[PATCH v3 1/2] LSM: add SafeSetID module that gates setid calls

[Micah Morton]

Patch set 1 of 2 was "Reviewed-by: Kees Cook <keescook at chromium.org>"
as well -- forgot to add that in the commit message above.

On Tue, Jan 22, 2019 at 2:42 PM <mortonm at chromium.org> wrote:
>
> From: Micah Morton <mortonm at chromium.org>
>
> This change ensures that the set*uid family of syscalls in kernel/sys.c
> (setreuid, setuid, setresuid, setfsuid) all call ns_capable_common with
> the CAP_OPT_INSETID flag, so capability checks in the security_capable
> hook can know whether they are being called from within a set*uid
> syscall. This change is a no-op by itself, but is needed for the
> proposed SafeSetID LSM.
>
> Signed-off-by: Micah Morton <mortonm at chromium.org>
> ---
> These changes used to be part of the main SafeSetID LSM patch set.
>
>  include/linux/capability.h |  5 +++++
>  kernel/capability.c        | 19 +++++++++++++++++++
>  kernel/sys.c               | 10 +++++-----
>  3 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index f640dcbc880c..c3f9a4d558a0 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -209,6 +209,7 @@ extern bool has_ns_capability_noaudit(struct task_struct *t,
>  extern bool capable(int cap);
>  extern bool ns_capable(struct user_namespace *ns, int cap);
>  extern bool ns_capable_noaudit(struct user_namespace *ns, int cap);
> +extern bool ns_capable_setid(struct user_namespace *ns, int cap);
>  #else
>  static inline bool has_capability(struct task_struct *t, int cap)
>  {
> @@ -240,6 +241,10 @@ static inline bool ns_capable_noaudit(struct user_namespace *ns, int cap)
>  {
>         return true;
>  }
> +static inline bool ns_capable_setid(struct user_namespace *ns, int cap)
> +{
> +       return true;
> +}
>  #endif /* CONFIG_MULTIUSER */
>  extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode);
>  extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 7718d7dcadc7..e0734ace5bc2 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -417,6 +417,25 @@ bool ns_capable_noaudit(struct user_namespace *ns, int cap)
>  }
>  EXPORT_SYMBOL(ns_capable_noaudit);
>
> +/**
> + * ns_capable_setid - Determine if the current task has a superior capability
> + * in effect, while signalling that this check is being done from within a
> + * setid syscall.
> + * @ns:  The usernamespace we want the capability in
> + * @cap: The capability to be tested for
> + *
> + * Return true if the current task has the given superior capability currently
> + * available for use, false if not.
> + *
> + * This sets PF_SUPERPRIV on the task if the capability is available on the
> + * assumption that it's about to be used.
> + */
> +bool ns_capable_setid(struct user_namespace *ns, int cap)
> +{
> +       return ns_capable_common(ns, cap, CAP_OPT_INSETID);
> +}
> +EXPORT_SYMBOL(ns_capable_setid);
> +
>  /**
>   * capable - Determine if the current task has a superior capability in effect
>   * @cap: The capability to be tested for
> diff --git a/kernel/sys.c b/kernel/sys.c
> index a48cbf1414b8..a98061c1a124 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -516,7 +516,7 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
>                 new->uid = kruid;
>                 if (!uid_eq(old->uid, kruid) &&
>                     !uid_eq(old->euid, kruid) &&
> -                   !ns_capable(old->user_ns, CAP_SETUID))
> +                   !ns_capable_setid(old->user_ns, CAP_SETUID))
>                         goto error;
>         }
>
> @@ -525,7 +525,7 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
>                 if (!uid_eq(old->uid, keuid) &&
>                     !uid_eq(old->euid, keuid) &&
>                     !uid_eq(old->suid, keuid) &&
> -                   !ns_capable(old->user_ns, CAP_SETUID))
> +                   !ns_capable_setid(old->user_ns, CAP_SETUID))
>                         goto error;
>         }
>
> @@ -584,7 +584,7 @@ long __sys_setuid(uid_t uid)
>         old = current_cred();
>
>         retval = -EPERM;
> -       if (ns_capable(old->user_ns, CAP_SETUID)) {
> +       if (ns_capable_setid(old->user_ns, CAP_SETUID)) {
>                 new->suid = new->uid = kuid;
>                 if (!uid_eq(kuid, old->uid)) {
>                         retval = set_user(new);
> @@ -646,7 +646,7 @@ long __sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
>         old = current_cred();
>
>         retval = -EPERM;
> -       if (!ns_capable(old->user_ns, CAP_SETUID)) {
> +       if (!ns_capable_setid(old->user_ns, CAP_SETUID)) {
>                 if (ruid != (uid_t) -1        && !uid_eq(kruid, old->uid) &&
>                     !uid_eq(kruid, old->euid) && !uid_eq(kruid, old->suid))
>                         goto error;
> @@ -814,7 +814,7 @@ long __sys_setfsuid(uid_t uid)
>
>         if (uid_eq(kuid, old->uid)  || uid_eq(kuid, old->euid)  ||
>             uid_eq(kuid, old->suid) || uid_eq(kuid, old->fsuid) ||
> -           ns_capable(old->user_ns, CAP_SETUID)) {
> +           ns_capable_setid(old->user_ns, CAP_SETUID)) {
>                 if (!uid_eq(kuid, old->fsuid)) {
>                         new->fsuid = kuid;
>                         if (security_task_fix_setuid(new, old, LSM_SETID_FS) == 0)
> --
> 2.20.1.97.g81188d93c3-goog
>