[PATCH v8] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Jeff Layton]

From: David Howells <dhowells@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
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@xxxxxxxxxx>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Acked-by: "Christian Brauner (Microsoft)" <brauner@xxxxxxxxxx>
Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1
Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2
Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v3
Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v4
Link: https://lore.kernel.org/r/217595.1662033775@xxxxxxxxxxxxxxxxxxxxxx/ # v5
---
ver #2)
- Added Smack support
- Made LSM parameter extraction dependent on reference != NULL.

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

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

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

ver #6)
- Rebase onto v6.5.0-rc4
- Link to v6: https://lore.kernel.org/r/20230802-master-v6-1-45d48299168b@xxxxxxxxxx

ver #7)
- Drop lsm_set boolean
- Link to v7: https://lore.kernel.org/r/20230804-master-v7-1-5d4e48407298@xxxxxxxxxx

ver #8)
- Remove spurious semicolon in smack_fs_context_init
- Make fs_context_init take a superblock as reference instead of dentry
- WARN_ON_ONCE's when fc->purpose != FS_CONTEXT_FOR_SUBMOUNT
- Call the security hook from fs_context_for_submount instead of alloc_fs_context
---
 fs/fs_context.c               | 23 +++++++++++++++++-
 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 +++++++++++++++++++++++++++++++++++++++++++
 6 files changed, 122 insertions(+), 1 deletion(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 851214d1d013..a76d7c82e091 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -315,10 +315,31 @@ struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
 }
 EXPORT_SYMBOL(fs_context_for_reconfigure);
 
+/**
+ * fs_context_for_submount: allocate a new fs_context for a submount
+ * @type: file_system_type of the new context
+ * @reference: reference dentry from which to copy relevant info
+ *
+ * Allocate a new fs_context suitable for a submount. This also ensures that
+ * the fc->security object is inherited from @reference (if needed).
+ */
 struct fs_context *fs_context_for_submount(struct file_system_type *type,
 					   struct dentry *reference)
 {
-	return alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
+	struct fs_context *fc;
+	int ret;
+
+	fc = alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
+	if (IS_ERR(fc))
+		return fc;
+
+	ret = security_fs_context_init(fc, reference->d_sb);
+	if (ret) {
+		put_fs_context(fc);
+		return ERR_PTR(ret);
+	}
+
+	return fc;
 }
 EXPORT_SYMBOL(fs_context_for_submount);
 
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 7308a1a7599b..2876dd6114c0 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 super_block *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..fe9bf5e805ee 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 super_block *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 super_block *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..f8a666d089f9 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
+ * @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_init(struct fs_context *fc, struct super_block *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..c8fb0d77104f 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 super_block *reference)
+{
+	const struct superblock_security_struct *sbsec;
+	struct selinux_mnt_opts *opts;
+
+	if (!reference || WARN_ON_ONCE(fc->purpose != FS_CONTEXT_FOR_SUBMOUNT))
+		return 0;
+
+	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 +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..6243cb54092d 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 superblock (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 (!reference || WARN_ON_ONCE(fc->purpose != FS_CONTEXT_FOR_SUBMOUNT))
+		return 0;
+
+	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.
@@ -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: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
change-id: 20230802-master-3082090e8d69

Best regards,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>