Re: [PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[Christian GÃttsche]

Am Fr., 24. Jan. 2020 um 19:53 Uhr schrieb Stephen Smalley <sds@xxxxxxxxxxxxx>:
>
> On 1/24/20 1:42 PM, Christian GÃttsche wrote:
> > Currently symlinks on kernel filesystems, like sysfs, are labeled on
> > creation with the parent fs root sid.
> >
> > Allow symlinks to inherit the parent directory context, so fine-grained
> > kernfs labeling can be applied to symlinks too and checking contexts
> > doesn't complain about them.
> >
> > For backward-compatibility this behavior is contained in a new policy
> > capability: kernfs_sovereign_symlinks
> >
> > Signed-off-by: Christian GÃttsche <cgzones@xxxxxxxxxxxxxx>
> > ---
> >   security/selinux/hooks.c            | 5 ++++-
> >   security/selinux/include/security.h | 8 ++++++++
> >   security/selinux/ss/services.c      | 3 ++-
> >   3 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index d9e8b2131..1303bc8c4 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -1475,7 +1475,10 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
> >               /* Default to the fs superblock SID. */
> >               sid = sbsec->sid;
> >
> > -             if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
> > +             if (((sbsec->flags & SE_SBGENFS) &&
> > +                  (!S_ISLNK(inode->i_mode))) ||
> > +                 (selinux_policycap_kernfs_sovereign_symlinks() &&
>
> Not fond of the name.  1) kernfs is a kernel implementation detail,
> shouldn't be exposed to policy; genfs is the policy construct 2)
> sovereign doesn't seem to fit the meaning of this capability; seclabel
> would be more appropriate.

Something like genfs_seclabel_symlinks?

> > +                  (sbsec->flags & SE_SBGENFS_XATTR))) {
>
> Why limit this to SE_SBGENFS_XATTR filesystems?  Why not just make the test:
>         if ((sbsec->flags & SE_SBGENFS) && (!S_ISLNK(inode->i_mode) ||
> selinux_policycap_genfs_symlinkseclabel()))
> or similar.

I somehow thought that this functionality is limited to filesystems
with SE_SBGENFS_XATTR;
so I can expand the check to SE_SBGENFS.

> >                       /* We must have a dentry to determine the label on
> >                        * procfs inodes */
> >                       if (opt_dentry) {
> > diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> > index a39f9565d..cc8217848 100644
> > --- a/security/selinux/include/security.h
> > +++ b/security/selinux/include/security.h
> > @@ -79,6 +79,7 @@ enum {
> >       POLICYDB_CAPABILITY_ALWAYSNETWORK,
> >       POLICYDB_CAPABILITY_CGROUPSECLABEL,
> >       POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> > +     POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS,
> >       __POLICYDB_CAPABILITY_MAX
> >   };
> >   #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> > @@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
> >       return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
> >   }
> >
> > +static inline bool selinux_policycap_kernfs_sovereign_symlinks(void)
> > +{
> > +     struct selinux_state *state = &selinux_state;
> > +
> > +     return state->policycap[POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS];
> > +}
> > +
> >   int security_mls_enabled(struct selinux_state *state);
> >   int security_load_policy(struct selinux_state *state,
> >                        void *data, size_t len);
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > index 216ce602a..b70380947 100644
> > --- a/security/selinux/ss/services.c
> > +++ b/security/selinux/ss/services.c
> > @@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
> >       "extended_socket_class",
> >       "always_check_network",
> >       "cgroup_seclabel",
> > -     "nnp_nosuid_transition"
> > +     "nnp_nosuid_transition",
> > +     "kernfs_sovereign_symlinks"
> >   };
> >
> >   static struct selinux_ss selinux_ss;
> >
>