squashfs: fix cache race with migration

Migration replaces the page in the mapping before copying the contents and
the flags over from the old page, so check that the page in the page cache
is really up to date before using it.  Without this, stressing squashfs
reads with parallel compaction sometimes results in squashfs reporting
data corruption.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: e994f5b ("squashfs: cache partial compressed blocks")
Signed-off-by: Vincent Whitchurch <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Phillip Lougher <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/squashfs/block.c

@@ -166,6 +166,26 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
 	return 0;
 }
 
+static struct page *squashfs_get_cache_page(struct address_space *mapping,
+					    pgoff_t index)
+{
+	struct page *page;
+
+	if (!mapping)
+		return NULL;
+
+	page = find_get_page(mapping, index);
+	if (!page)
+		return NULL;
+
+	if (!PageUptodate(page)) {
+		put_page(page);
+		return NULL;
+	}
+
+	return page;
+}
+
 static int squashfs_bio_read(struct super_block *sb, u64 index, int length,
 			     struct bio **biop, int *block_offset)
 {
@@ -190,11 +210,10 @@ static int squashfs_bio_read(struct super_block *sb, u64 index, int length,
 	for (i = 0; i < page_count; ++i) {
 		unsigned int len =
 			min_t(unsigned int, PAGE_SIZE - offset, total_len);
-		struct page *page = NULL;
+		pgoff_t index = (read_start >> PAGE_SHIFT) + i;
+		struct page *page;
 
-		if (cache_mapping)
-			page = find_get_page(cache_mapping,
-					     (read_start >> PAGE_SHIFT) + i);
+		page = squashfs_get_cache_page(cache_mapping, index);
 		if (!page)
 			page = alloc_page(GFP_NOIO);