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SubjectRe: INFO: task hung in blk_freeze_queue
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Date
A patch for this specific report is ready. I don't know whether other
"dup" reports will be solved by this patch. Thus, I "undup" this report.

#syz undup

From eed54c6ae475860a9c63b37b58f34735e792eef7 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Sat, 5 May 2018 12:59:12 +0900
Subject: [PATCH] block/loop: Add recursion check for LOOP_CHANGE_FD request.

syzbot is reporting hung tasks at blk_freeze_queue() [1]. This is
due to ioctl(loop_fd, LOOP_CHANGE_FD, loop_fd) request which should be
rejected. Fix this by adding same recursion check which is used by
LOOP_SET_FD request.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+2ab52b8d94df5e2eaa01@syzkaller.appspotmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
---
drivers/block/loop.c | 59 ++++++++++++++++++++++++++++++++--------------------
1 file changed, 37 insertions(+), 22 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 5d4e316..cee3c01 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -644,6 +644,34 @@ static void loop_reread_partitions(struct loop_device *lo,
__func__, lo->lo_number, lo->lo_file_name, rc);
}

+static inline int is_loop_device(struct file *file)
+{
+ struct inode *i = file->f_mapping->host;
+
+ return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
+}
+
+static int check_loop_recursion(struct file *f, struct block_device *bdev)
+{
+ /*
+ * FIXME: Traversing on other loop devices without corresponding
+ * lo_ctl_mutex is not safe. l->lo_state can become Lo_rundown and
+ * l->lo_backing_file can become NULL when raced with LOOP_CLR_FD.
+ */
+ while (is_loop_device(f)) {
+ struct loop_device *l;
+
+ if (f->f_mapping->host->i_bdev == bdev)
+ return -EBUSY;
+
+ l = f->f_mapping->host->i_bdev->bd_disk->private_data;
+ if (l->lo_state == Lo_unbound)
+ return -EINVAL;
+ f = l->lo_backing_file;
+ }
+ return 0;
+}
+
/*
* loop_change_fd switched the backing store of a loopback device to
* a new file. This is useful for operating system installers to free up
@@ -673,6 +701,11 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
if (!file)
goto out;

+ /* Avoid recursion */
+ error = check_loop_recursion(file, bdev);
+ if (error)
+ goto out_putf;
+
inode = file->f_mapping->host;
old_file = lo->lo_backing_file;

@@ -706,13 +739,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
return error;
}

-static inline int is_loop_device(struct file *file)
-{
- struct inode *i = file->f_mapping->host;
-
- return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
-}
-
/* loop sysfs attributes */

static ssize_t loop_attr_show(struct device *dev, char *page,
@@ -877,7 +903,7 @@ static int loop_prepare_queue(struct loop_device *lo)
static int loop_set_fd(struct loop_device *lo, fmode_t mode,
struct block_device *bdev, unsigned int arg)
{
- struct file *file, *f;
+ struct file *file;
struct inode *inode;
struct address_space *mapping;
int lo_flags = 0;
@@ -897,20 +923,9 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
goto out_putf;

/* Avoid recursion */
- f = file;
- while (is_loop_device(f)) {
- struct loop_device *l;
-
- if (f->f_mapping->host->i_bdev == bdev)
- goto out_putf;
-
- l = f->f_mapping->host->i_bdev->bd_disk->private_data;
- if (l->lo_state == Lo_unbound) {
- error = -EINVAL;
- goto out_putf;
- }
- f = l->lo_backing_file;
- }
+ error = check_loop_recursion(file, bdev);
+ if (error)
+ goto out_putf;

mapping = file->f_mapping;
inode = mapping->host;
--
1.8.3.1

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