SELinux: implement the new sb_remount LSM hook

For SELinux we do not allow security information to change during a remount
operation.  Thus this hook simply strips the security module options from
the data and verifies that those are the same options as exist on the
current superblock.

Signed-off-by: Eric Paris <eparis@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>

1 files changed, 86 insertions, 0 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 3decf07b8dc..f2f3cf2e806 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2363,6 +2363,91 @@ out:
         return rc;
 }
 
+static int selinux_sb_remount(struct super_block *sb, void *data)
+{
+        int rc, i, *flags;
+        struct security_mnt_opts opts;
+        char *secdata, **mount_options;
+        struct superblock_security_struct *sbsec = sb->s_security;
+
+        if (!(sbsec->flags & SE_SBINITIALIZED))
+                return 0;
+
+        if (!data)
+                return 0;
+
+        if (sb->s_type->fs_flags & FS_BINARY_MOUNTDATA)
+                return 0;
+
+        security_init_mnt_opts(&opts);
+        secdata = alloc_secdata();
+        if (!secdata)
+                return -ENOMEM;
+        rc = selinux_sb_copy_data(data, secdata);
+        if (rc)
+                goto out_free_secdata;
+
+        rc = selinux_parse_opts_str(secdata, &opts);
+        if (rc)
+                goto out_free_secdata;
+
+        mount_options = opts.mnt_opts;
+        flags = opts.mnt_opts_flags;
+
+        for (i = 0; i < opts.num_mnt_opts; i++) {
+                u32 sid;
+                size_t len;
+
+                if (flags[i] == SE_SBLABELSUPP)
+                        continue;
+                len = strlen(mount_options[i]);
+                rc = security_context_to_sid(mount_options[i], len, &sid);
+                if (rc) {
+                        printk(KERN_WARNING "SELinux: security_context_to_sid"
+                               "(%s) failed for (dev %s, type %s) errno=%d\n",
+                               mount_options[i], sb->s_id, sb->s_type->name, rc);
+                        goto out_free_opts;
+                }
+                rc = -EINVAL;
+                switch (flags[i]) {
+                case FSCONTEXT_MNT:
+                        if (bad_option(sbsec, FSCONTEXT_MNT, sbsec->sid, sid))
+                                goto out_bad_option;
+                        break;
+                case CONTEXT_MNT:
+                        if (bad_option(sbsec, CONTEXT_MNT, sbsec->mntpoint_sid, sid))
+                                goto out_bad_option;
+                        break;
+                case ROOTCONTEXT_MNT: {
+                        struct inode_security_struct *root_isec;
+                        root_isec = sb->s_root->d_inode->i_security;
+
+                        if (bad_option(sbsec, ROOTCONTEXT_MNT, root_isec->sid, sid))
+                                goto out_bad_option;
+                        break;
+                }
+                case DEFCONTEXT_MNT:
+                        if (bad_option(sbsec, DEFCONTEXT_MNT, sbsec->def_sid, sid))
+                                goto out_bad_option;
+                        break;
+                default:
+                        goto out_free_opts;
+                }
+        }
+
+        rc = 0;
+out_free_opts:
+        security_free_mnt_opts(&opts);
+out_free_secdata:
+        free_secdata(secdata);
+        return rc;
+out_bad_option:
+        printk(KERN_WARNING "SELinux: unable to change security options "
+               "during remount (dev %s, type=%s)\n", sb->s_id,
+               sb->s_type->name);
+        goto out_free_opts;
+}
+
 static int selinux_sb_kern_mount(struct super_block *sb, int flags, void *data)
 {
         const struct cred *cred = current_cred();
@@ -5356,6 +5441,7 @@ static struct security_operations selinux_ops = {
         .sb_alloc_security =            selinux_sb_alloc_security,
         .sb_free_security =             selinux_sb_free_security,
         .sb_copy_data =                 selinux_sb_copy_data,
+        .sb_remount =                   selinux_sb_remount,
         .sb_kern_mount =                selinux_sb_kern_mount,
         .sb_show_options =              selinux_sb_show_options,
         .sb_statfs =                    selinux_sb_statfs,