From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta0.migadu.com (out-171.mta0.migadu.com [91.218.175.171]) (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 E23FF372056 for ; Tue, 14 Jul 2026 01:52:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783993937; cv=none; b=hz2oQztLIyfnOI5TY3VSZr6Ixr9q1famzkrLPNNwWRULRqMoV3v6NfCCGt0amutfRVC9FNvnhykdhqA2noZ51vjZAGMrAjASD/RpoJ6SAmXgc2thfy8PxK3HkvjDf+tjNjgMHe/ur5OLqL+0yJIgF+SKlefnMY+56DZH7At98aY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783993937; c=relaxed/simple; bh=idQ9hkZPaWO5XRXHfLYCRc8ylO1WiBswmcQ/5Ywr6FE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Nm/yLUB6GZomUk+9I9FqS1KS/0zf8cWxNKhlAX+hi+dknxnEZig/VO+Z873OQnP4qt1zXyb8EkEwSV+tg5rF0MlO+3eEbNogiuOxeJDI9RGsbBwfjyhjy1FIGY+7Mzn0a2eEWLRVoencEnFjmn63wC+VUohG8P+1Gg664EHuHoo= 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=phiRJC08; arc=none smtp.client-ip=91.218.175.171 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="phiRJC08" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783993934; 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=2Pj2O1XV/iVkFDN8GVAoeYlkqg9Q6iDpIRfEcXtGxjw=; b=phiRJC08q5I5JcA3F6fMnZB1OyZRYKqEqiv6Y0aGluxJ7VzGnXQkfpPONgcPdItEWoGzZD 9BBqeYTV0ttmcWvgeZyk1QB1qPUFGkmxxSy0L2TEvXBiX/MMGMENJ1iuQ0M6Z7zXFyQ/31 vBthOxmjYMQXREFgpPs0km+9AM32NWE= From: Ye Liu To: Andrew Morton , Vlastimil Babka Cc: Ye Liu , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v6 9/9] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg() Date: Tue, 14 Jul 2026 09:51:08 +0800 Message-ID: <20260714015117.78351-10-ye.liu@linux.dev> In-Reply-To: <20260714015117.78351-1-ye.liu@linux.dev> References: <20260714015117.78351-1-ye.liu@linux.dev> 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=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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. This avoids both TOCTOU windows and the assertions entirely. Signed-off-by: Ye Liu --- 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: -- 2.43.0