Commit b4912557 authored by OGAWA Hirofumi's avatar OGAWA Hirofumi Committed by Jens Axboe
Browse files

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: default avatar <syzbot+a5d8c609c02f508672cc@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=a5d8c609c02f508672cc


Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
Signed-off-by: default avatarOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 46fd48ab

drivers/block/loop.c

+15 −15
Original line number Diff line number Diff line
@@ -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);

}