[PATCH 5.13 053/104] loop: reintroduce global lock for safe loop_validate_file() traversal

Greg Kroah-Hartman
2021-08-02T14:13:48Z

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

[ Upstream commit 3ce6e1f662a910970880188ea7bfd00542bd3934 ]

Commit 6cc8e7430801fa23 ("loop: scale loop device by introducing per
device lock") re-opened a race window for NULL pointer dereference at
loop_validate_file() where commit 310ca162d779efee ("block/loop: Use
global lock for ioctl() operation.") has closed.

Although we need to guarantee that other loop devices will not change
during traversal, we can't take remote "struct loop_device"->lo_mutex
inside loop_validate_file() in order to avoid AB-BA deadlock. Therefore,
introduce a global lock dedicated for loop_validate_file() which is
conditionally taken before local "struct loop_device"->lo_mutex is taken.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 6cc8e7430801fa23 ("loop: scale loop device by introducing per device lock")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/block/loop.c | 128 ++++++++++++++++++++++++++++++++-----------
1 file changed, 97 insertions(+), 31 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 8271df125153..e81298b91227 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -86,6 +86,47 @@

static DEFINE_IDR(loop_index_idr);
static DEFINE_MUTEX(loop_ctl_mutex);
+static DEFINE_MUTEX(loop_validate_mutex);
+
+/**
+ * loop_global_lock_killable() - take locks for safe loop_validate_file() test
+ *
+ * @lo: struct loop_device
+ * @global: true if @lo is about to bind another "struct loop_device", false otherwise
+ *
+ * Returns 0 on success, -EINTR otherwise.
+ *
+ * Since loop_validate_file() traverses on other "struct loop_device" if
+ * is_loop_device() is true, we need a global lock for serializing concurrent
+ * loop_configure()/loop_change_fd()/__loop_clr_fd() calls.
+ */
+static int loop_global_lock_killable(struct loop_device *lo, bool global)
+{
+ int err;
+
+ if (global) {
+ err = mutex_lock_killable(&loop_validate_mutex);
+ if (err)
+ return err;
+ }
+ err = mutex_lock_killable(&lo->lo_mutex);
+ if (err && global)
+ mutex_unlock(&loop_validate_mutex);
+ return err;
+}
+
+/**
+ * loop_global_unlock() - release locks taken by loop_global_lock_killable()
+ *
+ * @lo: struct loop_device
+ * @global: true if @lo was about to bind another "struct loop_device", false otherwise
+ */
+static void loop_global_unlock(struct loop_device *lo, bool global)
+{
+ mutex_unlock(&lo->lo_mutex);
+ if (global)
+ mutex_unlock(&loop_validate_mutex);
+}

static int max_part;
static int part_shift;
@@ -676,13 +717,15 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
while (is_loop_device(f)) {
struct loop_device *l;

+ lockdep_assert_held(&loop_validate_mutex);
if (f->f_mapping->host->i_rdev == bdev->bd_dev)
return -EBADF;

l = I_BDEV(f->f_mapping->host)->bd_disk->private_data;
- if (l->lo_state != Lo_bound) {
+ if (l->lo_state != Lo_bound)
return -EINVAL;
- }
+ /* Order wrt setting lo->lo_backing_file in loop_configure(). */
+ rmb();
f = l->lo_backing_file;
}
if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
@@ -701,13 +744,18 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
unsigned int arg)
{
- struct file *file = NULL, *old_file;
- int error;
- bool partscan;
+ struct file *file = fget(arg);
+ struct file *old_file;
+ int error;
+ bool partscan;
+ bool is_loop;

- error = mutex_lock_killable(&lo->lo_mutex);
+ if (!file)
+ return -EBADF;
+ is_loop = is_loop_device(file);
+ error = loop_global_lock_killable(lo, is_loop);
if (error)
- return error;
+ goto out_putf;
error = -ENXIO;
if (lo->lo_state != Lo_bound)
goto out_err;
@@ -717,11 +765,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
if (!(lo->lo_flags & LO_FLAGS_READ_ONLY))
goto out_err;

