This is a note to let you know that I've just added the patch titled
mm: cachestat: fix folio read-after-free in cache walk
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-cachestat-fix-folio-read-after-free-in-cache-walk.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 3a75cb05d53f4a6823a32deb078de1366954a804 Mon Sep 17 00:00:00 2001
From: Nhat Pham <nphamcs@xxxxxxxxx>
Date: Mon, 19 Feb 2024 19:01:21 -0800
Subject: mm: cachestat: fix folio read-after-free in cache walk
From: Nhat Pham <nphamcs@xxxxxxxxx>
commit 3a75cb05d53f4a6823a32deb078de1366954a804 upstream.
In cachestat, we access the folio from the page cache's xarray to compute
its page offset, and check for its dirty and writeback flags. However, we
do not hold a reference to the folio before performing these actions,
which means the folio can concurrently be released and reused as another
folio/page/slab.
Get around this altogether by just using xarray's existing machinery for
the folio page offsets and dirty/writeback states.
This changes behavior for tmpfs files to now always report zeroes in their
dirty and writeback counters. This is okay as tmpfs doesn't follow
conventional writeback cache behavior: its pages get "cleaned" during
swapout, after which they're no longer resident etc.
Link: https://lkml.kernel.org/r/20240220153409.GA216065@xxxxxxxxxxx
Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Tested-by: Jann Horn <jannh@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [6.4+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 51 ++++++++++++++++++++++++++-------------------------
1 file changed, 26 insertions(+), 25 deletions(-)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4159,28 +4159,40 @@ static void filemap_cachestat(struct add
rcu_read_lock();
xas_for_each(&xas, folio, last_index) {
+ int order;
unsigned long nr_pages;
pgoff_t folio_first_index, folio_last_index;
+ /*
+ * Don't deref the folio. It is not pinned, and might
+ * get freed (and reused) underneath us.
+ *
+ * We *could* pin it, but that would be expensive for
+ * what should be a fast and lightweight syscall.
+ *
+ * Instead, derive all information of interest from
+ * the rcu-protected xarray.
+ */
+
if (xas_retry(&xas, folio))
continue;
+ order = xa_get_order(xas.xa, xas.xa_index);
+ nr_pages = 1 << order;
+ folio_first_index = round_down(xas.xa_index, 1 << order);
+ folio_last_index = folio_first_index + nr_pages - 1;
+
+ /* Folios might straddle the range boundaries, only count covered pages */
+ if (folio_first_index < first_index)
+ nr_pages -= first_index - folio_first_index;
+
+ if (folio_last_index > last_index)
+ nr_pages -= folio_last_index - last_index;
+
if (xa_is_value(folio)) {
/* page is evicted */
void *shadow = (void *)folio;
bool workingset; /* not used */
- int order = xa_get_order(xas.xa, xas.xa_index);
-
- nr_pages = 1 << order;
- folio_first_index = round_down(xas.xa_index, 1 << order);
- folio_last_index = folio_first_index + nr_pages - 1;
-
- /* Folios might straddle the range boundaries, only count covered pages */
- if (folio_first_index < first_index)
- nr_pages -= first_index - folio_first_index;
-
- if (folio_last_index > last_index)
- nr_pages -= folio_last_index - last_index;
cs->nr_evicted += nr_pages;
@@ -4198,24 +4210,13 @@ static void filemap_cachestat(struct add
goto resched;
}
- nr_pages = folio_nr_pages(folio);
- folio_first_index = folio_pgoff(folio);
- folio_last_index = folio_first_index + nr_pages - 1;
-
- /* Folios might straddle the range boundaries, only count covered pages */
- if (folio_first_index < first_index)
- nr_pages -= first_index - folio_first_index;
-
- if (folio_last_index > last_index)
- nr_pages -= folio_last_index - last_index;
-
/* page is in cache */
cs->nr_cache += nr_pages;
- if (folio_test_dirty(folio))
+ if (xas_get_mark(&xas, PAGECACHE_TAG_DIRTY))
cs->nr_dirty += nr_pages;
- if (folio_test_writeback(folio))
+ if (xas_get_mark(&xas, PAGECACHE_TAG_WRITEBACK))
cs->nr_writeback += nr_pages;
resched:
Patches currently in stable-queue which might be from nphamcs@xxxxxxxxx are
queue-6.6/mm-cachestat-fix-folio-read-after-free-in-cache-walk.patch
