Re: [PATCH v2 1/2] LSM: SafeSetID: gate setgid transitions

[Micah Morton <mortonm@chromium.org>]

I'm fine with that. Sent patch 1/2, will send patch 2/2 in a bit (just
needs minor mods due to combining the hooks).

On Thu, Feb 28, 2019 at 8:50 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> On 2/27/2019 12:00 PM, mortonm@chromium.org wrote:
> > From: Micah Morton <mortonm@chromium.org>
> >
> > This patch adds a 'task_fix_setgid' LSM hook, which is analogous to the
> > existing 'task_fix_setuid' LSM hook, and calls this new hook from the
> > setgid functions in kernel/sys.c. This will allow the SafeSetID LSM to
> > govern setgid transitions in addition to setuid transitions. This change
> > also makes sure the setgid functions in kernel/sys.c call
> > security_capable_setid rather than the ordinary security_capable
> > function, so that the security_capable hook in the SafeSetID LSM knows
> > it is being invoked from a setid function.
> >
> > Signed-off-by: Micah Morton <mortonm@chromium.org>
> > ---
> > Changes since the last patch: took out the cap_task_fix_setgid stuff
> > from include/linux/security.h since this hook won't be hooked by
> > security/commoncap.c.
> >   include/linux/lsm_hooks.h | 12 ++++++++++++
> >   include/linux/security.h  |  9 +++++++++
> >   kernel/sys.c              | 27 +++++++++++++++++++++------
> >   security/security.c       |  6 ++++++
> >   4 files changed, 48 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index 22fc786d723a..f252ed3e95ef 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -603,6 +603,15 @@
> >    *  @old is the set of credentials that are being replaces
> >    *  @flags contains one of the LSM_SETID_* values.
> >    *  Return 0 on success.
> > + * @task_fix_setgid:
> > + *      Update the module's state after setting one or more of the group
> > + *      identity attributes of the current process.  The @flags parameter
> > + *      indicates which of the set*gid system calls invoked this hook.
> > + *      @new is the set of credentials that will be installed.  Modifications
> > + *      should be made to this rather than to @current->cred.
> > + *      @old is the set of credentials that are being replaced
> > + *      @flags contains one of the LSM_SETID_* values.
> > + *      Return 0 on success.
> >    * @task_setpgid:
> >    *  Check permission before setting the process group identifier of the
> >    *  process @p to @pgid.
> > @@ -1596,6 +1605,8 @@ union security_list_options {
> >                                    enum kernel_read_file_id id);
> >       int (*task_fix_setuid)(struct cred *new, const struct cred *old,
> >                               int flags);
> > +     int (*task_fix_setgid)(struct cred *new, const struct cred *old,
> > +                             int flags);
>
> Do you suppose that it be better to have one task_fix_id() hook and
> use the flags to differentiate between uid and gid requests? Since you're
> the only user of the hooks no one else should complain, and it will help
> keep the LSM infrastructure from expanding unnecessarily.
>
> >       int (*task_setpgid)(struct task_struct *p, pid_t pgid);
> >       int (*task_getpgid)(struct task_struct *p);
> >       int (*task_getsid)(struct task_struct *p);
> > @@ -1887,6 +1898,7 @@ struct security_hook_heads {
> >       struct hlist_head kernel_post_read_file;
> >       struct hlist_head kernel_module_request;
> >       struct hlist_head task_fix_setuid;
> > +     struct hlist_head task_fix_setgid;
> >       struct hlist_head task_setpgid;
> >       struct hlist_head task_getpgid;
> >       struct hlist_head task_getsid;
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index 13537a49ae97..e28ef6bf6280 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -326,6 +326,8 @@ int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
> >                                  enum kernel_read_file_id id);
> >   int security_task_fix_setuid(struct cred *new, const struct cred *old,
> >                            int flags);
> > +int security_task_fix_setgid(struct cred *new, const struct cred *old,
> > +                          int flags);
> >   int security_task_setpgid(struct task_struct *p, pid_t pgid);
> >   int security_task_getpgid(struct task_struct *p);
> >   int security_task_getsid(struct task_struct *p);
> > @@ -930,6 +932,13 @@ static inline int security_task_fix_setuid(struct cred *new,
> >       return cap_task_fix_setuid(new, old, flags);
> >   }
> >
