loop: Fix ABBA locking race

Current loop calls vfs_statfs() while holding the q->limits_lock. If
FS takes some locking in vfs_statfs callback, this may lead to ABBA
locking bug (at least, FAT fs has this issue actually).

So this patch calls vfs_statfs() outside q->limits_locks instead,
because looks like no reason to hold q->limits_locks while getting
discord configs.

Chain exists of:
  &sbi->fat_lock --> &q->q_usage_counter(io)#17 --> &q->limits_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->limits_lock);
                               lock(&q->q_usage_counter(io)#17);
                               lock(&q->limits_lock);
  lock(&sbi->fat_lock);

 *** DEADLOCK ***

Reported-by: syzbot+a5d8c609c02f508672cc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a5d8c609c02f508672cc
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index fe9bb4fb5f1ba743b3fd678922d5962ac838cb3d..8f6761c27c68b2a8a2088c2b1b33e85c291fedac 100644 (file)
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -770,12 +770,11 @@ static void loop_sysfs_exit(struct loop_device *lo)
                                   &loop_attribute_group);
 }
 
-static void loop_config_discard(struct loop_device *lo,
-               struct queue_limits *lim)
+static void loop_get_discard_config(struct loop_device *lo,
+                                   u32 *granularity, u32 *max_discard_sectors)
 {
        struct file *file = lo->lo_backing_file;
        struct inode *inode = file->f_mapping->host;
-       u32 granularity = 0, max_discard_sectors = 0;
        struct kstatfs sbuf;
 
        /*
@@ -788,24 +787,17 @@ static void loop_config_discard(struct loop_device *lo,
        if (S_ISBLK(inode->i_mode)) {
                struct block_device *bdev = I_BDEV(inode);
 
-               max_discard_sectors = bdev_write_zeroes_sectors(bdev);
-               granularity = bdev_discard_granularity(bdev);
+               *max_discard_sectors = bdev_write_zeroes_sectors(bdev);
+               *granularity = bdev_discard_granularity(bdev);
 
        /*
         * We use punch hole to reclaim the free space used by the
         * image a.k.a. discard.
         */
        } else if (file->f_op->fallocate && !vfs_statfs(&file->f_path, &sbuf)) {
-               max_discard_sectors = UINT_MAX >> 9;
-               granularity = sbuf.f_bsize;
+               *max_discard_sectors = UINT_MAX >> 9;
+               *granularity = sbuf.f_bsize;
        }
-
-       lim->max_hw_discard_sectors = max_discard_sectors;
-       lim->max_write_zeroes_sectors = max_discard_sectors;
-       if (max_discard_sectors)
-               lim->discard_granularity = granularity;
-       else
-               lim->discard_granularity = 0;
 }
 
 struct loop_worker {
@@ -991,6 +983,7 @@ static int loop_reconfigure_limits(struct loop_device *lo, unsigned int bsize)
        struct inode *inode = file->f_mapping->host;
        struct block_device *backing_bdev = NULL;
        struct queue_limits lim;
+       u32 granularity = 0, max_discard_sectors = 0;
 
        if (S_ISBLK(inode->i_mode))
                backing_bdev = I_BDEV(inode);
@@ -1000,6 +993,8 @@ static int loop_reconfigure_limits(struct loop_device *lo, unsigned int bsize)
        if (!bsize)
                bsize = loop_default_blocksize(lo, backing_bdev);
 
+       loop_get_discard_config(lo, &granularity, &max_discard_sectors);
+
        lim = queue_limits_start_update(lo->lo_queue);
        lim.logical_block_size = bsize;
        lim.physical_block_size = bsize;
@@ -1009,7 +1004,12 @@ static int loop_reconfigure_limits(struct loop_device *lo, unsigned int bsize)
                lim.features |= BLK_FEAT_WRITE_CACHE;
        if (backing_bdev && !bdev_nonrot(backing_bdev))
                lim.features |= BLK_FEAT_ROTATIONAL;
-       loop_config_discard(lo, &lim);
+       lim.max_hw_discard_sectors = max_discard_sectors;
+       lim.max_write_zeroes_sectors = max_discard_sectors;
+       if (max_discard_sectors)
+               lim.discard_granularity = granularity;
+       else
+               lim.discard_granularity = 0;
        return queue_limits_commit_update(lo->lo_queue, &lim);
 }