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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> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 6688B140009 X-Stat-Signature: oprzktm4xmtn1tri5o956cirmf1x17ye X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1783993935-775551 X-HE-Meta: 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 zaqqgVnq BF9tjdY8cksZWdzNwsPnr5rZc02VQs1/z856fEGWjHuoSm3LBwBHjGkFCXnqUCXp+RkrBSuJSUaBqkXOpxeX7bjrQfZuX/40tTnW0wUhFQ0aoPlUkIAYvD7gDNvxzSqT5JYKYyMcXoOqmLTfsyTLQ7K/3+HWVA36qcRLX/5O40cqEftbnVfPc+mV/Qa2eCwdMpgMzHfdACI4f5CiwyyQfUwaE+RuN11m3ti8diiVwivQpYJX6R+4aMxWdjkC+PxtY5cNn8hKBzdU2Hoq17k2Mtpw6Fwwi813RukkGE9/++Eh1e9TVFgyi3ssb1Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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