Re: [PATCH] namei: permit linking with CAP_FOWNER in userns

From: Kees Cook <keescook@chromium.org>
To: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Serge Hallyn <serge.hallyn@ubuntu.com>,
	Seth Forshee <seth.forshee@canonical.com>
Subject: Re: [PATCH] namei: permit linking with CAP_FOWNER in userns
Date: Tue, 3 Nov 2015 09:51:06 -0800	[thread overview]
Message-ID: <CAGXu5jLsYCmpR0=WhL3H6AL71ATJFtRRYwcko_eAA=Ve=0Skbg@mail.gmail.com> (raw)
In-Reply-To: <1444489163-24266-1-git-send-email-public@rsjtdrjgfuzkfg.com>

On Sat, Oct 10, 2015 at 7:59 AM, Dirk Steinmetz
<public@rsjtdrjgfuzkfg.com> wrote:
> Attempting to hardlink to an unsafe file (e.g. a setuid binary) from
> within an unprivileged user namespace fails, even if CAP_FOWNER is held
> within the namespace. This may cause various failures, such as a gentoo
> installation within a lxc container failing to build and install specific
> packages.
>
> This change permits hardlinking of files owned by mapped uids, if
> CAP_FOWNER is held for that namespace. Furthermore, it improves consistency
> by using the existing inode_owner_or_capable(), which is aware of
> namespaced capabilities as of 23adbe12ef7d3 ("fs,userns: Change
> inode_capable to capable_wrt_inode_uidgid").
>
> Signed-off-by: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>

Sorry for the delay: was travelling when I got put on CC. (FWIW, in
the future, please check the scripts/get_maintainer.pl script with
--git-blame to build CC lists, then I would have been CCed earlier.)

I think Eric's already taken this patch, but it looks correct to me:

Acked-by: Kees Cook <keescook@chromium.org>

I'll hop on the other thread to discuss the setgid issue.

-Kees

> ---
>  fs/namei.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 726d211..29fc6a6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -955,26 +955,23 @@ static bool safe_hardlink_source(struct inode *inode)
>   *  - sysctl_protected_hardlinks enabled
>   *  - fsuid does not match inode
>   *  - hardlink source is unsafe (see safe_hardlink_source() above)
> - *  - not CAP_FOWNER
> + *  - not CAP_FOWNER in a namespace with the inode owner uid mapped
>   *
>   * Returns 0 if successful, -ve on error.
>   */
>  static int may_linkat(struct path *link)
>  {
> -       const struct cred *cred;
>         struct inode *inode;
>
>         if (!sysctl_protected_hardlinks)
>                 return 0;
>
> -       cred = current_cred();
>         inode = link->dentry->d_inode;
>
>         /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
>          * otherwise, it must be a safe source.
>          */
> -       if (uid_eq(cred->fsuid, inode->i_uid) || safe_hardlink_source(inode) ||
> -           capable(CAP_FOWNER))
> +       if (inode_owner_or_capable(inode) || safe_hardlink_source(inode))
>                 return 0;
>
>         audit_log_link_denied("linkat", link);
> --
> 2.1.4
>
> --
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-- 
Kees Cook
Chrome OS Security