Re: [PATCH v3] nfs: Fix automount superblock LSM init problem, preventing sb sharing

From: Jeff Layton
Date: Mon Aug 08 2022 - 13:04:31 EST

nit: The problem is evident on NFS, but it's not really an "nfs:" patch,
IMO. I'd prefix this with vfs: since most of the fix is there and it
could be applicable elsewhere.


On Fri, 2022-08-05 at 14:36 +0100, David Howells wrote:
> 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 #2)
> - 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)
> 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: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> cc: linux-nfs@xxxxxxxxxxxxxxx
> cc: selinux@xxxxxxxxxxxxxxx
> cc: linux-security-module@xxxxxxxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> ---
>
> 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 | 29 +++++++++++++++++++
> security/smack/smack_lsm.c | 61 +++++++++++++++++++++++++++++++++++++++++
> 10 files changed, 119 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 60f57c7bc0a6..a1c440336fd9 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1519,10 +1519,12 @@ 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 eafa1d2489fd..6d1c738e4a84 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 91c8146649f5..1782814c7c5b 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 7fc4e9f49f54..94834f699b04 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);
> @@ -620,6 +621,11 @@ 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 188b8f782220..e683027f9424 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);
> +}
> +

It'd be good to have some LSM folks ack this change.

> 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 1bbd53321d13..ddeaff4f3bb1 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2768,6 +2768,34 @@ 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;
> + const struct inode_security_struct *root_isec;
> + struct selinux_mnt_opts *opts;
> +
> + if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
> + opts = kzalloc(sizeof(*opts), GFP_KERNEL);
> + if (!opts)
> + return -ENOMEM;
> +
> + root_isec = backing_inode_security(reference->d_sb->s_root);
> + 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 & ROOTCONTEXT_MNT)
> + opts->rootcontext_sid = root_isec->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)
> {
> @@ -7239,6 +7267,7 @@ 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 6207762dbdb1..6ba32bb097b5 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -612,6 +612,66 @@ 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;
> +
> + ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
> + if (!ctx)
> + return -ENOMEM;
> + fc->security = ctx;
> +
> + if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT) {
> + 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;
> + }
> + } else {
> + if (sbsp->smk_root) {
> + ctx->fsroot = kstrdup(sbsp->smk_root->smk_known, GFP_KERNEL);
> + if (!ctx->fsroot)
> + return -ENOMEM;
> + }
> + }
> + }
> +
> + return 0;
> +}
> +
> /**
> * smack_fs_context_dup - Duplicate the security data on fs_context duplication
> * @fc: The new filesystem context.
> @@ -4755,6 +4815,7 @@ 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),
>
>
>


Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>