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.jp
Signed-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>

2 files changed, 14 insertions, 2 deletions

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index 1bda2ab6745b..814ad2c2ba80 100644
--- a/fs/fat/dir.c
+++ b/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++;
diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index 265983635f2b..3647c65a0f48 100644
--- a/fs/fat/fatent.c
+++ b/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);