[PATCH] mm/filemap: Fix handling of THPs in generic_file_buffered_read()

[Matthew Wilcox (Oracle)]

When a THP is present in the page cache, we can return it several times,
leading to userspace seeing the same data repeatedly if doing a read()
that crosses a 64-page boundary.  This is probably not a security issue
(since the data all comes from the same file), but it can be interpreted
as a transient data corruption issue.  Fortunately, it is very rare as
it can only occur when CONFIG_READ_ONLY_THP_FOR_FS is enabled, and it can
only happen to executables.  We don't often call read() on executables.

This bug is fixed differently in v5.17 by commit 6b24ca4a1a8d
("mm: Use multi-index entries in the page cache").  That commit is
unsuitable for backporting, so fix this in the clearest way.  It
sacrifices a little performance for clarity, but this should never
be a performance path in these kernel versions.

Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Cc: stable@xxxxxxxxxxxxxxx # v5.15, v5.16
Link: https://lore.kernel.org/r/df3b5d1c-a36b-2c73-3e27-99e74983de3a@xxxxxxx/
Analyzed-by: Adam Majer <amajer@xxxxxxxx>
Analyzed-by: Dirk Mueller <dmueller@xxxxxxxx>
Bisected-by: Takashi Iwai <tiwai@xxxxxxx>
Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 82a17c35eb96..1293c3409e42 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2354,8 +2354,12 @@ static void filemap_get_read_batch(struct address_space *mapping,
 			break;
 		if (PageReadahead(head))
 			break;
-		xas.xa_index = head->index + thp_nr_pages(head) - 1;
-		xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
+		if (PageHead(head)) {
+			xas_set(&xas, head->index + thp_nr_pages(head));
+			/* Handle wrap correctly */
+			if (xas.xa_index - 1 >= max)
+				break;
+		}
 		continue;
 put_page:
 		put_page(head);
-- 
2.34.1