linux-kernel - Re: [PATCH 1/2] ecryptfs: fix uid translation for setxattr on security.capability

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Date:   Mon, 25 Jan 2021 14:25:38 +0100
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Tyler Hicks <code@...icks.com>
Cc:     "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>,
        "Serge E . Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 1/2] ecryptfs: fix uid translation for setxattr on security.capability

On Fri, Jan 22, 2021 at 7:31 PM Tyler Hicks <code@...icks.com> wrote:
>
> On 2021-01-19 17:22:03, Miklos Szeredi wrote:
> > Prior to commit 7c03e2cda4a5 ("vfs: move cap_convert_nscap() call into
> > vfs_setxattr()") the translation of nscap->rootid did not take stacked
> > filesystems (overlayfs and ecryptfs) into account.
> >
> > That patch fixed the overlay case, but made the ecryptfs case worse.
>
> Thanks for sending a fix!
>
> I know that you don't have an eCryptfs setup to test with but I'm at a
> loss about how to test this from the userns/fscaps side of things. Do
> you have a sequence of unshare/setcap/getcap commands that I can run on
> a file inside of an eCryptfs mount to verify that the bug exists after
> 7c03e2cda4a5 and then again to verify that this patch fixes the bug?

You need two terminals:
$ = <USER>
# = root

$ unshare -Um
$ echo $$
<PID>
# echo "0 1000 1" > uid_map
# cp uid_map gid_map
# echo 1000 2000 1 >> uid_map
# echo 2000 3000 1 >> uid_map
# cat uid_map > /proc/<PID>/uid_map
# cat gid_map > /proc/<PID>/gid_map
$ mkdir ~/tmp ~/mnt
$ mount -t tmpfs tmpfs ~/tmp
$ pwd
/home/<USER>
# nsenter -t <PID> -m
# [setup ecryptfs on /home/<USER>/mnt using /home/<USER>/tmp]
$ cd ~/mnt
$ touch test
$ /sbin/setcap -n 1000 cap_dac_override+eip test
$ /sbin/getcap -n test
test = cap_dac_override+eip [rootid=1000]

Without the patch, I'm thinking that it will do a double translate and
end up with rootid=2000 in the user namespace, but I might well have
messed it up...

Let me know how this goes.

Thanks,
Miklos