On Thu, 2022-08-04 at 16:34 +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)
>
> 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);
> +}
> +
> 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..6714cc592521 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 (reference) {
> + 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..6eaad28e9f0d 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 (reference) {
> + 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),
>
>
>
I hit a problem when testing this:
xfstest generic/294 fails on nfs with or without this patch. Without the
patch it seems to fail because the errors returned are different than
expected (EROFS instead of EEXIST). With this patch, it fails to remount
the filesystem at all, which I think is a regression.
We should probably fix the test to not be so specific about the error
being returned here too, but that's a different issue.
--
Jeff Layton <jlayton@xxxxxxxxxx>
