linux-kernel - Re: [PATCH v3 4/4] add listmount(2) syscall

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Date:   Wed, 4 Oct 2023 15:37:43 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        Matthew House <mattlloydhouse@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 4/4] add listmount(2) syscall

On Thu, Sep 28, 2023 at 9:04 AM Miklos Szeredi <mszeredi@...hat.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.
>
> 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@...hat.com>
> ---
>  arch/x86/entry/syscalls/syscall_32.tbl |  1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |  1 +
>  fs/namespace.c                         | 69 ++++++++++++++++++++++++++
>  include/linux/syscalls.h               |  3 ++
>  include/uapi/asm-generic/unistd.h      |  5 +-
>  include/uapi/linux/mount.h             |  3 ++
>  6 files changed, 81 insertions(+), 1 deletion(-)

...

> diff --git a/fs/namespace.c b/fs/namespace.c
> index 3326ba2b2810..050e2d2af110 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4970,6 +4970,75 @@ SYSCALL_DEFINE4(statmount, const struct __mount_arg __user *, req,
>         return ret;
>  }
>
> +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;
> +       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;
> +       }
> +
> +       if (reachable_only && !is_path_reachable(m, mnt->mnt_root, &rootmnt))
> +               return capable(CAP_SYS_ADMIN) ? 0 : -EPERM;
> +
> +       list_for_each_entry(r, &m->mnt_mounts, mnt_child) {
> +               if (reachable_only &&
> +                   !is_path_reachable(r, r->mnt.mnt_root, root))
> +                       continue;

I believe we would want to move the security_sb_statfs() call from
above to down here; something like this I think ...

  err = security_sb_statfs(r->mnt.mnt_root);
  if (err)
    /* if we can't access the mount, pretend it doesn't exist */
    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;
> +}

-- 
paul-moore.com