[PATCH v4 5/6] add listmount(2) syscall

[Paul Moore]

On Oct 25, 2023 Miklos Szeredi <mszeredi at redhat.com> wrote:
> 
> Add way to query the children of a particular mount.  This is a more
> flexible way to iterate the mount tree than having to parse the complete
> /proc/self/mountinfo.
> 
> Allow listing either
> 
>  - immediate child mounts only, or
> 
>  - recursively all descendant mounts (depth first).
> 
> Lookup the mount by the new 64bit mount ID.  If a mount needs to be queried
> based on path, then statx(2) can be used to first query the mount ID
> belonging to the path.
> 
> Return an array of new (64bit) mount ID's.  Without privileges only mounts
> are listed which are reachable from the task's root.
> 
> Signed-off-by: Miklos Szeredi <mszeredi at redhat.com>
> Reviewed-by: Ian Kent <raven at themaw.net>
> ---
>  fs/namespace.c             | 93 ++++++++++++++++++++++++++++++++++++++
>  include/linux/syscalls.h   |  3 ++
>  include/uapi/linux/mount.h |  9 ++++
>  3 files changed, 105 insertions(+)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index a980c250a3a6..0afe2344bba6 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4958,6 +4958,99 @@ SYSCALL_DEFINE4(statmount, const struct __mount_arg __user *, req,
>  	return ret;
>  }
>  
> +static struct mount *listmnt_first(struct mount *root)
> +{
> +	return list_first_entry_or_null(&root->mnt_mounts, struct mount, mnt_child);
> +}
> +
> +static struct mount *listmnt_next(struct mount *curr, struct mount *root, bool recurse)
> +{
> +	if (recurse)
> +		return next_mnt(curr, root);
> +	if (!list_is_head(curr->mnt_child.next, &root->mnt_mounts))
> +		return list_next_entry(curr, mnt_child);
> +	return NULL;
> +}
> +
> +static long do_listmount(struct vfsmount *mnt, u64 __user *buf, size_t bufsize,
> +			 const struct path *root, unsigned int flags)
> +{
> +	struct mount *r, *m = real_mount(mnt);
> +	struct path rootmnt = {
> +		.mnt = root->mnt,
> +		.dentry = root->mnt->mnt_root
> +	};
> +	long ctr = 0;
> +	bool reachable_only = true;
> +	bool recurse = flags & LISTMOUNT_RECURSIVE;
> +	int err;
> +
> +	err = security_sb_statfs(mnt->mnt_root);
> +	if (err)
> +		return err;
> +
> +	if (flags & LISTMOUNT_UNREACHABLE) {
> +		if (!capable(CAP_SYS_ADMIN))
> +			return -EPERM;
> +		reachable_only = false;
> +	}

Similar to my comment in patch 4/6, please move the LSM call after the
capability check.

> +	if (reachable_only && !is_path_reachable(m, mnt->mnt_root, &rootmnt))
> +		return capable(CAP_SYS_ADMIN) ? 0 : -EPERM;
> +
> +	for (r = listmnt_first(m); r; r = listmnt_next(r, m, recurse)) {
> +		if (reachable_only &&
> +		    !is_path_reachable(r, r->mnt.mnt_root, root))
> +			continue;
> +
> +		if (ctr >= bufsize)
> +			return -EOVERFLOW;
> +		if (put_user(r->mnt_id_unique, buf + ctr))
> +			return -EFAULT;
> +		ctr++;
> +		if (ctr < 0)
> +			return -ERANGE;
> +	}
> +	return ctr;
> +}

--
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