[PATCH v6] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Jeff Layton <jlayton@kernel.org>]

From: David Howells <dhowells@redhat.com>

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.

Fix this by adding a new LSM hook to load fc->security for submount
creation when alloc_fs_context() is creating the fs_context for it.

However, this uncovers a further bug: nfs_get_root() initialises the
superblock security manually by calling security_sb_set_mnt_opts() or
security_sb_clone_mnt_opts() - but then vfs_get_tree() calls
security_sb_set_mnt_opts(), which can lead to SELinux, at least,
complaining.

Fix that by adding a flag to the fs_context that suppresses the
security_sb_set_mnt_opts() call in vfs_get_tree().  This can be set by NFS
when it sets the LSM context on the new superblock.

The first bug leads to messages like the following appearing in dmesg:

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

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
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>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: "Christian Brauner (Microsoft)" <brauner@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
---
This patch was originally sent by David several months ago, but it
never got merged. I'm resending to resurrect the discussion. Can we
get this fixed?

ver #6)
 - Rebase onto v6.5.0-rc4

ver #5)
 - Removed unused variable.
 - Only allocate smack_mnt_opts if we're dealing with a submount.

ver #4)
 - When doing a FOR_SUBMOUNT mount, don't set the root label in SELinux or
   Smack.

ver #3)
 - Made LSM parameter extraction dependent on fc->purpose ==
   FS_CONTEXT_FOR_SUBMOUNT.  Shouldn't happen on FOR_RECONFIGURE.

ver #2)
 - Added Smack support
 - Made LSM parameter extraction dependent on reference != NULL.
---
 fs/fs_context.c               |  4 ++++
 fs/nfs/getroot.c              |  1 +
 fs/super.c                    | 10 ++++----
 include/linux/fs_context.h    |  1 +
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/security.h      |  6 +++++
 security/security.c           | 14 +++++++++++
 security/selinux/hooks.c      | 25 ++++++++++++++++++++
 security/smack/smack_lsm.c    | 54 +++++++++++++++++++++++++++++++++++++++++++
 9 files changed, 112 insertions(+), 4 deletions(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 851214d1d013..a523aea956c4 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -282,6 +282,10 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
 		break;
 	}
 
+	ret = security_fs_context_init(fc, reference);
+	if (ret < 0)
+		goto err_fc;
+
 	/* TODO: Make all filesystems support this unconditionally */
 	init_fs_context = fc->fs_type->init_fs_context;
 	if (!init_fs_context)
diff --git a/fs/nfs/getroot.c b/fs/nfs/getroot.c
index 11ff2b2e060f..651bffb0067e 100644
--- a/fs/nfs/getroot.c
+++ b/fs/nfs/getroot.c
@@ -144,6 +144,7 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
 	}
 	if (error)
 		goto error_splat_root;
+	fc->lsm_set = true;
 	if (server->caps & NFS_CAP_SECURITY_LABEL &&
 		!(kflags_out & SECURITY_LSM_NATIVE_LABELS))
 		server->caps &= ~NFS_CAP_SECURITY_LABEL;
diff --git a/fs/super.c b/fs/super.c
index e781226e2880..13adf43e2e5d 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1541,10 +1541,12 @@ int vfs_get_tree(struct fs_context *fc)
 	smp_wmb();
 	sb->s_flags |= SB_BORN;
 
-	error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
-	if (unlikely(error)) {
-		fc_drop_locked(fc);
-		return error;
+	if (!(fc->lsm_set)) {
+		error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
+		if (unlikely(error)) {
+			fc_drop_locked(fc);
+			return error;
+		}
 	}
 