- error = -EBADF;
- file = fget(arg);
- if (!file)
- goto out_err;
-
error = loop_validate_file(file, bdev);
if (error)
goto out_err;
@@ -744,7 +787,16 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
loop_update_dio(lo);
blk_mq_unfreeze_queue(lo->lo_queue);
partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
- mutex_unlock(&lo->lo_mutex);
+ loop_global_unlock(lo, is_loop);
+
+ /*
+ * Flush loop_validate_file() before fput(), for l->lo_backing_file
+ * might be pointing at old_file which might be the last reference.
+ */
+ if (!is_loop) {
+ mutex_lock(&loop_validate_mutex);
+ mutex_unlock(&loop_validate_mutex);
+ }
/*
* We must drop file reference outside of lo_mutex as dropping
* the file ref can take bd_mutex which creates circular locking
@@ -756,9 +808,9 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
return 0;

out_err:
- mutex_unlock(&lo->lo_mutex);
- if (file)
- fput(file);
+ loop_global_unlock(lo, is_loop);
+out_putf:
+ fput(file);
return error;
}

@@ -1067,22 +1119,22 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
struct block_device *bdev,
const struct loop_config *config)
{
- struct file *file;
- struct inode *inode;
+ struct file *file = fget(config->fd);
+ struct inode *inode;
struct address_space *mapping;
- int error;
- loff_t size;
- bool partscan;
- unsigned short bsize;
+ int error;
+ loff_t size;
+ bool partscan;
+ unsigned short bsize;
+ bool is_loop;
+
+ if (!file)
+ return -EBADF;
+ is_loop = is_loop_device(file);

/* This is safe, since we have a reference from open(). */
__module_get(THIS_MODULE);

- error = -EBADF;
- file = fget(config->fd);
- if (!file)
- goto out;
-
/*
* If we don't hold exclusive handle for the device, upgrade to it
* here to avoid changing device under exclusive owner.
@@ -1093,7 +1145,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
goto out_putf;
}

- error = mutex_lock_killable(&lo->lo_mutex);
+ error = loop_global_lock_killable(lo, is_loop);
if (error)
goto out_bdev;

@@ -1162,6 +1214,9 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
size = get_loop_size(lo, file);
loop_set_size(lo, size);

+ /* Order wrt reading lo_state in loop_validate_file(). */
+ wmb();
+
lo->lo_state = Lo_bound;
if (part_shift)
lo->lo_flags |= LO_FLAGS_PARTSCAN;
@@ -1173,7 +1228,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
* put /dev/loopXX inode. Later in __loop_clr_fd() we bdput(bdev).
*/
bdgrab(bdev);
- mutex_unlock(&lo->lo_mutex);
+ loop_global_unlock(lo, is_loop);
if (partscan)
loop_reread_partitions(lo, bdev);
if (!(mode & FMODE_EXCL))
@@ -1181,13 +1236,12 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
return 0;

out_unlock:
- mutex_unlock(&lo->lo_mutex);
+ loop_global_unlock(lo, is_loop);
out_bdev:
if (!(mode & FMODE_EXCL))
bd_abort_claiming(bdev, loop_configure);
out_putf:
fput(file);
-out:
/* This is safe: open() is still holding a reference. */
module_put(THIS_MODULE);
return error;
@@ -1202,6 +1256,18 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
bool partscan = false;
int lo_number;

+ /*
+ * Flush loop_configure() and loop_change_fd(). It is acceptable for
+ * loop_validate_file() to succeed, for actual clear operation has not
+ * started yet.
+ */
+ mutex_lock(&loop_validate_mutex);
+ mutex_unlock(&loop_validate_mutex);
+ /*
+ * loop_validate_file() now fails because l->lo_state != Lo_bound
+ * became visible.
+ */
+
mutex_lock(&lo->lo_mutex);
if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
err = -ENXIO;
--
2.30.2