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Date:   Fri, 5 Jan 2018 20:24:07 +0100
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Dongsu Park <dongsu@...volk.io>
Cc:     linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        Alban Crequy <alban@...volk.io>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Sargun Dhillon <sargun@...gun.me>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 03/11] fs: Allow superblock owner to change ownership of
 inodes

On Fri, Dec 22, 2017 at 03:32:27PM +0100, Dongsu Park wrote:
> diff --git a/fs/attr.c b/fs/attr.c
> index 12ffdb6f..bf8e94f3 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -18,6 +18,30 @@
>  #include <linux/evm.h>
>  #include <linux/ima.h>
>  
> +static bool chown_ok(const struct inode *inode, kuid_t uid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    uid_eq(uid, inode->i_uid))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
> +static bool chgrp_ok(const struct inode *inode, kgid_t gid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
>  /**
>   * setattr_prepare - check if attribute changes to a dentry are allowed
>   * @dentry:	dentry to check
> @@ -52,17 +76,11 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
>  		goto kill_priv;
>  
>  	/* Make sure a caller can chown. */
> -	if ((ia_valid & ATTR_UID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	     !uid_eq(attr->ia_uid, inode->i_uid)) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
>  		return -EPERM;

I think this patch would read much better and easier to review if it was
split up by first adding the helpers, and then extending them afterwards.

>  
>  	/* Make sure caller can chgrp. */
> -	if ((ia_valid & ATTR_GID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	    (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
>  		return -EPERM;
>  
>  	/* Make sure a caller can chmod. */
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 31934cb9..9d50ec92 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -665,10 +665,17 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>  	int error;
>  	struct inode *inode = d_inode(dentry);
> +	struct user_namespace *s_user_ns;
>  
>  	if (attr->ia_valid & ATTR_MODE)
>  		return -EPERM;
>  
> +	/* Don't let anyone mess with weird proc files */
> +	s_user_ns = inode->i_sb->s_user_ns;
> +	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
> +	    !kgid_has_mapping(s_user_ns, inode->i_gid))
> +		return -EPERM;
> +
>  	error = setattr_prepare(dentry, attr);
>  	if (error)
>  		return error;

Are we sure proc is the only special one? How was it observed first that this was
require for proc? Has anyone tried fuzzing by trying this op with a slew of other
filesystems on all files?

  Luis

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