Re: [PATCH v3] scripts/selinux: add basic mls support to mdp

[Stephen Smalley <sds@xxxxxxxxxxxxx>]

On 2/15/19 10:05 AM, Stephen Smalley wrote:
> On 2/15/19 10:03 AM, Stephen Smalley wrote:
> > On 2/15/19 10:00 AM, Paul Moore wrote:
> > > On Fri, Feb 15, 2019 at 9:51 AM Stephen Smalley <sds@xxxxxxxxxxxxx> 
> > > wrote:
> > > > Add basic MLS policy support to mdp.  Declares
> > > > two sensitivities and two categories, defines
> > > > mls constraints for all permissions requiring
> > > > dominance (ala MCS), assigns the system-high
> > > > level to initial SID contexts and the default user
> > > > level, and assigns system-low level to filesystems.
> > > >
> > > > Also reworks the fs_use and genfscon rules to only
> > > > generate rules for filesystems that are configured
> > > > in the kernel.  In some cases this depends on a specific
> > > > config option for security xattrs, in other cases security
> > > > xattrs are unconditionally supported by a given filesystem
> > > > if the filesystem is enabled, and in some cases the filesystem
> > > > is always enabled in the kernel.  Dropped obsolete pseudo
> > > > filesystems.
> > > >
> > > > NB The list of fs_use_* and genfscon rules emitted by mdp
> > > > is very incomplete compared to refpolicy or Android sepolicy.
> > > > We should probably expand it.
> > > >
> > > > Usage:
> > > > scripts/selinux/mdp/mdp -m policy.conf file_contexts
> > > > checkpolicy -M -o policy policy.conf
> > > >
> > > > Then install the resulting policy and file_contexts as usual.
> > > >
> > > > Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
> > > > ---
> > > > v3 fixes up the file contexts generation code to also use SYSTEMLOW and
> > > > collapse down to a single fprintf call per line.
> > > >   scripts/selinux/mdp/mdp.c | 131 
> > > > ++++++++++++++++++++++++++++++--------
> > > >   1 file changed, 103 insertions(+), 28 deletions(-)
> > >
> > > This is great Stephen, thanks for working on this - and rather quickly
> > > too!  For those who don't follow the GitHub issues, I just opened an
> > > issue yesterday mentioning it would be nice to add MLS support to the
> > > mdp tool.
> > >
> > > Are you planning to keep playing with this?  I'm asking not because I
> > > think it needs more work to be worthwhile, but rather I don't want to
> > > merge something that you want to continue working on.  If you are
> > > happy with this latest patch I think it is okay to merge this into
> > > selinux/next, even at this late stage, simply because it is not part
> > > of a built kernel, but rather a developer's tool.
> >
> > No, I think I'm done for now unless you find a problem with it.  
> > Absent some compelling use case for mdp it is hard to justify spending 
> > any more time on it.
>
> Note however that the instructions in 
> Documentation/admin-guide/LSM/SELinux.rst just say to run 
> scripts/selinux/install_policy.sh and since that doesn't pass -m to mdp 
> or -M to checkpolicy, no one will use this support unless they do it all 
> by hand.

FWIW, a Fedora system wouldn't come up cleanly with this policy.  Partly 
appears to be due to systemd having embedded security contexts specific 
to Fedora/refpolicy into its own configurations and partly due to MLS 
denials.  I don't even know if it would work before this change though...

