selinux: never allow relabeling on context mounts

In the SECURITY_FS_USE_MNTPOINT case we never want to allow relabeling
files/directories, so we should never set the SBLABEL_MNT flag. The
'special handling' in selinux_is_sblabel_mnt() is only intended for when
the behavior is set to SECURITY_FS_USE_GENFS.

While there, make the logic in selinux_is_sblabel_mnt() more explicit
and add a BUILD_BUG_ON() to make sure that introducing a new
SECURITY_FS_USE_* forces a review of the logic.

Fixes: d5f3a5f6e7e7 ("selinux: add security in-core xattr support for pstore and debugfs")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 31 insertions, 9 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index f08a0f201967..f8931c5d252a 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -534,16 +534,10 @@ static int may_context_mount_inode_relabel(u32 sid,
         return rc;
 }
 
-static int selinux_is_sblabel_mnt(struct super_block *sb)
+static int selinux_is_genfs_special_handling(struct super_block *sb)
 {
-        struct superblock_security_struct *sbsec = sb->s_security;
-
-        return sbsec->behavior == SECURITY_FS_USE_XATTR ||
-                sbsec->behavior == SECURITY_FS_USE_TRANS ||
-                sbsec->behavior == SECURITY_FS_USE_TASK ||
-                sbsec->behavior == SECURITY_FS_USE_NATIVE ||
-                /* Special handling. Genfs but also in-core setxattr handler */
-                !strcmp(sb->s_type->name, "sysfs") ||
+        /* Special handling. Genfs but also in-core setxattr handler */
+        return  !strcmp(sb->s_type->name, "sysfs") ||
                 !strcmp(sb->s_type->name, "pstore") ||
                 !strcmp(sb->s_type->name, "debugfs") ||
                 !strcmp(sb->s_type->name, "tracefs") ||
@@ -553,6 +547,34 @@ static int selinux_is_sblabel_mnt(struct super_block *sb)
                   !strcmp(sb->s_type->name, "cgroup2")));
 }
 
+static int selinux_is_sblabel_mnt(struct super_block *sb)
+{
+        struct superblock_security_struct *sbsec = sb->s_security;
+
+        /*
+         * IMPORTANT: Double-check logic in this function when adding a new
+         * SECURITY_FS_USE_* definition!
+         */
+        BUILD_BUG_ON(SECURITY_FS_USE_MAX != 7);
+
+        switch (sbsec->behavior) {
+        case SECURITY_FS_USE_XATTR:
+        case SECURITY_FS_USE_TRANS:
+        case SECURITY_FS_USE_TASK:
+        case SECURITY_FS_USE_NATIVE:
+                return 1;
+
+        case SECURITY_FS_USE_GENFS:
+                return selinux_is_genfs_special_handling(sb);
+
+        /* Never allow relabeling on context mounts */
+        case SECURITY_FS_USE_MNTPOINT:
+        case SECURITY_FS_USE_NONE:
+        default:
+                return 0;
+        }
+}
+
 static int sb_finish_set_opts(struct super_block *sb)
 {
         struct superblock_security_struct *sbsec = sb->s_security;