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Subject[PATCH] fat: Workaround the race with userspace's read via blockdev while mounting
Date
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)

The 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.

Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---

fs/fat/dir.c | 13 +++++++++++--
fs/fat/fatent.c | 3 +++
2 files changed, 14 insertions(+), 2 deletions(-)

diff -puN fs/fat/dir.c~fat-workaround-getblk fs/fat/dir.c
--- linux/fs/fat/dir.c~fat-workaround-getblk 2019-09-10 09:29:51.137292020 +0900
+++ linux-hirofumi/fs/fat/dir.c 2019-09-10 09:39:15.366295152 +0900
@@ -1100,8 +1100,11 @@ static int fat_zeroed_cluster(struct ino
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,
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,
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 in

/* 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 -puN fs/fat/fatent.c~fat-workaround-getblk fs/fat/fatent.c
--- linux/fs/fat/fatent.c~fat-workaround-getblk 2019-09-10 09:36:20.247225406 +0900
+++ linux-hirofumi/fs/fat/fatent.c 2019-09-10 09:36:43.847100048 +0900
@@ -388,8 +388,11 @@ static int fat_mirror_bhs(struct super_b
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);
_
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