixed regardless of its likelihood. This patch fixes the out-of-bounds >> access by refactoring the logic into a new helper, folio_unmap_pte_batch(). >> >> The new helper correctly calculates the safe batch size by capping the scan >> at both the VMA and PMD boundaries. To simplify the code, it also supports >> partial batching (i.e., any number of pages from 1 up to the calculated >> safe maximum), as there is no strong reason to special-case for fully >> mapped folios. >> >> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com >> >> Cc: >> Reported-by: David Hildenbrand >> Closes: https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com >> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") >> Suggested-by: Barry Song >> Acked-by: Barry Song >> Reviewed-by: Lorenzo Stoakes >> Acked-by: David Hildenbrand >> Signed-off-by: Lance Yang >> --- > > LGTM, > Reviewed-by: Harry Yoo Hi Harry, Thanks for taking time to review! > > With a minor comment below. > >> diff --git a/mm/rmap.c b/mm/rmap.c >> index fb63d9256f09..1320b88fab74 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2206,13 +2213,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >> hugetlb_remove_rmap(folio); >> } else { >> folio_remove_rmap_ptes(folio, subpage, nr_pages, vma); >> - folio_ref_sub(folio, nr_pages - 1); >> } >> if (vma->vm_flags & VM_LOCKED) >> mlock_drain_local(); >> - folio_put(folio); >> - /* We have already batched the entire folio */ >> - if (nr_pages > 1) >> + folio_put_refs(folio, nr_pages); >> + >> + /* >> + * If we are sure that we batched the entire folio and cleared >> + * all PTEs, we can just optimize and stop right here. >> + */ >> + if (nr_pages == folio_nr_pages(folio)) >> goto walk_done; > > Just a minor comment. > > We should probably teach page_vma_mapped_walk() to skip nr_pages pages, > or just rely on next_pte: do { ... } while (pte_none(ptep_get(pvmw->pte))) > loop in page_vma_mapped_walk() to skip those ptes? Good point. We handle partially-mapped folios by relying on the "next_pte" loop to skip those ptes. The common case we expect to handle is fully-mapped folios. > > Taking different paths depending on (nr_pages == folio_nr_pages(folio)) > doesn't seem sensible. Adding more logic to page_vma_mapped_walk() for the rare partial-folio case seems like an over-optimization that would complicate the walker. So, I'd prefer to keep it as is for now ;)