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mm: cachestat: fix folio read-after-free in cache walk

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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@cmpxchg.org
Fixes: cf264e1329 ("cachestat: implement cachestat syscall")
Reported-by: Jann Horn <jannh@google.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org>	[6.4+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Nhat Pham 2024-02-19 19:01:21 -08:00 committed by Andrew Morton
parent 00130266f6
commit 3a75cb05d5
1 changed files with 26 additions and 25 deletions
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51
mm/filemap.c
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@ -4111,28 +4111,40 @@ static void filemap_cachestat(struct address_space *mapping,
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;
@ -4150,24 +4162,13 @@ static void filemap_cachestat(struct address_space *mapping,
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: