ext4: fix race when setting bitmap_uptodate flag

In ext4_read_{inode,block}_bitmap() we were setting bitmap_uptodate()
before submitting the buffer for read.  The is bad, since we check
bitmap_uptodate() without locking the buffer, and so if another
process is racing with us, it's possible that they will think the
bitmap is uptodate even though the read has not completed yet,
resulting in inodes and blocks potentially getting allocated more than
once if we get really unlucky.

Addresses-Google-Bug: 2828254

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 15 insertions, 64 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index cb990b21c698..545fa0256606 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -782,7 +782,7 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
 	int groups_per_page;
 	int err = 0;
 	int i;
-	ext4_group_t first_group;
+	ext4_group_t first_group, group;
 	int first_block;
 	struct super_block *sb;
 	struct buffer_head *bhs;
@@ -806,24 +806,23 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
 
 	/* allocate buffer_heads to read bitmaps */
 	if (groups_per_page > 1) {
-		err = -ENOMEM;
 		i = sizeof(struct buffer_head *) * groups_per_page;
 		bh = kzalloc(i, GFP_NOFS);
-		if (bh == NULL)
+		if (bh == NULL) {
+			err = -ENOMEM;
 			goto out;
+		}
 	} else
 		bh = &bhs;
 
 	first_group = page->index * blocks_per_page / 2;
 
 	/* read all groups the page covers into the cache */
-	for (i = 0; i < groups_per_page; i++) {
-		struct ext4_group_desc *desc;
-
-		if (first_group + i >= ngroups)
+	for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
+		if (group >= ngroups)
 			break;
 
-		grinfo = ext4_get_group_info(sb, first_group + i);
+		grinfo = ext4_get_group_info(sb, group);
 		/*
 		 * If page is uptodate then we came here after online resize
 		 * which added some new uninitialized group info structs, so
@@ -834,69 +833,21 @@ static int ext4_mb_init_cache(struct page *page, char *incore)
 			bh[i] = NULL;
 			continue;
 		}
-
-		err = -EIO;
-		desc = ext4_get_group_desc(sb, first_group + i, NULL);
-		if (desc == NULL)
-			goto out;
-
-		err = -ENOMEM;
-		bh[i] = sb_getblk(sb, ext4_block_bitmap(sb, desc));
-		if (bh[i] == NULL)
+		if (!(bh[i] = ext4_read_block_bitmap_nowait(sb, group))) {
+			err = -ENOMEM;
 			goto out;
-
-		if (bitmap_uptodate(bh[i]))
-			continue;
-
-		lock_buffer(bh[i]);
-		if (bitmap_uptodate(bh[i])) {
-			unlock_buffer(bh[i]);
-			continue;
 		}
-		ext4_lock_group(sb, first_group + i);
-		if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
-			ext4_init_block_bitmap(sb, bh[i],
-						first_group + i, desc);
-			set_bitmap_uptodate(bh[i]);
-			set_buffer_uptodate(bh[i]);
-			ext4_unlock_group(sb, first_group + i);
-			unlock_buffer(bh[i]);
-			continue;
-		}
-		ext4_unlock_group(sb, first_group + i);
-		if (buffer_uptodate(bh[i])) {
-			/*
-			 * if not uninit if bh is uptodate,
-			 * bitmap is also uptodate
-			 */
-			set_bitmap_uptodate(bh[i]);
-			unlock_buffer(bh[i]);
-			continue;
-		}
-		get_bh(bh[i]);
-		/*
-		 * submit the buffer_head for read. We can
-		 * safely mark the bitmap as uptodate now.
-		 * We do it here so the bitmap uptodate bit
-		 * get set with buffer lock held.
-		 */
-		set_bitmap_uptodate(bh[i]);
-		bh[i]->b_end_io = end_buffer_read_sync;
-		submit_bh(READ, bh[i]);
-		mb_debug(1, "read bitmap for group %u\n", first_group + i);
+		mb_debug(1, "read bitmap for group %u\n", group);
 	}
 
 	/* wait for I/O completion */
-	for (i = 0; i < groups_per_page; i++)
-		if (bh[i])
-			wait_on_buffer(bh[i]);
-
-	err = -EIO;
-	for (i = 0; i < groups_per_page; i++)
-		if (bh[i] && !buffer_uptodate(bh[i]))
+	for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
+		if (bh[i] && ext4_wait_block_bitmap(sb, group, bh[i])) {
+			err = -EIO;
 			goto out;
+		}
+	}
 
-	err = 0;
 	first_block = page->index * blocks_per_page;
 	for (i = 0; i < blocks_per_page; i++) {
 		int group;