[PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

[Andy Lutomirski]

If a process gets access to a mount from a descendent or unrelated
user namespace, that process should not be able to take advantage of
setuid files or selinux entrypoints from that filesystem.

This will make it safer to allow more complex filesystems to be
mounted in non-root user namespaces.

This does not remove the need for MNT_LOCK_NOSUID.  The setuid,
setgid, and file capability bits can no longer be abused if code in
a user namespace were to clear nosuid on an untrusted filesystem,
but this patch, by itself, is insufficient to protect the system
from abuse of files that, when execed, would increase MAC privilege.

As a more concrete explanation, any task that can manipulate a
vfsmount associated with a given user namespace already has
capabilities in that namespace and all of its descendents.  If they
can cause a malicious setuid, setgid, or file-caps executable to
appear in that mount, then that executable will only allow them to
elevate privileges in exactly the set of namespaces in which they
are already privileges.

On the other hand, if they can cause a malicious executable to
appear with a dangerous MAC label, running it could change the
caller's security context in a way that should not have been
possible, even inside the namespace in which the task is confined.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---

Seth, this should address a problem that's related to yours.  If a
userns creates and untrusted fs (by any means, although admittedly fuse
and user namespaces don't work all that well together right now), then
this prevents shenanigans that could happen when the userns passes an fd
pointing at the filesystem out to the root ns.

 fs/exec.c                |  2 +-
 fs/namespace.c           | 21 +++++++++++++++++++++
 include/linux/mount.h    |  1 +
 security/commoncap.c     |  2 +-
 security/selinux/hooks.c |  4 ++--
 5 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index a2b42a98c743..ac0bb22aa3ed 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1267,7 +1267,7 @@ int prepare_binprm(struct linux_binprm *bprm)
 	bprm->cred->euid = current_euid();
 	bprm->cred->egid = current_egid();
 
-	if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) &&
+	if (mnt_may_suid(bprm->file->f_path.mnt) &&
 	    !task_no_new_privs(current) &&
 	    kuid_has_mapping(bprm->cred->user_ns, inode->i_uid) &&
 	    kgid_has_mapping(bprm->cred->user_ns, inode->i_gid)) {
diff --git a/fs/namespace.c b/fs/namespace.c
index a01c7730e9af..53301680ea7e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3011,6 +3011,27 @@ found:
 	return visible;
 }
 
+bool mnt_may_suid(struct vfsmount *mnt)
+{
+	struct user_namespace *mount_userns = real_mount(mnt)->mnt_ns->user_ns;
+	struct user_namespace *ns;
+
+	if (mnt->mnt_flags & MNT_NOSUID)
+		return false;
+
+	/*
+	 * We only trust mounts in our own namespace or its parents; we
+	 * treat untrusted mounts as MNT_NOSUID regardless of whether
+	 * they have MNT_NOSUID set.
+	 */
+	for (ns = current_user_ns(); ns; ns = ns->parent) {
+		if (ns == mount_userns)
+			return true;
+	}
+
+	return false;
+}
+
 static void *mntns_get(struct task_struct *task)
 {
 	struct mnt_namespace *ns = NULL;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 9262e4bf0cc3..b7b84bafe09b 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -80,6 +80,7 @@ extern void mntput(struct vfsmount *mnt);
 extern struct vfsmount *mntget(struct vfsmount *mnt);
 extern struct vfsmount *mnt_clone_internal(struct path *path);
 extern int __mnt_is_readonly(struct vfsmount *mnt);
+extern bool mnt_may_suid(struct vfsmount *mnt);
 
 struct file_system_type;
 extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
diff --git a/security/commoncap.c b/security/commoncap.c
index bab0611afc1e..52b3eed065e0 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -443,7 +443,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
 	if (!file_caps_enabled)
 		return 0;
 
-	if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+	if (!mnt_may_suid(bprm->file->f_path.mnt))
 		return 0;
 
 	dentry = dget(bprm->file->f_dentry);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index b0e940497e23..2089fd0d539e 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2139,7 +2139,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
 		 */
 		if (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)
 			return -EPERM;
-		if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+		if (!mnt_may_suid(bprm->file->f_path.mnt))
 			return -EACCES;
 	} else {
 		/* Check for a default transition on this program. */
@@ -2153,7 +2153,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
 	ad.type = LSM_AUDIT_DATA_PATH;
 	ad.u.path = bprm->file->f_path;
 
-	if ((bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
+	if (!mnt_may_suid(bprm->file->f_path.mnt) ||
 	    (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS))
 		new_tsec->sid = old_tsec->sid;
 
-- 
1.9.3

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