diff mbox

[Xenial,SRU] fs: Better permission checking for submounts

Message ID 1487167036-110530-2-git-send-email-seth.forshee@canonical.com
State New
Headers show

Commit Message

Seth Forshee Feb. 15, 2017, 1:57 p.m. UTC 
From: "Eric W. Biederman" <ebiederm@xmission.com>

BugLink: http://bugs.launchpad.net/bugs/1649292

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>
(cherry picked from commit 93faccbbfa958a9668d3ab4e30f38dd205cee8d8 linux-next)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 fs/afs/mntpt.c          |  2 +-
 fs/autofs4/waitq.c      |  4 ++--
 fs/cifs/cifs_dfs_ref.c  |  7 ++++---
 fs/debugfs/inode.c      |  8 ++++----
 fs/namei.c              |  3 ---
 fs/namespace.c          | 17 ++++++++++++++++-
 fs/nfs/namespace.c      |  2 +-
 fs/nfs/nfs4namespace.c  |  2 +-
 fs/super.c              | 13 ++++++++++---
 include/linux/debugfs.h |  3 ++-
 include/linux/mount.h   |  3 +++
 include/uapi/linux/fs.h |  1 +
 kernel/trace/trace.c    |  4 ++--
 13 files changed, 47 insertions(+), 22 deletions(-)
diff mbox

Patch

diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index ccd0b212e82a..05ba277e6269 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -202,7 +202,7 @@  static struct vfsmount *afs_mntpt_do_automount(struct dentry *mntpt)
 
 	/* try and do the mount */
 	_debug("--- attempting mount %s -o %s ---", devname, options);
-	mnt = vfs_kern_mount(&afs_fs_type, 0, devname, options);
+	mnt = vfs_submount(mntpt, &afs_fs_type, devname, options);
 	_debug("--- mount result %p ---", mnt);
 
 	free_page((unsigned long) devname);
diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
index 2b369c8ccfab..e17bf04d483f 100644
--- a/fs/autofs4/waitq.c
+++ b/fs/autofs4/waitq.c
@@ -427,8 +427,8 @@  int autofs4_wait(struct autofs_sb_info *sbi, struct dentry *dentry,
 		memcpy(&wq->name, &qstr, sizeof(struct qstr));
 		wq->dev = autofs4_get_dev(sbi);
 		wq->ino = autofs4_get_ino(sbi);
-		wq->uid = current_real_cred()->uid;
-		wq->gid = current_real_cred()->gid;
+		wq->uid = current_cred()->uid;
+		wq->gid = current_cred()->gid;
 		wq->pid = pid;
 		wq->tgid = tgid;
 		wq->status = -EINTR; /* Status return if interrupted */
diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
index 7dc886c9a78f..d2e35b1eb118 100644
--- a/fs/cifs/cifs_dfs_ref.c
+++ b/fs/cifs/cifs_dfs_ref.c
@@ -241,7 +241,8 @@  compose_mount_options_err:
  * @fullpath:		full path in UNC format
  * @ref:		server's referral
  */
-static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb,
+static struct vfsmount *cifs_dfs_do_refmount(struct dentry *mntpt,
+		struct cifs_sb_info *cifs_sb,
 		const char *fullpath, const struct dfs_info3_param *ref)
 {
 	struct vfsmount *mnt;
@@ -255,7 +256,7 @@  static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb,
 	if (IS_ERR(mountdata))
 		return (struct vfsmount *)mountdata;
 
-	mnt = vfs_kern_mount(&cifs_fs_type, 0, devname, mountdata);
+	mnt = vfs_submount(mntpt, &cifs_fs_type, devname, mountdata);
 	kfree(mountdata);
 	kfree(devname);
 	return mnt;
@@ -330,7 +331,7 @@  static struct vfsmount *cifs_dfs_do_automount(struct dentry *mntpt)
 			mnt = ERR_PTR(-EINVAL);
 			break;
 		}
-		mnt = cifs_dfs_do_refmount(cifs_sb,
+		mnt = cifs_dfs_do_refmount(mntpt, cifs_sb,
 				full_path, referrals + i);
 		cifs_dbg(FYI, "%s: cifs_dfs_do_refmount:%s , mnt:%p\n",
 			 __func__, referrals[i].node_name, mnt);
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 0f5d05bf2131..96198d193611 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -181,9 +181,9 @@  static const struct super_operations debugfs_super_operations = {
 
 static struct vfsmount *debugfs_automount(struct path *path)
 {
-	struct vfsmount *(*f)(void *);
-	f = (struct vfsmount *(*)(void *))path->dentry->d_fsdata;
-	return f(d_inode(path->dentry)->i_private);
+	debugfs_automount_t f;
+	f = (debugfs_automount_t)path->dentry->d_fsdata;
+	return f(path->dentry, d_inode(path->dentry)->i_private);
 }
 
 static const struct dentry_operations debugfs_dops = {
@@ -444,7 +444,7 @@  EXPORT_SYMBOL_GPL(debugfs_create_dir);
  */
 struct dentry *debugfs_create_automount(const char *name,
 					struct dentry *parent,
-					struct vfsmount *(*f)(void *),
+					debugfs_automount_t f,
 					void *data)
 {
 	struct dentry *dentry = start_creating(name, parent);
diff --git a/fs/namei.c b/fs/namei.c
index f251a018e79f..119a211b1c38 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1095,7 +1095,6 @@  static int follow_automount(struct path *path, struct nameidata *nd,
 			    bool *need_mntput)
 {
 	struct vfsmount *mnt;
-	const struct cred *old_cred;
 	int err;
 
 	if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
@@ -1124,9 +1123,7 @@  static int follow_automount(struct path *path, struct nameidata *nd,
 	if (nd->total_link_count >= 40)
 		return -ELOOP;
 
