[Devel] [PATCH vz10 2/2] fs: enforce cgroup permissions for bdevs on mount

[Aleksei Oladko]

In mount operations, cgroup device access permissions are checked only
when a new super_block is created. If the device has already been mounted,
no permission check is performed since the super_block already exists.
As a result, a process belonging to a cgroup without access permissions
to the device can still perform a mount operation successfully.

To reproduce:
 # mount /dev/loop0 /mnt/test0
 # echo $$ > /sys/fs/cgroup/machine.slice/CTID/cgroup.procs
 # mount /dev/loop0 /mnt/test1
 #

If the device was not mounted before, the mount operation fails
as expected:
 # echo $$ > /sys/fs/cgroup/machine.slice/CTID/cgroup.procs
 # mount /dev/loop0 /mnt/test1
mount: /mnt/test1: permission denied.

This patch adds a cgroup permission check for the mounting process
before obtaining the device's super_block. As a result, even if the
device has already been mounted and the super_block exists, the mount
operation will be denied when the process lacks the appropriate cgroup
permissions.

https://virtuozzo.atlassian.net/browse/VSTOR-105978

Signed-off-by: Aleksei Oladko <aleksey.oladko at virtuozzo.com>
---
 block/blk.h            |  1 -
 fs/nilfs2/super.c      |  2 +-
 fs/super.c             | 14 +++++++++++++-
 include/linux/blkdev.h |  1 +
 include/linux/fs.h     |  1 +
 5 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/block/blk.h b/block/blk.h
index c718e4291db0..8610d87e5050 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -727,7 +727,6 @@ static inline void bio_issue_init(struct bio_issue *issue,
 void bdev_release(struct file *bdev_file);
 int bdev_open(struct block_device *bdev, blk_mode_t mode, void *holder,
 	      const struct blk_holder_ops *hops, struct file *bdev_file);
-int bdev_permission(dev_t dev, blk_mode_t mode, void *holder);
 
 void blk_integrity_generate(struct bio *bio);
 void blk_integrity_verify(struct bio *bio);
diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
index eca79cca3803..9ee186478575 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -1210,7 +1210,7 @@ nilfs_get_tree(struct fs_context *fc)
 	if (err)
 		return err;
 
-	s = sget_dev(fc, dev);
+	s = sget_dev_check(fc, dev);
 	if (IS_ERR(s))
 		return PTR_ERR(s);
 
diff --git a/fs/super.c b/fs/super.c
index b8da7df103b3..514573da3ecb 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1405,6 +1405,18 @@ struct super_block *sget_dev(struct fs_context *fc, dev_t dev)
 }
 EXPORT_SYMBOL(sget_dev);
 
+struct super_block *sget_dev_check(struct fs_context *fc, dev_t dev)
+{
+	int ret;
+
+	blk_mode_t mode = sb_open_mode(fc->sb_flags) | BLK_OPEN_MOUNT;
+	ret = bdev_permission(dev, mode, fc);
+	if (ret)
+		return ERR_PTR(ret);
+	return sget_dev(fc, dev);
+}
+EXPORT_SYMBOL(sget_dev_check);
+
 #ifdef CONFIG_BLOCK
 /*
  * Lock the superblock that is holder of the bdev. Returns the superblock
@@ -1662,7 +1674,7 @@ int get_tree_bdev_flags(struct fs_context *fc,
 		return error;
 	}
 	fc->sb_flags |= SB_NOSEC;
-	s = sget_dev(fc, dev);
+	s = sget_dev_check(fc, dev);
 	if (IS_ERR(s))
 		return PTR_ERR(s);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 030242b56e04..0bbb65885692 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1660,6 +1660,7 @@ struct file *bdev_file_open_by_path(const char *path, blk_mode_t mode,
 int bd_prepare_to_claim(struct block_device *bdev, void *holder,
 		const struct blk_holder_ops *hops);
 void bd_abort_claiming(struct block_device *bdev, void *holder);
+int bdev_permission(dev_t dev, blk_mode_t mode, void *holder);
 
 /* just for blk-cgroup, don't use elsewhere */
 struct block_device *blkdev_get_no_open(dev_t dev);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b6f7809cad29..1f535909524c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2608,6 +2608,7 @@ struct super_block *sget(struct file_system_type *type,
 			int (*set)(struct super_block *,void *),
 			int flags, void *data);
 struct super_block *sget_dev(struct fs_context *fc, dev_t dev);
+struct super_block *sget_dev_check(struct fs_context *fc, dev_t dev);
 
 /* Alas, no aliases. Too much hassle with bringing module.h everywhere */
 #define fops_get(fops) ({						\
-- 
2.43.0