On Wed, 2017-02-01 at 19:38 +1300, Eric W. Biederman wrote:
> To support unprivileged users mounting filesystems two permission
> checks have to be performed: a test to see if the user allowed to
> create a mount in the mount namespace, and a test to see if
> the user is allowed to access the specified filesystem.
>
> The automount case is special in that mounting the original
> filesystem
> grants permission to mount the sub-filesystems, to any user who
> happens to stumble across the their mountpoint and satisfies the
> ordinary filesystem permission checks.
>
> Attempting to handle the automount case by using override_creds
> almost works. It preserves the idea that permission to mount
> the original filesystem is permission to mount the sub-filesystem.
> Unfortunately using override_creds messes up the filesystems
> ordinary permission checks.
>
> Solve this by being explicit that a mount is a submount by
> introducing
> vfs_submount, and using it where appropriate.
>
> vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to
> let
> sget and friends know that a mount is a submount so they can take
> appropriate
> action.
>
> sget and sget_userns are modified to not perform any permission
> checks
> on submounts.
>
> follow_automount is modified to stop using override_creds as that
> has proven problemantic.
>
> do_mount is modified to always remove the new MS_SUBMOUNT flag so
> that we know userspace will never by able to specify it.
>
> autofs4 is modified to stop using current_real_cred that was put in
> there to handle the previous version of submount permission checking.
>
> cifs is modified to pass the mountpoint all of the way down to
> vfs_submount.
>
> debugfs is modified to pass the mountpoint all of the way down to
> trace_automount by adding a new parameter. To make this change
> easier
> a new typedef debugfs_automount_t is introduced to capture the type
> of
> the debugfs automount function.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using
> current_real_cred()->uid")
> Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems
> creds")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> fs/afs/mntpt.c | 2 +-
> fs/autofs4/waitq.c | 4 ++--
> fs/cifs/cifs_dfs_ref.c | 7 ++++---
> fs/debugfs/inode.c | 8 ++++----
> fs/namei.c | 3 ---
> fs/namespace.c | 17 ++++++++++++++++-
> fs/nfs/namespace.c | 2 +-
> fs/nfs/nfs4namespace.c | 2 +-
> fs/super.c | 13 ++++++++++---
> include/linux/debugfs.h | 3 ++-
> include/linux/mount.h | 3 +++
> include/uapi/linux/fs.h | 1 +
> kernel/trace/trace.c | 4 ++--
> 13 files changed, 47 insertions(+), 22 deletions(-)
>
> diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
> index 81dd075356b9..d4fb0afc0097 100644
> --- a/fs/afs/mntpt.c
> +++ b/fs/afs/mntpt.c
> @@ -202,7 +202,7 @@ static struct vfsmount
> *afs_mntpt_do_automount(struct dentry *mntpt)
>
> /* try and do the mount */
> _debug("--- attempting mount %s -o %s ---", devname,
> options);
> - mnt = vfs_kern_mount(&afs_fs_type, 0, devname, options);
> + mnt = vfs_submount(mntpt, &afs_fs_type, devname, options);
> _debug("--- mount result %p ---", mnt);
>
> free_page((unsigned long) devname);
> diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
> index 1278335ce366..79fbd85db4ba 100644
> --- a/fs/autofs4/waitq.c
> +++ b/fs/autofs4/waitq.c
> @@ -436,8 +436,8 @@ int autofs4_wait(struct autofs_sb_info *sbi,
> memcpy(&wq->name, &qstr, sizeof(struct qstr));
> wq->dev = autofs4_get_dev(sbi);
> wq->ino = autofs4_get_ino(sbi);
> - wq->uid = current_real_cred()->uid;
> - wq->gid = current_real_cred()->gid;
> + wq->uid = current_cred()->uid;
> + wq->gid = current_cred()->gid;
> wq->pid = pid;
> wq->tgid = tgid;
> wq->status = -EINTR; /* Status return if interrupted
> */
> diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
> index ec9dbbcca3b9..9156be545b0f 100644
> --- a/fs/cifs/cifs_dfs_ref.c
> +++ b/fs/cifs/cifs_dfs_ref.c
> @@ -245,7 +245,8 @@ char *cifs_compose_mount_options(const char
> *sb_mountdata,
> * @fullpath: full path in UNC format
> * @ref: server's referral
> */
> -static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info
> *cifs_sb,
> +static struct vfsmount *cifs_dfs_do_refmount(struct dentry *mntpt,
> + struct cifs_sb_info *cifs_sb,
> const char *fullpath, const struct dfs_info3_param
> *ref)
> {
> struct vfsmount *mnt;
> @@ -259,7 +260,7 @@ static struct vfsmount
> *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb,
> if (IS_ERR(mountdata))
> return (struct vfsmount *)mountdata;
>
> - mnt = vfs_kern_mount(&cifs_fs_type, 0, devname, mountdata);
> + mnt = vfs_submount(mntpt, &cifs_fs_type, devname,
> mountdata);
> kfree(mountdata);
> kfree(devname);
> return mnt;
> @@ -334,7 +335,7 @@ static struct vfsmount
> *cifs_dfs_do_automount(struct dentry *mntpt)
> mnt = ERR_PTR(-EINVAL);
> break;
> }
> - mnt = cifs_dfs_do_refmount(cifs_sb,
> + mnt = cifs_dfs_do_refmount(mntpt, cifs_sb,
> full_path, referrals + i);
> cifs_dbg(FYI, "%s: cifs_dfs_do_refmount:%s ,
> mnt:%p\n",
> __func__, referrals[i].node_name, mnt);
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index f17fcf89e18e..1e30f74a9527 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -187,9 +187,9 @@ static const struct super_operations
> debugfs_super_operations = {
>
> static struct vfsmount *debugfs_automount(struct path *path)
> {
> - struct vfsmount *(*f)(void *);
> - f = (struct vfsmount *(*)(void *))path->dentry->d_fsdata;
> - return f(d_inode(path->dentry)->i_private);
> + debugfs_automount_t f;
> + f = (debugfs_automount_t)path->dentry->d_fsdata;
> + return f(path->dentry, d_inode(path->dentry)->i_private);
> }
>
> static const struct dentry_operations debugfs_dops = {
> @@ -504,7 +504,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_dir);
> */
> struct dentry *debugfs_create_automount(const char *name,
> struct dentry *parent,
> - struct vfsmount *(*f)(void
> *),
> + debugfs_automount_t f,
> void *data)
> {
> struct dentry *dentry = start_creating(name, parent);
> diff --git a/fs/namei.c b/fs/namei.c
> index 6fa3e9138fe4..da689c9c005e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1100,7 +1100,6 @@ static int follow_automount(struct path *path,
> struct nameidata *nd,
> bool *need_mntput)
> {
> struct vfsmount *mnt;
> - const struct cred *old_cred;
> int err;
>
> if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
> @@ -1129,9 +1128,7 @@ static int follow_automount(struct path *path,
> struct nameidata *nd,
> if (nd->total_link_count >= 40)
> return -ELOOP;
>
> - old_cred = override_creds(&init_cred);
> mnt = path->dentry->d_op->d_automount(path);
> - revert_creds(old_cred);
> if (IS_ERR(mnt)) {
> /*
> * The filesystem is allowed to return -EISDIR here
> to indicate
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 487ba30bb5c6..089a6b23135a 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -989,6 +989,21 @@ vfs_kern_mount(struct file_system_type *type,
> int flags, const char *name, void
> }
> EXPORT_SYMBOL_GPL(vfs_kern_mount);
>
> +struct vfsmount *
> +vfs_submount(const struct dentry *mountpoint, struct
> file_system_type *type,
> + const char *name, void *data)
> +{
> + /* Until it is worked out how to pass the user namespace
> + * through from the parent mount to the submount don't
> support
> + * unprivileged mounts with submounts.
> + */
> + if (mountpoint->d_sb->s_user_ns != &init_user_ns)
> + return ERR_PTR(-EPERM);
> +
> + return vfs_kern_mount(type, MS_SUBMOUNT, name, data);
> +}
> +EXPORT_SYMBOL_GPL(vfs_submount);
> +
> static struct mount *clone_mnt(struct mount *old, struct dentry
> *root,
> int flag)
> {
> @@ -2794,7 +2809,7 @@ long do_mount(const char *dev_name, const char
> __user *dir_name,
>
> flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
> MS_BORN |
> MS_NOATIME | MS_NODIRATIME | MS_RELATIME|
> MS_KERNMOUNT |
> - MS_STRICTATIME | MS_NOREMOTELOCK);
> + MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);
>
> if (flags & MS_REMOUNT)
> retval = do_remount(&path, flags & ~MS_REMOUNT,
> mnt_flags,
> diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
> index 5551e8ef67fd..e49d831c4e85 100644
> --- a/fs/nfs/namespace.c
> +++ b/fs/nfs/namespace.c
> @@ -226,7 +226,7 @@ static struct vfsmount *nfs_do_clone_mount(struct
> nfs_server *server,
> const char *devname,
> struct nfs_clone_mount
> *mountdata)
> {
> - return vfs_kern_mount(&nfs_xdev_fs_type, 0, devname,
> mountdata);
> + return vfs_submount(mountdata->dentry, &nfs_xdev_fs_type,
> devname, mountdata);
> }
>
> /**
> diff --git a/fs/nfs/nfs4namespace.c b/fs/nfs/nfs4namespace.c
> index d21104912676..d8b040bd9814 100644
> --- a/fs/nfs/nfs4namespace.c
> +++ b/fs/nfs/nfs4namespace.c
> @@ -279,7 +279,7 @@ static struct vfsmount *try_location(struct
> nfs_clone_mount *mountdata,
> mountdata->hostname,
> mountdata->mnt_path);
>
> - mnt = vfs_kern_mount(&nfs4_referral_fs_type, 0,
> page, mountdata);
> + mnt = vfs_submount(mountdata->dentry,
> &nfs4_referral_fs_type, page, mountdata);
> if (!IS_ERR(mnt))
> break;
> }
> diff --git a/fs/super.c b/fs/super.c
> index 1709ed029a2c..4185844f7a12 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -469,7 +469,7 @@ struct super_block *sget_userns(struct
> file_system_type *type,
> struct super_block *old;
> int err;
>
> - if (!(flags & MS_KERNMOUNT) &&
> + if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) &&
> !(type->fs_flags & FS_USERNS_MOUNT) &&
> !capable(CAP_SYS_ADMIN))
> return ERR_PTR(-EPERM);
> @@ -499,7 +499,7 @@ struct super_block *sget_userns(struct
> file_system_type *type,
> }
> if (!s) {
> spin_unlock(&sb_lock);
> - s = alloc_super(type, flags, user_ns);
> + s = alloc_super(type, (flags & ~MS_SUBMOUNT),
> user_ns);
> if (!s)
> return ERR_PTR(-ENOMEM);
> goto retry;
> @@ -540,8 +540,15 @@ struct super_block *sget(struct file_system_type
> *type,
> {
> struct user_namespace *user_ns = current_user_ns();
>
> + /* We don't yet pass the user namespace of the parent
> + * mount through to here so always use &init_user_ns
> + * until that changes.
> + */
> + if (flags & MS_SUBMOUNT)
> + user_ns = &init_user_ns;
> +
> /* Ensure the requestor has permissions over the target
> filesystem */
> - if (!(flags & MS_KERNMOUNT) && !ns_capable(user_ns,
> CAP_SYS_ADMIN))
> + if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) &&
> !ns_capable(user_ns, CAP_SYS_ADMIN))
> return ERR_PTR(-EPERM);
>
> return sget_userns(type, test, set, flags, user_ns, data);
> diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> index 014cc564d1c4..233006be30aa 100644
> --- a/include/linux/debugfs.h
> +++ b/include/linux/debugfs.h
> @@ -97,9 +97,10 @@ struct dentry *debugfs_create_dir(const char
> *name, struct dentry *parent);
> struct dentry *debugfs_create_symlink(const char *name, struct
> dentry *parent,
> const char *dest);
>
> +typedef struct vfsmount *(*debugfs_automount_t)(struct dentry *,
> void *);
> struct dentry *debugfs_create_automount(const char *name,
> struct dentry *parent,
> - struct vfsmount *(*f)(void
> *),
> + debugfs_automount_t f,
> void *data);
>
> void debugfs_remove(struct dentry *dentry);
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index c6f55158d5e5..8e0352af06b7 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -90,6 +90,9 @@ struct file_system_type;
> extern struct vfsmount *vfs_kern_mount(struct file_system_type
> *type,
> int flags, const char *name,
> void *data);
> +extern struct vfsmount *vfs_submount(const struct dentry
> *mountpoint,
> + struct file_system_type *type,
> + const char *name, void *data);
>
> extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head
> *expiry_list);
> extern void mark_mounts_for_expiry(struct list_head *mounts);
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 36da93fbf188..048a85e9f017 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -132,6 +132,7 @@ struct inodes_stat_t {
> #define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times
> lazily */
>
> /* These sb flags are internal to the kernel */
> +#define MS_SUBMOUNT (1<<26)
> #define MS_NOREMOTELOCK (1<<27)
> #define MS_NOSEC (1<<28)
> #define MS_BORN (1<<29)
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d7449783987a..310f0ea0d1a2 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -7503,7 +7503,7 @@ init_tracer_tracefs(struct trace_array *tr,
> struct dentry *d_tracer)
> ftrace_init_tracefs(tr, d_tracer);
> }
>
> -static struct vfsmount *trace_automount(void *ingore)
> +static struct vfsmount *trace_automount(struct dentry *mntpt, void
> *ingore)
> {
> struct vfsmount *mnt;
> struct file_system_type *type;
> @@ -7516,7 +7516,7 @@ static struct vfsmount *trace_automount(void
> *ingore)
> type = get_fs_type("tracefs");
> if (!type)
> return NULL;
> - mnt = vfs_kern_mount(type, 0, "tracefs", NULL);
> + mnt = vfs_submount(mntpt, type, "tracefs", NULL);
> put_filesystem(type);
> if (IS_ERR(mnt))
> return NULL;
Yes. That looks like it might be workable.
Reviewed-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
--
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@xxxxxxxxxxxxxxx
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