 	/*
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index ff6341e09925..26a9fcdb10cc 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -109,6 +109,7 @@ struct fs_context {
 	bool			need_free:1;	/* Need to call ops->free() */
 	bool			global:1;	/* Goes into &init_user_ns */
 	bool			oldapi:1;	/* Coming from mount(2) */
+	bool			lsm_set:1;	/* security_sb_set/clone_mnt_opts() already done */
 };
 
 struct fs_context_operations {
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 7308a1a7599b..7ce3550154b1 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -54,6 +54,7 @@ LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, struct file *f
 LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
 LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
 LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
+LSM_HOOK(int, 0, fs_context_init, struct fs_context *fc, struct dentry *reference)
 LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
 	 struct fs_context *src_sc)
 LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
diff --git a/include/linux/security.h b/include/linux/security.h
index 32828502f09e..61fda06fac9d 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -293,6 +293,7 @@ int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
 int security_bprm_check(struct linux_binprm *bprm);
 void security_bprm_committing_creds(struct linux_binprm *bprm);
 void security_bprm_committed_creds(struct linux_binprm *bprm);
+int security_fs_context_init(struct fs_context *fc, struct dentry *reference);
 int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
 int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
 int security_sb_alloc(struct super_block *sb);
@@ -629,6 +630,11 @@ static inline void security_bprm_committed_creds(struct linux_binprm *bprm)
 {
 }
 
+static inline int security_fs_context_init(struct fs_context *fc,
+					   struct dentry *reference)
+{
+	return 0;
+}
 static inline int security_fs_context_dup(struct fs_context *fc,
 					  struct fs_context *src_fc)
 {
diff --git a/security/security.c b/security/security.c
index b720424ca37d..8a6dc6f7cda0 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1138,6 +1138,20 @@ void security_bprm_committed_creds(struct linux_binprm *bprm)
 	call_void_hook(bprm_committed_creds, bprm);
 }
 
+/**
+ * security_fs_context_init() - Initialise fc->security
+ * @fc: new filesystem context
+ * @dentry: 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_init(struct fs_context *fc, struct dentry *reference)
+{
+	return call_int_hook(fs_context_init, 0, fc, reference);
+}
+
 /**
  * security_fs_context_dup() - Duplicate a fs_context LSM blob
  * @fc: destination filesystem context
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d06e350fedee..29cce0fadbeb 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2745,6 +2745,30 @@ static int selinux_umount(struct vfsmount *mnt, int flags)
 				   FILESYSTEM__UNMOUNT, NULL);
 }
 
+static int selinux_fs_context_init(struct fs_context *fc,
+				   struct dentry *reference)
+{
+	const struct superblock_security_struct *sbsec;
+	struct selinux_mnt_opts *opts;
+
+	if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
+		opts = kzalloc(sizeof(*opts), GFP_KERNEL);
+		if (!opts)
+			return -ENOMEM;
+
+		sbsec = selinux_superblock(reference->d_sb);
+		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 +7206,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
 	/*
 	 * PUT "CLONING" (ACCESSING + ALLOCATING) HOOKS HERE
 	 */
+	LSM_HOOK_INIT(fs_context_init, selinux_fs_context_init),
 	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..938c8259c5e7 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -614,6 +614,59 @@ static int smack_add_opt(int token, const char *s, void **mnt_opts)
 	return -EINVAL;
 }
 
+/**
+ * smack_fs_context_init - Initialise security data for a filesystem context
+ * @fc: The filesystem context.
+ * @reference: Reference dentry (automount/reconfigure) or NULL
+ *
+ * Returns 0 on success or -ENOMEM on error.
+ */
+static int smack_fs_context_init(struct fs_context *fc,
+				 struct dentry *reference)
+{
+	struct superblock_smack *sbsp;
+	struct smack_mnt_opts *ctx;
+	struct inode_smack *isp;
+
+	if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
+		ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+		if (!ctx)
+			return -ENOMEM;
+		fc->security = ctx;
+
+		sbsp = smack_superblock(reference->d_sb);
+		isp = smack_inode(reference->d_sb->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.
@@ -4876,6 +4929,7 @@ 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_init, smack_fs_context_init),
 	LSM_HOOK_INIT(fs_context_dup, smack_fs_context_dup),
 	LSM_HOOK_INIT(fs_context_parse_param, smack_fs_context_parse_param),
 

---
base-commit: 5d0c230f1de8c7515b6567d9afba1f196fb4e2f4
change-id: 20230802-master-3082090e8d69

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>