> > > > diff --git a/scripts/selinux/mdp/mdp.c b/scripts/selinux/mdp/mdp.c
> > > > index 073fe7537f6c..4223e2fea441 100644
> > > > --- a/scripts/selinux/mdp/mdp.c
> > > > +++ b/scripts/selinux/mdp/mdp.c
> > > > @@ -33,6 +33,7 @@
> > > >   #include <unistd.h>
> > > >   #include <string.h>
> > > >   #include <sys/socket.h>
> > > > +#include <linux/kconfig.h>
> > > >
> > > >   static void usage(char *name)
> > > >   {
> > > > @@ -95,10 +96,31 @@ int main(int argc, char *argv[])
> > > >          }
> > > >          fprintf(fout, "\n");
> > > >
> > > > -       /* NOW PRINT OUT MLS STUFF */
> > > > +       /* print out mls declarations and constraints */
> > > >          if (mls) {
> > > > -               printf("MLS not yet implemented\n");
> > > > -               exit(1);
> > > > +               fprintf(fout, "sensitivity s0;\n");
> > > > +               fprintf(fout, "sensitivity s1;\n");
> > > > +               fprintf(fout, "dominance { s0 s1 }\n");
> > > > +               fprintf(fout, "category c0;\n");
> > > > +               fprintf(fout, "category c1;\n");
> > > > +               fprintf(fout, "level s0:c0.c1;\n");
> > > > +               fprintf(fout, "level s1:c0.c1;\n");
> > > > +#define SYSTEMLOW "s0"
> > > > +#define SYSTEMHIGH "s1:c0.c1"
> > > > +               for (i = 0; secclass_map[i].name; i++) {
> > > > +                       struct security_class_mapping *map = 
> > > > &secclass_map[i];
> > > > +
> > > > +                       fprintf(fout, "mlsconstrain %s {\n", 
> > > > map->name);
> > > > +                       for (j = 0; map->perms[j]; j++)
> > > > +                               fprintf(fout, "\t%s\n", map->perms[j]);
> > > > +                       /*
> > > > +                        * This requires all subjects and objects to be
> > > > +                        * single-level (l2 eq h2), and that the 
> > > > subject
> > > > +                        * level dominate the object level (h1 dom h2)
> > > > +                        * in order to have any permissions to it.
> > > > +                        */
> > > > +                       fprintf(fout, "} (l2 eq h2 and h1 dom 
> > > > h2);\n\n");
> > > > +               }
> > > >          }
> > > >
> > > >          /* types, roles, and allows */
> > > > @@ -108,34 +130,87 @@ int main(int argc, char *argv[])
> > > >          for (i = 0; secclass_map[i].name; i++)
> > > >                  fprintf(fout, "allow base_t base_t:%s *;\n",
> > > >                          secclass_map[i].name);
> > > > -       fprintf(fout, "user user_u roles { base_r };\n");
> > > > -       fprintf(fout, "\n");
> > > > +       fprintf(fout, "user user_u roles { base_r }");
> > > > +       if (mls)
> > > > +               fprintf(fout, " level %s range %s - %s", SYSTEMHIGH,
> > > > +                       SYSTEMLOW, SYSTEMHIGH);
> > > > +       fprintf(fout, ";\n");
> > > > +
> > > > +#define USERROLETYPE "user_u:base_r:base_t"
> > > >
> > > >          /* default sids */
> > > >          for (i = 1; i < initial_sid_to_string_len; i++)
> > > > -               fprintf(fout, "sid %s user_u:base_r:base_t\n", 
> > > > initial_sid_to_string[i]);
> > > > +               fprintf(fout, "sid %s " USERROLETYPE "%s\n",
> > > > +                       initial_sid_to_string[i], mls ? ":" 
> > > > SYSTEMHIGH : "");
> > > >          fprintf(fout, "\n");
> > > >
> > > > -       fprintf(fout, "fs_use_xattr ext2 user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr ext3 user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr ext4 user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr jfs user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr xfs user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr reiserfs user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr jffs2 user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_xattr gfs2 user_u:base_r:base_t;\n");
> > > > -
> > > > -       fprintf(fout, "fs_use_task eventpollfs 
> > > > user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_task pipefs user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_task sockfs user_u:base_r:base_t;\n");
> > > > -
> > > > -       fprintf(fout, "fs_use_trans mqueue user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_trans devpts user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_trans hugetlbfs 
> > > > user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_trans tmpfs user_u:base_r:base_t;\n");
> > > > -       fprintf(fout, "fs_use_trans shm user_u:base_r:base_t;\n");
> > > > -
> > > > -       fprintf(fout, "genfscon proc / user_u:base_r:base_t\n");
> > > > +#define FS_USE(behavior, fstype)                           \
> > > > +       fprintf(fout, "fs_use_%s %s " USERROLETYPE "%s;\n", \
> > > > +               behavior, fstype, mls ? ":" SYSTEMLOW : "")
> > > > +
> > > > +       /*
> > > > +        * Filesystems whose inode labels can be fetched via getxattr.
