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Subject[PATCH 3.18 09/29] loop: add recursion validation to LOOP_CHANGE_FD
Date
3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@mit.edu>

commit d2ac838e4cd7e5e9891ecc094d626734b0245c99 upstream.

Refactor the validation code used in LOOP_SET_FD so it is also used in
LOOP_CHANGE_FD. Otherwise it is possible to construct a set of loop
devices that all refer to each other. This can lead to a infinite
loop in starting with "while (is_loop_device(f)) .." in loop_set_fd().

Fix this by refactoring out the validation code and using it for
LOOP_CHANGE_FD as well as LOOP_SET_FD.

Reported-by: syzbot+4349872271ece473a7c91190b68b4bac7c5dbc87@syzkaller.appspotmail.com
Reported-by: syzbot+40bd32c4d9a3cc12a339@syzkaller.appspotmail.com
Reported-by: syzbot+769c54e66f994b041be7@syzkaller.appspotmail.com
Reported-by: syzbot+0a89a9ce473936c57065@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/block/loop.c | 68 ++++++++++++++++++++++++++++-----------------------
1 file changed, 38 insertions(+), 30 deletions(-)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -628,6 +628,36 @@ out:
}


+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 loop_validate_file(struct file *file, struct block_device *bdev)
+{
+ struct inode *inode = file->f_mapping->host;
+ struct file *f = file;
+
+ /* Avoid recursion */
+ while (is_loop_device(f)) {
+ struct loop_device *l;
+
+ if (f->f_mapping->host->i_bdev == bdev)
+ return -EBADF;
+
+ l = f->f_mapping->host->i_bdev->bd_disk->private_data;
+ if (l->lo_state == Lo_unbound) {
+ return -EINVAL;
+ }
+ f = l->lo_backing_file;
+ }
+ if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
+ return -EINVAL;
+ 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
@@ -657,14 +687,15 @@ static int loop_change_fd(struct loop_de
if (!file)
goto out;

+ error = loop_validate_file(file, bdev);
+ if (error)
+ goto out_putf;
+
inode = file->f_mapping->host;
old_file = lo->lo_backing_file;

error = -EINVAL;

- if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
- goto out_putf;
-
/* size of the new backing store needs to be the same */
if (get_loop_size(lo, file) != get_loop_size(lo, old_file))
goto out_putf;
@@ -685,13 +716,6 @@ static int loop_change_fd(struct loop_de
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,
@@ -823,7 +847,7 @@ static void loop_config_discard(struct l
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;
unsigned lo_blocksize;
@@ -843,29 +867,13 @@ static int loop_set_fd(struct loop_devic
if (lo->lo_state != Lo_unbound)
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 = loop_validate_file(file, bdev);
+ if (error)
+ goto out_putf;

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

- error = -EINVAL;
- if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
- goto out_putf;
-
if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) ||
!file->f_op->write)
lo_flags |= LO_FLAGS_READ_ONLY;

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