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 AAEEF3FAE1A for ; Tue, 14 Jul 2026 08:24:41 +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=1784017488; cv=none; b=lC9tGuofC1/sdYq8svJkqIL7ds3EXFkuCiXO0f/p4El+rxYqhar33mj3+N11R7riRmh1IgFYt95Aoe7Jx1NMaGm12cANn9uyYa2qa9yvDvBevfVhpRMRe1Xlbq2O2segSRgkbF7N7q3jVfac0dAd8mI2r1zvbp/u5vipjolPxV0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784017488; c=relaxed/simple; bh=ff/6cHnN50Ab28JVhnFVU4t/rd73LPljFKUjP/DGmPk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hlfgme1Tn72XWywGQ4axYk6rfTj5O4FVefWFnUqqGIPEHVudrzGqnNj3mdxpKDSVq9N9U5ISb0ZDV36EdWPa5sf33F9Mi4f57bKBG2JPTORoBbS5/d59RKt4mtiSwX2RxPSPc4bl8o/ybFDcW8dZe6gk7PUHBz8qNff4qUuiZfo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NqG70l5h; 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="NqG70l5h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7A3B41F000E9; Tue, 14 Jul 2026 08:24:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784017478; bh=bbuLAjCoX2Tf2xn8G6n1Bh/HGLghJUpzugxizUSUcMU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NqG70l5hvvJ5SeDUUhXjDmxaW+OF2x1POiWfWXDReY39GbSmsmzqf2C9koMNrg3lm H7eYQChV3ZrCOmxu2p0sCBYTkJJ6JC5Nov2VZJ9dK5S5rXxltZTlDkn/wTk1PmaIyh x+qQ0HAMfQBUe5fvdTB5R2TWmvQoBMVDLofQ7MjdHi9yG42ksE0I3PKIzkR5+jwYCt 3aaGJw21Bvf3sjQg8begs7HhaTIFlE5C/e6+ZzbLGJNu0v6Sv+Tuwc9witph46K4nu s8U70sRdEBRvb9Dkci26XqJSrT3Mxz/Fw90KdjoxP1VQjag35pUzXJFifZotgZpjvE P+dIJ71kQ1YZw== Message-ID: <21007313-40dc-46fb-bbf2-453c8eedf3e8@kernel.org> Date: Tue, 14 Jul 2026 10:24:33 +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 v6 9/9] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg() To: Ye Liu , Andrew Morton Cc: Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260714015117.78351-1-ye.liu@linux.dev> <20260714015117.78351-10-ye.liu@linux.dev> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 7/14/26 03:51, Ye Liu wrote: > print_page_owner_memcg() takes a snapshot of page->memcg_data via > READ_ONCE at the top of the function and guards against tail pages and > NULL memcg_data. However, it later calls two functions that re-read > page->memcg_data locklessly: > > 1) page_memcg_check(page) — re-reads page->memcg_data; > 2) PageMemcgKmem(page) — calls folio_memcg_kmem(), which re-reads > folio->memcg_data and folio->page->compound_head, wrapping both > in VM_BUG_ON assertions: > > VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page); > VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio); > > If the page is concurrently freed and reallocated as a THP tail page > or a slab page between the initial guards and these later calls, the > VM_BUG_ON assertions can fire on debug builds (CONFIG_DEBUG_VM=y), > causing a kernel panic. > > Fix both TOCTOU issues by using the memcg_data snapshot throughout: > - Extract objcg from the snapshot via objcg = (void *)(memcg_data & > ~OBJEXTS_FLAGS_MASK) instead of calling page_memcg_check(page); > - Test (memcg_data & MEMCG_DATA_KMEM) instead of calling > PageMemcgKmem(page), which is semantically equivalent: > PageMemcgKmem()->folio_memcg_kmem()->folio->memcg_data & > MEMCG_DATA_KMEM. > - When memcg_data has MEMCG_DATA_OBJEXTS set, early-return after > printing "Slab cache page\n" since objcg != memcg for slab pages > and there is no meaningful cgroup to look up. These points have too much detail that's already in the code. Would just mention that we opencode applicable parts of page_memcg_check() and PageMemcgKmem() using the snapshot? > This avoids both TOCTOU windows and the assertions entirely. > > Signed-off-by: Ye Liu Reviewed-by: Vlastimil Babka (SUSE) Was all of this reported by sashiko? At least the new-in-v6 was? Then: Reported-by: Sashiko But it's no longer a cleanup but a fix, so probably this? Fixes: fcf8935832b8 ("mm/page_owner: print memcg information") Cc: stable@vger.kernel.org It's not fixing a new regression so I think it's fine to keep it part of this series for next release and not need to split out for mm-hotfixes. > --- > Changes in v6: > - Rename patch to cover both TOCTOU fixes rather than only > PageMemcgKmem(). > - Also replace page_memcg_check(page) with extracting objcg from the > memcg_data snapshot to fix a second TOCTOU issue. > - Add early return for the MEMCG_DATA_OBJEXTS (slab) case since > objcg != memcg for slab pages and there is no cgroup to look up. > - Update commit message to cover all changes. > - Link: https://lore.kernel.org/all/20260701061101.344679-10-ye.liu@linux.dev/ > mm/page_owner.c | 10 +++++++--- > 1 file changed, 7 insertions(+), 3 deletions(-) > > diff --git a/mm/page_owner.c b/mm/page_owner.c > index 2e3880053a34..e18512a49e38 100644 > --- a/mm/page_owner.c > +++ b/mm/page_owner.c > @@ -540,6 +540,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, > struct page *page) > { > unsigned long memcg_data; > + struct obj_cgroup *objcg; > struct mem_cgroup *memcg; > bool online; > char name[80]; > @@ -549,11 +550,14 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, > if (!memcg_data || PageTail(page)) > goto out_unlock; > > - if (memcg_data & MEMCG_DATA_OBJEXTS) > + if (memcg_data & MEMCG_DATA_OBJEXTS) { > ret += scnprintf(kbuf + ret, count - ret, > "Slab cache page\n"); > + goto out_unlock; > + } > > - memcg = page_memcg_check(page); > + objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK); > + memcg = objcg ? obj_cgroup_memcg(objcg) : NULL; > if (!memcg) > goto out_unlock; > > @@ -561,7 +565,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, > cgroup_name(memcg->css.cgroup, name, sizeof(name)); > ret += scnprintf(kbuf + ret, count - ret, > "Charged %sto %smemcg %s\n", > - PageMemcgKmem(page) ? "(via objcg) " : "", > + (memcg_data & MEMCG_DATA_KMEM) ? "(via objcg) " : "", > online ? "" : "offline ", > name); > out_unlock: