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    SubjectRe: [PATCH v3] nfs: Fix automount superblock LSM init problem, preventing sb sharing
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    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@redhat.com>
    > 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@hammerspace.com>
    > cc: Anna Schumaker <anna@kernel.org>
    > cc: Alexander Viro <viro@zeniv.linux.org.uk>
    > cc: Scott Mayhew <smayhew@redhat.com>
    > cc: Jeff Layton <jlayton@kernel.org>
    > cc: Paul Moore <paul@paul-moore.com>
    > cc: Casey Schaufler <casey@schaufler-ca.com>
    > cc: linux-nfs@vger.kernel.org
    > cc: selinux@vger.kernel.org
    > cc: linux-security-module@vger.kernel.org
    > cc: linux-fsdevel@vger.kernel.org
    > ---
    >
    > 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@kernel.org>
    Tested-by: Jeff Layton <jlayton@kernel.org>

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