[PATCH 4.19 90/99] selinux: never allow relabeling on context mounts

[Greg Kroah-Hartman]

From: Ondrej Mosnacek <omosn...@redhat.com>

commit a83d6ddaebe541570291205cb538e35ad4ff94f9 upstream.

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 <omosn...@redhat.com>
Reviewed-by: Stephen Smalley <s...@tycho.nsa.gov>
Signed-off-by: Paul Moore <p...@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 security/selinux/hooks.c |   40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -497,16 +497,10 @@ static int may_context_mount_inode_relab
        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") ||
@@ -516,6 +510,34 @@ static int selinux_is_sblabel_mnt(struct
                  !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;