bpf: Use mount_nodev not mount_ns to mount the bpf filesystem

ebiederm · davem330 · commit e27f4a942a0e · 2016-05-20T19:45:40.000-04:00
While reviewing the filesystems that set FS_USERNS_MOUNT I spotted the bpf filesystem. Looking at the code I saw a broken usage of mount_ns with current->nsproxy->mnt_ns. As the code does not acquire a reference to the mount namespace it can not possibly be correct to store the mount namespace on the superblock as it does. Replace mount_ns with mount_nodev so that each mount of the bpf filesystem returns a distinct instance, and the code is not buggy. In discussion with Hannes Frederic Sowa it was reported that the use of mount_ns was an attempt to have one bpf instance per mount namespace, in an attempt to keep resources that pin resources from hiding. That intent simply does not work, the vfs is not built to allow that kind of behavior. Which means that the bpf filesystem really is buggy both semantically and in it's implemenation as it does not nor can it implement the original intent. This change is userspace visible, but my experience with similar filesystems leads me to believe nothing will break with a model of each mount of the bpf filesystem is distinct from all others. Fixes: b219775 ("bpf: add support for persistent maps/progs") Cc: Hannes Frederic Sowa <hannes@stressinduktion.org> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
@@ -357,7 +357,7 @@ static int bpf_fill_super(struct super_block *sb, void *data, int silent)
 static struct dentry *bpf_mount(struct file_system_type *type, int flags,
 				const char *dev_name, void *data)
 {
-	return mount_ns(type, flags, current->nsproxy->mnt_ns, bpf_fill_super);
+	return mount_nodev(type, flags, data, bpf_fill_super);
 }
 
 static struct file_system_type bpf_fs_type = {

Original file line number	Diff line number	Diff line change
`@@ -357,7 +357,7 @@ static int bpf_fill_super(struct super_block sb, void data, int silent)`
`357`	`357`	`static struct dentry bpf_mount(struct file_system_type type, int flags,`
`358`	`358`	`const char dev_name, void data)`
`359`	`359`	`{`
`360`		`- return mount_ns(type, flags, current->nsproxy->mnt_ns, bpf_fill_super);`
	`360`	`+ return mount_nodev(type, flags, data, bpf_fill_super);`
`361`	`361`	`}`
`362`	`362`
`363`	`363`	`static struct file_system_type bpf_fs_type = {`