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imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=NqG70l5h; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf06.hostedemail.com: domain of vbabka@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=vbabka@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1784017479; b=rzjEO3Sfs3GPeKkpqgY8Lvvw99MPoNmFlNad8bhbxXQS7su4nCnr5AzM0aR1MFbvl4usc4 lnR59SNW51JtK6iXymN+QBO1VYJ7DHsTMP5uG25qZ41oL635Pc0jjZgtCDacsLz/yIMrO6 pWErTBR1ZJU4I6ksjgWXqipf3gOQEKA= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 7C977601EC; Tue, 14 Jul 2026 08:24:38 +0000 (UTC) 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 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; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20260714015117.78351-10-ye.liu@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: stsaoj4u68y9nnbq1iwi1aecdexc8yu6 X-Rspamd-Queue-Id: 241FB180004 X-Rspamd-Server: rspam01 X-Rspam-User: X-HE-Tag: 1784017478-1099 X-HE-Meta: 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 SFM24XIt d1+GyS85sCrnMGs1NtqKRdvKpVigGwIGXozeyWuD4t6OAcplBVBPIqN1i2MPo6Y2Qixsn8fsLnYLiE3JF6f5adqd9BM8uuOfZPSLNzxX7mPiiyo/9iiNxzcpVyO8PNBYtrYG9viZitkUjNdPLYc+p/jyLQjQtHVnmNSjrCkUKOC05yfWVdD3nuKhuaW8vbqXhYgO12Q66WhGse8lIW1IxH8ruo5Q48W15ACnfql5D3sS4tjx6hDWosIhsRcm3ECgeENyZx2i0Q8oRuMnZMB+dtMciig74fAkJf7JML4tS5a9OsNxym/YSYsNeG+mwMUmBLeNNRxxiWVGUCd8+NfOGsTDLDYfHjh9C/QLE Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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: