vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

When NFS superblocks are created by automounting, their LSM parameters
aren't set in the fs_context struct prior to sget_fc() being called,
leading to failure to match existing superblocks.

This bug leads to messages like the following appearing in dmesg when
fscache is enabled:

    NFS: Cache volume key already in use (nfs,4.2,2,108,106a8c0,1,,,,100000,100000,2ee,3a98,1d4c,3a98,1)

Fix this by adding a new LSM hook to load fc->security for submount
creation.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/217595.1662033775@warthog.procyon.org.uk/ # v5
Fixes: 9bc61ab18b1d ("vfs: Introduce fs_context, switch vfs_kern_mount() to it.")
Fixes: 779df6a5480f ("NFS: Ensure security label is set for root inode")
Tested-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: "Christian Brauner (Microsoft)" <brauner@kernel.org>
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Message-Id: <20230808-master-v9-1-e0ecde888221@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

3 files changed, 87 insertions, 0 deletions

diff --git a/security/security.c b/security/security.c
index b720424ca37d..549104a447e3 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1139,6 +1139,20 @@ void security_bprm_committed_creds(struct linux_binprm *bprm)
 }
 
 /**
+ * security_fs_context_submount() - Initialise fc->security
+ * @fc: new filesystem context
+ * @reference: dentry reference for submount/remount
+ *
+ * Fill out the ->security field for a new fs_context.
+ *
+ * Return: Returns 0 on success or negative error code on failure.
+ */
+int security_fs_context_submount(struct fs_context *fc, struct super_block *reference)
+{
+	return call_int_hook(fs_context_submount, 0, fc, reference);
+}
+
+/**
  * security_fs_context_dup() - Duplicate a fs_context LSM blob
  * @fc: destination filesystem context
  * @src_fc: source filesystem context
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d06e350fedee..afd663744041 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2745,6 +2745,27 @@ static int selinux_umount(struct vfsmount *mnt, int flags)
 				   FILESYSTEM__UNMOUNT, NULL);
 }
 
+static int selinux_fs_context_submount(struct fs_context *fc,
+				   struct super_block *reference)
+{
+	const struct superblock_security_struct *sbsec;
+	struct selinux_mnt_opts *opts;
+
+	opts = kzalloc(sizeof(*opts), GFP_KERNEL);
+	if (!opts)
+		return -ENOMEM;
+
+	sbsec = selinux_superblock(reference);
+	if (sbsec->flags & FSCONTEXT_MNT)
+		opts->fscontext_sid = sbsec->sid;
+	if (sbsec->flags & CONTEXT_MNT)
+		opts->context_sid = sbsec->mntpoint_sid;
+	if (sbsec->flags & DEFCONTEXT_MNT)
+		opts->defcontext_sid = sbsec->def_sid;
+	fc->security = opts;
+	return 0;
+}
+
 static int selinux_fs_context_dup(struct fs_context *fc,
 				  struct fs_context *src_fc)
 {
@@ -7182,6 +7203,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
 	/*
 	 * PUT "CLONING" (ACCESSING + ALLOCATING) HOOKS HERE
 	 */
+	LSM_HOOK_INIT(fs_context_submount, selinux_fs_context_submount),
 	LSM_HOOK_INIT(fs_context_dup, selinux_fs_context_dup),
 	LSM_HOOK_INIT(fs_context_parse_param, selinux_fs_context_parse_param),
 	LSM_HOOK_INIT(sb_eat_lsm_opts, selinux_sb_eat_lsm_opts),
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 6e270cf3fd30..a8201cf22f20 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -615,6 +615,56 @@ out_opt_err:
 }
 
 /**
+ * smack_fs_context_submount - Initialise security data for a filesystem context
+ * @fc: The filesystem context.
+ * @reference: reference superblock
+ *
+ * Returns 0 on success or -ENOMEM on error.
+ */
+static int smack_fs_context_submount(struct fs_context *fc,
+				 struct super_block *reference)
+{
+	struct superblock_smack *sbsp;
+	struct smack_mnt_opts *ctx;
+	struct inode_smack *isp;
+
+	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+	if (!ctx)
+		return -ENOMEM;
+	fc->security = ctx;
+
+	sbsp = smack_superblock(reference);
+	isp = smack_inode(reference->s_root->d_inode);
+
+	if (sbsp->smk_default) {
+		ctx->fsdefault = kstrdup(sbsp->smk_default->smk_known, GFP_KERNEL);
+		if (!ctx->fsdefault)
+			return -ENOMEM;
+	}
+
+	if (sbsp->smk_floor) {
+		ctx->fsfloor = kstrdup(sbsp->smk_floor->smk_known, GFP_KERNEL);
+		if (!ctx->fsfloor)
+			return -ENOMEM;
+	}
+
+	if (sbsp->smk_hat) {
+		ctx->fshat = kstrdup(sbsp->smk_hat->smk_known, GFP_KERNEL);
+		if (!ctx->fshat)
+			return -ENOMEM;
+	}
+
+	if (isp->smk_flags & SMK_INODE_TRANSMUTE) {
+		if (sbsp->smk_root) {
+			ctx->fstransmute = kstrdup(sbsp->smk_root->smk_known, GFP_KERNEL);
+			if (!ctx->fstransmute)
+				return -ENOMEM;
+		}
+	}
+	return 0;
+}
+
+/**
  * smack_fs_context_dup - Duplicate the security data on fs_context duplication
  * @fc: The new filesystem context.
  * @src_fc: The source filesystem context being duplicated.
@@ -4876,6 +4926,7 @@ static struct security_hook_list smack_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(ptrace_traceme, smack_ptrace_traceme),
 	LSM_HOOK_INIT(syslog, smack_syslog),
 
+	LSM_HOOK_INIT(fs_context_submount, smack_fs_context_submount),
 	LSM_HOOK_INIT(fs_context_dup, smack_fs_context_dup),
 	LSM_HOOK_INIT(fs_context_parse_param, smack_fs_context_parse_param),