Re: [syzbot] [exfat?] possible deadlock in fat_count_free_clusters

[OGAWA Hirofumi]

Hi,

syzbot <syzbot+a5d8c609c02f508672cc@xxxxxxxxxxxxxxxxxxxxxxxxx> writes:

> syzbot found the following issue on:
>
> HEAD commit:    929beafbe7ac Add linux-next specific files for 20241108
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1621bd87980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=75175323f2078363
> dashboard link: https://syzkaller.appspot.com/bug?extid=a5d8c609c02f508672cc
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

This patch is to fix the above race. Please check this.

Thanks


From: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
Subject: [PATCH] loop: Fix ABBA locking race
Date: Mon, 11 Nov 2024 21:53:36 +0900

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 there is no reason to hold q->limits_locks while
getting discard 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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=a5d8c609c02f508672cc
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
---
 drivers/block/loop.c |   31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 78a7bb2..5f3ce51 100644
--- a/drivers/block/loop.c	2024-09-16 13:45:20.253220178 +0900
+++ b/drivers/block/loop.c	2024-11-11 21:51:00.910135443 +0900
@@ -770,12 +770,11 @@ static void loop_sysfs_exit(struct loop_
 				   &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;
 
 	/*
@@ -788,8 +787,9 @@ static void loop_config_discard(struct l
 	if (S_ISBLK(inode->i_mode)) {
 		struct request_queue *backingq = bdev_get_queue(I_BDEV(inode));
 
-		max_discard_sectors = backingq->limits.max_write_zeroes_sectors;
-		granularity = bdev_discard_granularity(I_BDEV(inode)) ?:
+		*max_discard_sectors =
+			backingq->limits.max_write_zeroes_sectors;
+		*granularity = bdev_discard_granularity(I_BDEV(inode)) ?:
 			queue_physical_block_size(backingq);
 
 	/*
@@ -797,16 +797,9 @@ static void loop_config_discard(struct l
 	 * 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 {
@@ -992,6 +985,7 @@ static int loop_reconfigure_limits(struc
 	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);
@@ -1001,6 +995,8 @@ static int loop_reconfigure_limits(struc
 	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;
@@ -1010,7 +1006,12 @@ static int loop_reconfigure_limits(struc
 		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);
 }
 
_
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>