squashfs metadata 2: electric boogaloo

Anatoly continues to find issues with fuzzed squashfs images.

This time, corrupt, missing, or undersized data for the page filling
wasn't checked for, because the squashfs_{copy,read}_cache() functions
did the squashfs_copy_data() call without checking the resulting data
size.

Which could result in the page cache pages being incompletely filled in,
and no error indication to the user space reading garbage data.

So make a helper function for the "fill in pages" case, because the
exact same incomplete sequence existed in two places.

[ I should have made a squashfs branch for these things, but I didn't
  intend to start doing them in the first place.

  My historical connection through cramfs is why I got into looking at
  these issues at all, and every time I (continue to) think it's a
  one-off.

  Because _this_ time is always the last time. Right?   - Linus ]
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Tested-by: Willy Tarreau <w@1wt.eu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/squashfs/file.c

@@ -374,13 +374,29 @@ static int read_blocklist(struct inode *inode, int index, u64 *block)
 	return squashfs_block_size(size);
 }
 
+void squashfs_fill_page(struct page *page, struct squashfs_cache_entry *buffer, int offset, int avail)
+{
+	int copied;
+	void *pageaddr;
+
+	pageaddr = kmap_atomic(page);
+	copied = squashfs_copy_data(pageaddr, buffer, offset, avail);
+	memset(pageaddr + copied, 0, PAGE_SIZE - copied);
+	kunmap_atomic(pageaddr);
+
+	flush_dcache_page(page);
+	if (copied == avail)
+		SetPageUptodate(page);
+	else
+		SetPageError(page);
+}
+
 /* Copy data into page cache  */
 void squashfs_copy_cache(struct page *page, struct squashfs_cache_entry *buffer,
 	int bytes, int offset)
 {
 	struct inode *inode = page->mapping->host;
 	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
-	void *pageaddr;
 	int i, mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
 	int start_index = page->index & ~mask, end_index = start_index | mask;
 
@@ -406,12 +422,7 @@ void squashfs_copy_cache(struct page *page, struct squashfs_cache_entry *buffer,
 		if (PageUptodate(push_page))
 			goto skip_page;
 
-		pageaddr = kmap_atomic(push_page);
-		squashfs_copy_data(pageaddr, buffer, offset, avail);
-		memset(pageaddr + avail, 0, PAGE_SIZE - avail);
-		kunmap_atomic(pageaddr);
-		flush_dcache_page(push_page);
-		SetPageUptodate(push_page);
+		squashfs_fill_page(push_page, buffer, offset, avail);
 skip_page:
 		unlock_page(push_page);
 		if (i != page->index)

fs/squashfs/file_direct.c

@@ -144,7 +144,6 @@ static int squashfs_read_cache(struct page *target_page, u64 block, int bsize,
 	struct squashfs_cache_entry *buffer = squashfs_get_datablock(i->i_sb,
 						 block, bsize);
 	int bytes = buffer->length, res = buffer->error, n, offset = 0;
-	void *pageaddr;
 
 	if (res) {
 		ERROR("Unable to read page, block %llx, size %x\n", block,
@@ -159,12 +158,7 @@ static int squashfs_read_cache(struct page *target_page, u64 block, int bsize,
 		if (page[n] == NULL)
 			continue;
 
-		pageaddr = kmap_atomic(page[n]);
-		squashfs_copy_data(pageaddr, buffer, offset, avail);
-		memset(pageaddr + avail, 0, PAGE_SIZE - avail);
-		kunmap_atomic(pageaddr);
-		flush_dcache_page(page[n]);
-		SetPageUptodate(page[n]);
+		squashfs_fill_page(page[n], buffer, offset, avail);
 		unlock_page(page[n]);
 		if (page[n] != target_page)
 			put_page(page[n]);

fs/squashfs/squashfs.h

@@ -67,6 +67,7 @@ extern __le64 *squashfs_read_fragment_index_table(struct super_block *,
 				u64, u64, unsigned int);
 
 /* file.c */
+void squashfs_fill_page(struct page *, struct squashfs_cache_entry *, int, int);
 void squashfs_copy_cache(struct page *, struct squashfs_cache_entry *, int,
 				int);