linux-kernel - Re: [PATCH v2 01/10] vfs: move cap_convert_nscap() call into vfs

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Message-ID: <87blduf0t3.fsf@x220.int.ebiederm.org>
Date:   Tue, 12 Jan 2021 12:49:12 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org, "Serge E. Hallyn" <serge@...lyn.com>,
        <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 01/10] vfs: move cap_convert_nscap() call into vfs_setxattr()

ebiederm@...ssion.com (Eric W. Biederman) writes:

> So there is the basic question do we want to read the raw bytes on disk
> or do we want to return something meaningful to the reader.  As the
> existing tools use the xattr interface to set/clear fscaps returning
> data to user space rather than raw bytes seems the perfered interface.
>
> My ideal semantics would be:
>
> - If current_user_ns() == sb->s_user_ns return the raw data.
>
>   I don't know how to implement this first scenario while permitting
>   stacked filesystems.

    After a little more thought I do.

    In getxattr if the get_cap method is not implemented by the
    filesystem if current_user_ns() == sb->s_user_ns simply treat it as
    an ordinary xattr read/write.

    Otherwise call vfs_get_cap and translate the result as described
    below.
    
    The key point of this is it allows for seeing what is actually on
    disk (when it is not confusing).

> - Calculate the cpu_vfs_cap_data as get_vfs_caps_from_disk does.
>   That gives the meaning of the xattr.
>
> - If "from_kuid(current_userns(), krootid) == 0" return a v2 cap.
>
> - If "rootid_owns_currentns()" return a v2 cap.
>
> - Else return an error.  Probably a permission error.
>
>   The fscap simply can not make sense to the user if the rootid does not
>   map.  Return a v2 cap would imply that the caps are present on the
>   executable (in the current context) which they are not.


Eric