| Message ID | 1449070821-73820-4-git-send-email-seth.forshee@canonical.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
On Wed, Dec 02, 2015 at 09:40:03AM -0600, Seth Forshee wrote: > From: Andy Lutomirski <luto@amacapital.net> > > If a process gets access to a mount from a different user > namespace, that process should not be able to take advantage of > setuid files or selinux entrypoints from that filesystem. Prevent > this by treating mounts from other mount namespaces and those not > owned by current_user_ns() or an ancestor as nosuid. > > 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> > Signed-off-by: Seth Forshee <seth.forshee@canonical.com> > Acked-by: James Morris <james.l.morris@oracle.com> Acked-by: Serge Hallyn <serge.hallyn@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 da70f7c4ece1..2101ce7b96ab 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -3276,6 +3276,19 @@ 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 !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) && > + in_userns(current_user_ns(), mnt->mnt_sb->s_user_ns); > +} > + > 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 400aa224b491..6243aef5860e 100644 > --- a/security/commoncap.c > +++ b/security/commoncap.c > @@ -448,7 +448,7 @@ 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 d0cfaa9f19d0..a5b93df6553f 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -2171,7 +2171,7 @@ 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) > -- > 1.9.1
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 da70f7c4ece1..2101ce7b96ab 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3276,6 +3276,19 @@ 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 !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) && + in_userns(current_user_ns(), mnt->mnt_sb->s_user_ns); +} + 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 400aa224b491..6243aef5860e 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -448,7 +448,7 @@ 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 d0cfaa9f19d0..a5b93df6553f 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -2171,7 +2171,7 @@ 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)