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authorEric W. Biederman <ebiederm@xmission.com>2017-02-01 06:06:16 +1300
committerEric W. Biederman <ebiederm@xmission.com>2017-02-02 04:36:12 +1300
commit93faccbbfa958a9668d3ab4e30f38dd205cee8d8 (patch)
treef4a102b92d86d19a52a88a57bdde19ad25250ca1 /fs/namespace.c
parentc6c70f4455d1eda91065e93cc4f7eddf4499b105 (diff)
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 'fs/namespace.c')
-rw-r--r--fs/namespace.c17
1 files changed, 16 insertions, 1 deletions
diff --git a/fs/namespace.c b/fs/namespace.c
index 487ba30bb5c6..089a6b23135a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -989,6 +989,21 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
}
EXPORT_SYMBOL_GPL(vfs_kern_mount);
+struct vfsmount *
+vfs_submount(const struct dentry *mountpoint, struct file_system_type *type,
+ const char *name, void *data)
+{
+ /* Until it is worked out how to pass the user namespace
+ * through from the parent mount to the submount don't support
+ * unprivileged mounts with submounts.
+ */
+ if (mountpoint->d_sb->s_user_ns != &init_user_ns)
+ return ERR_PTR(-EPERM);
+
+ return vfs_kern_mount(type, MS_SUBMOUNT, name, data);
+}
+EXPORT_SYMBOL_GPL(vfs_submount);
+
static struct mount *clone_mnt(struct mount *old, struct dentry *root,
int flag)
{
@@ -2794,7 +2809,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
- MS_STRICTATIME | MS_NOREMOTELOCK);
+ MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);
if (flags & MS_REMOUNT)
retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,