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authorEric W. Biederman <ebiederm@xmission.com>2017-01-31 20:06:16 +0300
committerEric W. Biederman <ebiederm@xmission.com>2017-02-01 18:36:12 +0300
commit93faccbbfa958a9668d3ab4e30f38dd205cee8d8 (patch)
treef4a102b92d86d19a52a88a57bdde19ad25250ca1 /include
parentc6c70f4455d1eda91065e93cc4f7eddf4499b105 (diff)
downloadlinux-93faccbbfa958a9668d3ab4e30f38dd205cee8d8.tar.xz
fs: Better permission checking for submounts
To support unprivileged users mounting filesystems two permission checks have to be performed: a test to see if the user allowed to create a mount in the mount namespace, and a test to see if the user is allowed to access the specified filesystem. The automount case is special in that mounting the original filesystem grants permission to mount the sub-filesystems, to any user who happens to stumble across the their mountpoint and satisfies the ordinary filesystem permission checks. Attempting to handle the automount case by using override_creds almost works. It preserves the idea that permission to mount the original filesystem is permission to mount the sub-filesystem. Unfortunately using override_creds messes up the filesystems ordinary permission checks. Solve this by being explicit that a mount is a submount by introducing vfs_submount, and using it where appropriate. vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let sget and friends know that a mount is a submount so they can take appropriate action. sget and sget_userns are modified to not perform any permission checks on submounts. follow_automount is modified to stop using override_creds as that has proven problemantic. do_mount is modified to always remove the new MS_SUBMOUNT flag so that we know userspace will never by able to specify it. autofs4 is modified to stop using current_real_cred that was put in there to handle the previous version of submount permission checking. cifs is modified to pass the mountpoint all of the way down to vfs_submount. debugfs is modified to pass the mountpoint all of the way down to trace_automount by adding a new parameter. To make this change easier a new typedef debugfs_automount_t is introduced to capture the type of the debugfs automount function. Cc: stable@vger.kernel.org Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using current_real_cred()->uid") Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds") Reviewed-by: Trond Myklebust <trond.myklebust@primarydata.com> Reviewed-by: Seth Forshee <seth.forshee@canonical.com> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Diffstat (limited to 'include')
-rw-r--r--include/linux/debugfs.h3
-rw-r--r--include/linux/mount.h3
-rw-r--r--include/uapi/linux/fs.h1
3 files changed, 6 insertions, 1 deletions
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 014cc564d1c4..233006be30aa 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -97,9 +97,10 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
const char *dest);
+typedef struct vfsmount *(*debugfs_automount_t)(struct dentry *, void *);
struct dentry *debugfs_create_automount(const char *name,
struct dentry *parent,
- struct vfsmount *(*f)(void *),
+ debugfs_automount_t f,
void *data);
void debugfs_remove(struct dentry *dentry);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index c6f55158d5e5..8e0352af06b7 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -90,6 +90,9 @@ struct file_system_type;
extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
int flags, const char *name,
void *data);
+extern struct vfsmount *vfs_submount(const struct dentry *mountpoint,
+ struct file_system_type *type,
+ const char *name, void *data);
extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list);
extern void mark_mounts_for_expiry(struct list_head *mounts);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 36da93fbf188..048a85e9f017 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -132,6 +132,7 @@ struct inodes_stat_t {
#define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
/* These sb flags are internal to the kernel */
+#define MS_SUBMOUNT (1<<26)
#define MS_NOREMOTELOCK (1<<27)
#define MS_NOSEC (1<<28)
#define MS_BORN (1<<29)