[PATCH v6 9/9] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()

[Ye Liu <ye.liu@linux.dev>]

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 <ye.liu@linux.dev>
---
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