[PATCH v2] mm/huge_memory: fix a folio_split() race condition with folio_try_get()

[Zi Yan <ziy@nvidia.com>]

During a pagecache folio split, the values in the related xarray should not
be changed from the original folio at xarray split time until all
after-split folios are well formed and stored in the xarray. Current use
of xas_try_split() in __split_unmapped_folio() lets some after-split folios
show up at wrong indices in the xarray. When these misplaced after-split
folios are unfrozen, before correct folios are stored via __xa_store(), and
grabbed by folio_try_get(), they are returned to userspace at wrong file
indices, causing data corruption. More detailed explanation is at the
bottom.

The reproducer is at: https://github.com/dfinity/thp-madv-remove-test
It
1. creates a memfd,
2. forks,
3. in the child process, maps the file with large folios (via shmem code
   path) and reads the mapped file continuously with 16 threads,
4. in the parent process, uses madvise(MADV_REMOVE) to punch poles in the
   large folio.

Data corruption can be observed without the fix. Basically, data from a
wrong page->index is returned.

Fix it by using the original folio in xas_try_split() calls, so that
folio_try_get() can get the right after-split folios after the original
folio is unfrozen.

Uniform split, split_huge_page*(), is not affected, since it uses
xas_split_alloc() and xas_split() only once and stores the original folio
in the xarray. Change xas_split() used in uniform split branch to use
the original folio to avoid confusion.

Fixes below points to the commit introduces the code, but folio_split() is
used in a later commit 7460b470a131f ("mm/truncate: use folio_split() in
truncate operation").

More details:

For example, a folio f is split non-uniformly into f, f2, f3, f4 like
below:
+----------------+---------+----+----+
|       f        |    f2   | f3 | f4 |
+----------------+---------+----+----+
but the xarray would look like below after __split_unmapped_folio() is
done:
+----------------+---------+----+----+
|       f        |    f2   | f3 | f3 |
+----------------+---------+----+----+

After __split_unmapped_folio(), the code changes the xarray and unfreezes
after-split folios:

1. unfreezes f2, __xa_store(f2)
2. unfreezes f3, __xa_store(f3)
3. unfreezes f4, __xa_store(f4), which overwrites the second f3 to f4.
4. unfreezes f.

Meanwhile, a parallel filemap_get_entry() can read the second f3 from the
xarray and use folio_try_get() on it at step 2 when f3 is unfrozen. Then,
f3 is wrongly returned to user.

After the fix, the xarray looks like below after __split_unmapped_folio():
+----------------+---------+----+----+
|       f        |    f    | f  | f  |
+----------------+---------+----+----+
so that the race window no longer exists.

Fixes: 00527733d0dc8 ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reported-by: Bas van Dijk <bas@dfinity.org>
Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@mail.gmail.com/
Tested-by: Lance Yang <lance.yang@linux.dev>
Cc: <stable@vger.kernel.org>
---
 mm/huge_memory.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 56db54fa48181..f0bdac3f270b5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3647,6 +3647,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	const bool is_anon = folio_test_anon(folio);
 	int old_order = folio_order(folio);
 	int start_order = split_type == SPLIT_TYPE_UNIFORM ? new_order : old_order - 1;
+	struct folio *old_folio = folio;
 	int split_order;
 
 	/*
@@ -3668,11 +3669,17 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 			 * irq is disabled to allocate enough memory, whereas
 			 * non-uniform split can handle ENOMEM.
 			 */
-			if (split_type == SPLIT_TYPE_UNIFORM)
-				xas_split(xas, folio, old_order);
-			else {
+			if (split_type == SPLIT_TYPE_UNIFORM) {
+				xas_split(xas, old_folio, old_order);
+			} else {
 				xas_set_order(xas, folio->index, split_order);
-				xas_try_split(xas, folio, old_order);
+				/*
+				 * use the to-be-split folio, so that a parallel
+				 * folio_try_get() waits on it until xarray is
+				 * updated with after-split folios and
+				 * the original one is unfrozen.
+				 */
+				xas_try_split(xas, old_folio, old_order);
 				if (xas_error(xas))
 					return xas_error(xas);
 			}
-- 
2.51.0