-	old_cred = override_creds(&init_cred);
 	mnt = path->dentry->d_op->d_automount(path);
-	revert_creds(old_cred);
 	if (IS_ERR(mnt)) {
 		/*
 		 * The filesystem is allowed to return -EISDIR here to indicate
diff --git a/fs/namespace.c b/fs/namespace.c
index 61d3e34b0be6..475457737647 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -994,6 +994,21 @@  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)
 {
@@ -2783,7 +2798,7 @@  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_STRICTATIME | MS_SUBMOUNT);
 
 	if (flags & MS_REMOUNT)
 		retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index c8162c660c44..7363c2b25059 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -226,7 +226,7 @@  static struct vfsmount *nfs_do_clone_mount(struct nfs_server *server,
 					   const char *devname,
 					   struct nfs_clone_mount *mountdata)
 {
-	return vfs_kern_mount(&nfs_xdev_fs_type, 0, devname, mountdata);
+	return vfs_submount(mountdata->dentry, &nfs_xdev_fs_type, devname, mountdata);
 }
 
 /**
diff --git a/fs/nfs/nfs4namespace.c b/fs/nfs/nfs4namespace.c
index f592672373cb..b2a7f72eb116 100644
--- a/fs/nfs/nfs4namespace.c
+++ b/fs/nfs/nfs4namespace.c
@@ -279,7 +279,7 @@  static struct vfsmount *try_location(struct nfs_clone_mount *mountdata,
 				mountdata->hostname,
 				mountdata->mnt_path);
 
-		mnt = vfs_kern_mount(&nfs4_referral_fs_type, 0, page, mountdata);
+		mnt = vfs_submount(mountdata->dentry, &nfs4_referral_fs_type, page, mountdata);
 		if (!IS_ERR(mnt))
 			break;
 	}
diff --git a/fs/super.c b/fs/super.c
index 4f5aef8ecdfe..d28b7a9f5e74 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -466,7 +466,7 @@  struct super_block *sget_userns(struct file_system_type *type,
 	struct super_block *old;
 	int err;
 
-	if (!(flags & MS_KERNMOUNT) &&
+	if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) &&
 	    !(type->fs_flags & FS_USERNS_MOUNT) &&
 	    !capable(CAP_SYS_ADMIN))
 		return ERR_PTR(-EPERM);
@@ -496,7 +496,7 @@  retry:
 	}
 	if (!s) {
 		spin_unlock(&sb_lock);
-		s = alloc_super(type, flags, user_ns);
+		s = alloc_super(type, (flags & ~MS_SUBMOUNT), user_ns);
 		if (!s)
 			return ERR_PTR(-ENOMEM);
 		goto retry;
@@ -537,8 +537,15 @@  struct super_block *sget(struct file_system_type *type,
 {
 	struct user_namespace *user_ns = current_user_ns();
 
+	/* We don't yet pass the user namespace of the parent
+	 * mount through to here so always use &init_user_ns
+	 * until that changes.
+	 */
+	if (flags & MS_SUBMOUNT)
+		user_ns = &init_user_ns;
+
 	/* Ensure the requestor has permissions over the target filesystem */
-	if (!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
+	if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
 		return ERR_PTR(-EPERM);
 
 	return sget_userns(type, test, set, flags, user_ns, data);
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 19c066dce1da..51362ff607fc 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -60,9 +60,10 @@  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 1172cce949a4..e0f3a82eee6d 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 f15d980249b5..612f566198aa 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -93,6 +93,7 @@  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_NOSEC	(1<<28)
 #define MS_BORN		(1<<29)
 #define MS_ACTIVE	(1<<30)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 059233abcfcf..7cb0fd37b4f6 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6816,7 +6816,7 @@  init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
 
 }
 
-static struct vfsmount *trace_automount(void *ingore)
+static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 {
 	struct vfsmount *mnt;
 	struct file_system_type *type;
@@ -6829,7 +6829,7 @@  static struct vfsmount *trace_automount(void *ingore)
 	type = get_fs_type("tracefs");
 	if (!type)
 		return NULL;
-	mnt = vfs_kern_mount(type, 0, "tracefs", NULL);
+	mnt = vfs_submount(mntpt, type, "tracefs", NULL);
 	put_filesystem(type);
 	if (IS_ERR(mnt))
 		return NULL;