> > +static inline int security_task_fix_setgid(struct cred *new,
> > +                                        const struct cred *old,
> > +                                        int flags)
> > +{
> > +     return 0;
> > +}
> > +
> >   static inline int security_task_setpgid(struct task_struct *p, pid_t pgid)
> >   {
> >       return 0;
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index c5f875048aef..76f1c46ac66f 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -372,7 +372,7 @@ long __sys_setregid(gid_t rgid, gid_t egid)
> >       if (rgid != (gid_t) -1) {
> >               if (gid_eq(old->gid, krgid) ||
> >                   gid_eq(old->egid, krgid) ||
> > -                 ns_capable(old->user_ns, CAP_SETGID))
> > +                 ns_capable_setid(old->user_ns, CAP_SETGID))
> >                       new->gid = krgid;
> >               else
> >                       goto error;
> > @@ -381,7 +381,7 @@ long __sys_setregid(gid_t rgid, gid_t egid)
> >               if (gid_eq(old->gid, kegid) ||
> >                   gid_eq(old->egid, kegid) ||
> >                   gid_eq(old->sgid, kegid) ||
> > -                 ns_capable(old->user_ns, CAP_SETGID))
> > +                 ns_capable_setid(old->user_ns, CAP_SETGID))
> >                       new->egid = kegid;
> >               else
> >                       goto error;
> > @@ -392,6 +392,10 @@ long __sys_setregid(gid_t rgid, gid_t egid)
> >               new->sgid = new->egid;
> >       new->fsgid = new->egid;
> >
> > +     retval = security_task_fix_setgid(new, old, LSM_SETID_RE);
> > +     if (retval < 0)
> > +             goto error;
> > +
> >       return commit_creds(new);
> >
> >   error:
> > @@ -427,13 +431,17 @@ long __sys_setgid(gid_t gid)
> >       old = current_cred();
> >
> >       retval = -EPERM;
> > -     if (ns_capable(old->user_ns, CAP_SETGID))
> > +     if (ns_capable_setid(old->user_ns, CAP_SETGID))
> >               new->gid = new->egid = new->sgid = new->fsgid = kgid;
> >       else if (gid_eq(kgid, old->gid) || gid_eq(kgid, old->sgid))
> >               new->egid = new->fsgid = kgid;
> >       else
> >               goto error;
> >
> > +     retval = security_task_fix_setgid(new, old, LSM_SETID_ID);
> > +     if (retval < 0)
> > +             goto error;
> > +
> >       return commit_creds(new);
> >
> >   error:
> > @@ -735,7 +743,7 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
> >       old = current_cred();
> >
> >       retval = -EPERM;
> > -     if (!ns_capable(old->user_ns, CAP_SETGID)) {
> > +     if (!ns_capable_setid(old->user_ns, CAP_SETGID)) {
> >               if (rgid != (gid_t) -1        && !gid_eq(krgid, old->gid) &&
> >                   !gid_eq(krgid, old->egid) && !gid_eq(krgid, old->sgid))
> >                       goto error;
> > @@ -755,6 +763,10 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
> >               new->sgid = ksgid;
> >       new->fsgid = new->egid;
> >
> > +     retval = security_task_fix_setgid(new, old, LSM_SETID_RES);
> > +     if (retval < 0)
> > +             goto error;
> > +
> >       return commit_creds(new);
> >
> >   error:
> > @@ -858,10 +870,13 @@ long __sys_setfsgid(gid_t gid)
> >
> >       if (gid_eq(kgid, old->gid)  || gid_eq(kgid, old->egid)  ||
> >           gid_eq(kgid, old->sgid) || gid_eq(kgid, old->fsgid) ||
> > -         ns_capable(old->user_ns, CAP_SETGID)) {
> > +         ns_capable_setid(old->user_ns, CAP_SETGID)) {
> >               if (!gid_eq(kgid, old->fsgid)) {
> >                       new->fsgid = kgid;
> > -                     goto change_okay;
> > +                     if (security_task_fix_setgid(new,
> > +                                             old,
> > +                                             LSM_SETID_FS) == 0)
> > +                             goto change_okay;
> >               }
> >       }
> >
> > diff --git a/security/security.c b/security/security.c
> > index ed9b8cbf21cf..7e936f944a66 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1574,6 +1574,12 @@ int security_task_fix_setuid(struct cred *new, const struct cred *old,
> >       return call_int_hook(task_fix_setuid, 0, new, old, flags);
> >   }
> >
> > +int security_task_fix_setgid(struct cred *new, const struct cred *old,
> > +                          int flags)
> > +{
> > +     return call_int_hook(task_fix_setgid, 0, new, old, flags);
> > +}
> > +
> >   int security_task_setpgid(struct task_struct *p, pid_t pgid)
> >   {
> >       return call_int_hook(task_setpgid, 0, p, pgid);