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

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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









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