fat: work around race with userspace's read via blockdev while mounting

If userspace reads the buffer via blockdev while mounting,
sb_getblk()+modify can race with buffer read via blockdev.

For example,

            FS                               userspace
    bh = sb_getblk()
    modify bh->b_data
                                  read
				    ll_rw_block(bh)
				      fill bh->b_data by on-disk data
				      /* lost modified data by FS */
				      set_buffer_uptodate(bh)
    set_buffer_uptodate(bh)

Userspace should not use the blockdev while mounting though, the udev
seems to be already doing this.  Although I think the udev should try to
avoid this, workaround the race by small overhead.

Link: http://lkml.kernel.org/r/87pnk7l3sw.fsf_-_@mail.parknet.co.jpSigned-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/dir.c

@@ -1100,8 +1100,11 @@ static int fat_zeroed_cluster(struct inode *dir, sector_t blknr, int nr_used,
 			err = -ENOMEM;
 			goto error;
 		}
+		/* Avoid race with userspace read via bdev */
+		lock_buffer(bhs[n]);
 		memset(bhs[n]->b_data, 0, sb->s_blocksize);
 		set_buffer_uptodate(bhs[n]);
+		unlock_buffer(bhs[n]);
 		mark_buffer_dirty_inode(bhs[n], dir);
 
 		n++;
@@ -1158,6 +1161,8 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts)
 	fat_time_unix2fat(sbi, ts, &time, &date, &time_cs);
 
 	de = (struct msdos_dir_entry *)bhs[0]->b_data;
+	/* Avoid race with userspace read via bdev */
+	lock_buffer(bhs[0]);
 	/* filling the new directory slots ("." and ".." entries) */
 	memcpy(de[0].name, MSDOS_DOT, MSDOS_NAME);
 	memcpy(de[1].name, MSDOS_DOTDOT, MSDOS_NAME);
@@ -1180,6 +1185,7 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts)
 	de[0].size = de[1].size = 0;
 	memset(de + 2, 0, sb->s_blocksize - 2 * sizeof(*de));
 	set_buffer_uptodate(bhs[0]);
+	unlock_buffer(bhs[0]);
 	mark_buffer_dirty_inode(bhs[0], dir);
 
 	err = fat_zeroed_cluster(dir, blknr, 1, bhs, MAX_BUF_PER_PAGE);
@@ -1237,11 +1243,14 @@ static int fat_add_new_entries(struct inode *dir, void *slots, int nr_slots,
 
 			/* fill the directory entry */
 			copy = min(size, sb->s_blocksize);
+			/* Avoid race with userspace read via bdev */
+			lock_buffer(bhs[n]);
 			memcpy(bhs[n]->b_data, slots, copy);
-			slots += copy;
-			size -= copy;
 			set_buffer_uptodate(bhs[n]);
+			unlock_buffer(bhs[n]);
 			mark_buffer_dirty_inode(bhs[n], dir);
+			slots += copy;
+			size -= copy;
 			if (!size)
 				break;
 			n++;

fs/fat/fatent.c

@@ -388,8 +388,11 @@ static int fat_mirror_bhs(struct super_block *sb, struct buffer_head **bhs,
 				err = -ENOMEM;
 				goto error;
 			}
+			/* Avoid race with userspace read via bdev */
+			lock_buffer(c_bh);
 			memcpy(c_bh->b_data, bhs[n]->b_data, sb->s_blocksize);
 			set_buffer_uptodate(c_bh);
+			unlock_buffer(c_bh);
 			mark_buffer_dirty_inode(c_bh, sbi->fat_inode);
 			if (sb->s_flags & SB_SYNCHRONOUS)
 				err = sync_dirty_buffer(c_bh);