> > > > +        */
> > > > +#ifdef CONFIG_EXT2_FS_SECURITY
> > > > +       FS_USE("xattr", "ext2");
> > > > +#endif
> > > > +#ifdef CONFIG_EXT3_FS_SECURITY
> > > > +       FS_USE("xattr", "ext3");
> > > > +#endif
> > > > +#ifdef CONFIG_EXT4_FS_SECURITY
> > > > +       FS_USE("xattr", "ext4");
> > > > +#endif
> > > > +#ifdef CONFIG_JFS_SECURITY
> > > > +       FS_USE("xattr", "jfs");
> > > > +#endif
> > > > +#ifdef CONFIG_REISERFS_FS_SECURITY
> > > > +       FS_USE("xattr", "reiserfs");
> > > > +#endif
> > > > +#ifdef CONFIG_JFFS2_FS_SECURITY
> > > > +       FS_USE("xattr", "jffs2");
> > > > +#endif
> > > > +#ifdef CONFIG_XFS_FS
> > > > +       FS_USE("xattr", "xfs");
> > > > +#endif
> > > > +#ifdef CONFIG_GFS2_FS
> > > > +       FS_USE("xattr", "gfs2");
> > > > +#endif
> > > > +
> > > > +       /*
> > > > +        * Filesystems whose inodes are labeled from allocating task.
> > > > +        */
> > > > +       FS_USE("task", "pipefs");
> > > > +       FS_USE("task", "sockfs");
> > > > +#ifdef CONFIG_POSIX_MQUEUE
> > > > +       FS_USE("task", "mqueue");
> > > > +#endif
> > > > +
> > > > +       /*
> > > > +        * Filesystems whose inode labels are computed from both
> > > > +        * the allocating task and the superblock label.
> > > > +        */
> > > > +#ifdef CONFIG_UNIX98_PTYS
> > > > +       FS_USE("trans", "devpts");
> > > > +#endif
> > > > +#ifdef CONFIG_HUGETLBFS
> > > > +       FS_USE("trans", "hugetlbfs");
> > > > +#endif
> > > > +#ifdef CONFIG_TMPFS
> > > > +       FS_USE("trans", "tmpfs");
> > > > +#endif
> > > > +
> > > > +
> > > > +#define GENFSCON(fstype, prefix)                            \
> > > > +       fprintf(fout, "genfscon %s %s " USERROLETYPE "%s\n", \
> > > > +               fstype, prefix, mls ? ":" SYSTEMLOW : "")
> > > > +
> > > > +       /*
> > > > +        * Filesystems whose inodes are labeled from path prefix match
> > > > +        * relative to the filesystem root.  Depending on the 
> > > > filesystem,
> > > > +        * only a single label for all inodes may be supported.
> > > > +        */
> > > > +       GENFSCON("proc", "/");
> > > > +       GENFSCON("selinuxfs", "/");
> > > >
> > > >          fclose(fout);
> > > >
> > > > @@ -144,8 +219,8 @@ int main(int argc, char *argv[])
> > > >                  printf("Wrote policy, but cannot open %s for 
> > > > writing\n", ctxout);
> > > >                  usage(argv[0]);
> > > >          }
> > > > -       fprintf(fout, "/ user_u:base_r:base_t\n");
> > > > -       fprintf(fout, "/.* user_u:base_r:base_t\n");
> > > > +       fprintf(fout, "/ " USERROLETYPE "%s\n", mls ? ":" SYSTEMLOW 
> > > > : "");
> > > > +       fprintf(fout, "/.* " USERROLETYPE "%s\n", mls ? ":" 
> > > > SYSTEMLOW : "");
> > > >          fclose(fout);
> > > >
> > > >          return 0;
> > > > --
> > > > 2.20.1