linux-kernel - Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode

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Message-ID: <7998ae36-662b-91f7-c42a-8a4d35d333c1@android.com>
Date:   Tue, 28 Aug 2018 10:44:11 -0700
From:   Mark Salyzyn <salyzyn@...roid.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Jonathan Corbet <corbet@....net>,
        Vivek Goyal <vgoyal@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before
 issuing exportfs_decode_fh

On 08/28/2018 10:34 AM, Amir Goldstein wrote:
> On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@...roid.com> wrote:
>> Assumption never checked, should fail if the mounter creds are not
>> sufficient.
>>
>> Signed-off-by: Mark Salyzyn <salyzyn@...roid.com>
>> Cc: Miklos Szeredi <miklos@...redi.hu>
>> Cc: Jonathan Corbet <corbet@....net>
>> Cc: Vivek Goyal <vgoyal@...hat.com>
>> Cc: Eric W. Biederman <ebiederm@...ssion.com>
>> Cc: Amir Goldstein <amir73il@...il.com>
>> Cc: Randy Dunlap <rdunlap@...radead.org>
>> Cc: Stephen Smalley <sds@...ho.nsa.gov>
>> Cc: linux-unionfs@...r.kernel.org
>> Cc: linux-doc@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>>
>> v5:
>> - dependency of "overlayfs: override_creds=off option bypass creator_cred"
>> ---
>>   fs/overlayfs/namei.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
>> index c993dd8db739..84982b6525fb 100644
>> --- a/fs/overlayfs/namei.c
>> +++ b/fs/overlayfs/namei.c
>> @@ -193,6 +193,11 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
>>          if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
>>                  return NULL;
>>
>> +       if (!capable(CAP_DAC_READ_SEARCH)) {
>> +               origin = ERR_PTR(-EPERM);
>> +               goto out;
> Which branch is this works based on?
> I don't see any out label in current code.

<sigh> I can only truly test this on 4.14 (android's current top of 
tree) and on Hikey with that. Lack of due diligence for Top of Linux.
>
>> +       }
>> +
>>          bytes = (fh->len - offsetof(struct ovl_fh, fid));
>>          real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
>>                                    bytes >> 2, (int)fh->type,
>> --
> Please add same test in ovl_can_decode_fh().

Ahhhh
> Problem: none of the ovl_export_operations functions override creds.
> I guess things are working now because nfsd is privileged enough.
> IOW, the capability check you added doesn't check mounter creds
> when coming from nfs export ops - I guess that is not what you want
> although you probably don'r enable nfs export.
NFS export/import blocked on Android devices.
> Thanks,
> Amir.