[4/7] fs: Treat foreign mounts as nosuid
diff mbox

Message ID 1436989569-69582-5-git-send-email-seth.forshee@canonical.com
State New
Headers show

Commit Message

Seth Forshee July 15, 2015, 7:46 p.m. UTC
From: Andy Lutomirski <luto@amacapital.net>

If a process gets access to a mount from a different namespace user
namespace, that process should not be able to take advantage of
setuid files or selinux entrypoints from that filesystem.
Technically, trusting mounts created by the same or ancestor user
namespaces ought to be safe, but it's simpler to distrust all
foreign mounts.

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.

As a hardening measure, this would have made CVE-2014-5207 much
more difficult to exploit.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
[ saf: Forward ported to 4.2 ]
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 fs/exec.c                |  2 +-
 fs/namespace.c           | 13 +++++++++++++
 include/linux/mount.h    |  1 +
 security/commoncap.c     |  2 +-
 security/selinux/hooks.c |  2 +-
 5 files changed, 17 insertions(+), 3 deletions(-)

Comments

Nikolay Borisov July 17, 2015, 6:46 a.m. UTC | #1
On 07/15/2015 10:46 PM, Seth Forshee wrote:
> From: Andy Lutomirski <luto@amacapital.net>
> 
> If a process gets access to a mount from a different namespace user
> namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem.
> Technically, trusting mounts created by the same or ancestor user
> namespaces ought to be safe, but it's simpler to distrust all
> foreign mounts.
> 
> 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.
> 
> As a hardening measure, this would have made CVE-2014-5207 much
> more difficult to exploit.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> [ saf: Forward ported to 4.2 ]
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> ---
>  fs/exec.c                |  2 +-
>  fs/namespace.c           | 13 +++++++++++++
>  include/linux/mount.h    |  1 +
>  security/commoncap.c     |  2 +-
>  security/selinux/hooks.c |  2 +-
>  5 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index b06623a9347f..ea7311d72cc3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1295,7 +1295,7 @@ static void bprm_fill_uid(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))
>  		return;
>  
>  	if (task_no_new_privs(current))
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 423001de32a2..2bfd7ca92247 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3252,6 +3252,19 @@ found:
>  	return visible;
>  }
>  
> +bool mnt_may_suid(struct vfsmount *mnt)
> +{
> +	/*
> +	 * Foreign mounts (accessed via fchdir or through /proc
> +	 * symlinks) are always treated as if they are nosuid.  This
> +	 * prevents namespaces from trusting potentially unsafe
> +	 * suid/sgid bits, file caps, or security labels that originate
> +	 * in other namespaces.
> +	 */
> +	return real_mount(mnt)->mnt_ns == current->nsproxy->mnt_ns &&
> +		!(mnt->mnt_flags & MNT_NOSUID);

Maybe check_mnt() from fs/namespace.c can be exported and used here,
instead of open coding it.

> +}
> +
>  static struct ns_common *mntns_get(struct task_struct *task)
>  {
>  	struct ns_common *ns = NULL;
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index f822c3c11377..54a594d49733 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -81,6 +81,7 @@ 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 path;
>  extern struct vfsmount *clone_private_mount(struct path *path);
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 175ab497e810..858d86a1b73c 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -437,7 +437,7 @@ 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;
>  	if (!in_userns(current_user_ns(), bprm->file->f_path.mnt->mnt_sb->s_user_ns))
>  		return 0;
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 564079c5c49d..459e71ddbc9d 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2137,7 +2137,7 @@ static int check_nnp_nosuid(const struct linux_binprm *bprm,
>  			    const struct task_security_struct *new_tsec)
>  {
>  	int nnp = (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS);
> -	int nosuid = (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID);
> +	int nosuid = !mnt_may_suid(bprm->file->f_path.mnt);
>  	int rc;
>  
>  	if (!nnp && !nosuid)
> 
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Patch
diff mbox

diff --git a/fs/exec.c b/fs/exec.c
index b06623a9347f..ea7311d72cc3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1295,7 +1295,7 @@  static void bprm_fill_uid(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))
 		return;
 
 	if (task_no_new_privs(current))
diff --git a/fs/namespace.c b/fs/namespace.c
index 423001de32a2..2bfd7ca92247 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3252,6 +3252,19 @@  found:
 	return visible;
 }
 
+bool mnt_may_suid(struct vfsmount *mnt)
+{
+	/*
+	 * Foreign mounts (accessed via fchdir or through /proc
+	 * symlinks) are always treated as if they are nosuid.  This
+	 * prevents namespaces from trusting potentially unsafe
+	 * suid/sgid bits, file caps, or security labels that originate
+	 * in other namespaces.
+	 */
+	return real_mount(mnt)->mnt_ns == current->nsproxy->mnt_ns &&
+		!(mnt->mnt_flags & MNT_NOSUID);
+}
+
 static struct ns_common *mntns_get(struct task_struct *task)
 {
 	struct ns_common *ns = NULL;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index f822c3c11377..54a594d49733 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -81,6 +81,7 @@  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 path;
 extern struct vfsmount *clone_private_mount(struct path *path);
diff --git a/security/commoncap.c b/security/commoncap.c
index 175ab497e810..858d86a1b73c 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -437,7 +437,7 @@  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;
 	if (!in_userns(current_user_ns(), bprm->file->f_path.mnt->mnt_sb->s_user_ns))
 		return 0;
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 564079c5c49d..459e71ddbc9d 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2137,7 +2137,7 @@  static int check_nnp_nosuid(const struct linux_binprm *bprm,
 			    const struct task_security_struct *new_tsec)
 {
 	int nnp = (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS);
-	int nosuid = (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID);
+	int nosuid = !mnt_may_suid(bprm->file->f_path.mnt);
 	int rc;
 
 	if (!nnp && !nosuid)