[PATCH v5] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

From: David Howells
Date: Thu Sep 01 2022 - 08:03:54 EST


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)

Changes
=======
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.

Signed-off-by: David Howells <dhowells@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>
Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
cc: Anna Schumaker <anna@xxxxxxxxxx>
cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: Scott Mayhew <smayhew@xxxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: Paul Moore <paul@xxxxxxxxxxxxxx>
cc: linux-nfs@xxxxxxxxxxxxxxx
cc: selinux@xxxxxxxxxxxxxxx
cc: linux-security-module@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
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
---
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/lsm_hooks.h | 6 +++-
include/linux/security.h | 6 ++++
security/security.c | 5 +++
security/selinux/hooks.c | 25 +++++++++++++++++++
security/smack/smack_lsm.c | 54 ++++++++++++++++++++++++++++++++++++++++++
10 files changed, 108 insertions(+), 5 deletions(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 24ce12f0db32..22248b8a88a8 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 734ed584a946..94666c0c92a4 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1552,10 +1552,12 @@ 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 13fa6f3df8e4..3876dd96bb20 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -110,6 +110,7 @@ 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 60fff133c0b1..a0cf11cfce8d 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/lsm_hooks.h b/include/linux/lsm_hooks.h
index 3aa6030302f5..099528f6a91c 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -87,8 +87,12 @@
* Security hooks for mount using fs_context.
* [See also Documentation/filesystems/mount_api.rst]
*
+ * @fs_context_init:
+ * Initialise fc->security. This is initialised to NULL by the caller.
+ * @fc indicates the new filesystem context.
+ * @dentry indicates a reference for submount/remount
* @fs_context_dup:
- * Allocate and attach a security structure to sc->security. This pointer
+ * Allocate and attach a security structure to fc->security. This pointer
* is initialised to NULL by the caller.
* @fc indicates the new filesystem context.
* @src_fc indicates the original filesystem context.
diff --git a/include/linux/security.h b/include/linux/security.h
index 7bd0c490703d..eb865af42b36 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -291,6 +291,7 @@ 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);
@@ -622,6 +623,11 @@ 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 4b95de24bc8d..d04d01a57ee2 100644
--- a/security/security.c
+++ b/security/security.c
@@ -880,6 +880,11 @@ void security_bprm_committed_creds(struct linux_binprm *bprm)
call_void_hook(bprm_committed_creds, bprm);
}

+int security_fs_context_init(struct fs_context *fc, struct dentry *reference)
+{
+ return call_int_hook(fs_context_init, 0, fc, reference);
+}
+
int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
{
return call_int_hook(fs_context_dup, 0, fc, src_fc);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 03bca97c8b29..9b48d15c9eab 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2766,6 +2766,30 @@ 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)
{
@@ -7260,6 +7284,7 @@ static struct security_hook_list selinux_hooks[] __lsm_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 bffccdc494cb..3396ecebd791 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -613,6 +613,59 @@ 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.
@@ -4779,6 +4832,7 @@ static struct security_hook_list smack_hooks[] __lsm_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),