Re: [PATCH 0/7] Initial support for user namespace owned mounts

From: Seth Forshee <seth.forshee@canonical.com>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	SELinux-NSA <selinux@tycho.nsa.gov>,
	Serge Hallyn <serge.hallyn@canonical.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/7] Initial support for user namespace owned mounts
Date: Wed, 22 Jul 2015 14:32:23 -0500	[thread overview]
Message-ID: <20150722193223.GD124342@ubuntu-hedt> (raw)
In-Reply-To: <55AFDCA6.10201@schaufler-ca.com>

On Wed, Jul 22, 2015 at 11:10:46AM -0700, Casey Schaufler wrote:
> On 7/22/2015 8:56 AM, Seth Forshee wrote:
> > On Tue, Jul 21, 2015 at 06:52:31PM -0700, Casey Schaufler wrote:
> >> On 7/21/2015 1:35 PM, Seth Forshee wrote:
> >>> On Thu, Jul 16, 2015 at 05:59:22PM -0700, Andy Lutomirski wrote:
> >>>> On Thu, Jul 16, 2015 at 5:45 PM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> >>>>> On 7/16/2015 4:29 PM, Andy Lutomirski wrote:
> >>>>>> I really don't see the benefit of making up extra rules that apply to
> >>>>>> users outside a userns who try to access specifically a filesystem
> >>>>>> with backing store.  They wouldn't make sense for filesystems without
> >>>>>> backing store.
> >>>>> Sure it would. For Smack, it would be the label a file would be
> >>>>> created with, which would be the label of the process creating
> >>>>> the memory based filesystem. For SELinux the rules are more a
> >>>>> touch more sophisticated, but I'm sure that Paul or Stephen could
> >>>>> come up with how to determine it.
> >>>>>
> >>>>> The point, looping all the way back to the beginning, where we
> >>>>> were talking about just ignoring the labels on the filesystem,
> >>>>> is that if you use the same Smack label on the files in the
> >>>>> filesystem as the backing store file has, we'll all be happy.
> >>>>> If that label isn't something user can write to, he won't be
> >>>>> able to write to the mounted objects, either. If there is no
> >>>>> backing store then use the label of the process creating the
> >>>>> filesystem, which will be the user, which will mean everything
> >>>>> will work hunky dory.
> >>>>>
> >>>>> Yes, there's work involved, but I doubt there's a lot. Getting
> >>>>> the label from the backing store or the creating process is
> >>>>> simple enough.
> >>>>>
> >>> So something like the diff below (untested)?
> >> I think that this is close, and quite good for someone
> >> who isn't very familiar with Smack. It's definitely headed
> >> in the right direction.
> >>
> >>> All I'm really doing is setting smk_default as you describe above and
> >>> then using it instead of smk_of_current() in
> >>> smack_inode_alloc_security() and instead of the label from the disk in
> >>> smack_d_instantiate().
> >> Let's say your backing store is a file labeled Rubble.
> >>
> >> mount -o smackfsroot=Rubble,smackfsdef=Rubble ...
> >>
> >> It is completely reasonable for a process labeled Flintstone to
> >> have rwxa access to a file labeled Rubble.
> >>
> >> Smack rule: Flintstone Rubble rwxa
> >>
> >> In the case of writing to an existing Rubble file, what you
> >> have looks fine. What's not so great is that if the Flintstone
> >> process creates a file, it should be labeled Flintstone. Your
> >> use of the smk_default, which is going to violate the principle
> >> of least astonishment, and break the Smack policy as well.
> >>
> >> Let's make a minor change. Instead of using smackfsroot let's
> >> use smackfstransmute and a slightly different access rule:
> >>
> >> mount -o smackfstransmute=Rubble,smackfsdef=Rubble ...
> >>
> >> Smack rule: Flintstone Rubble rwxat
> >>
> >> Now the only change we have to make to the Smack code is
> >> that we don't want to create any files unless either the
> >> process is labeled Rubble or the rule allowing the creation
> >> has the "t" for transmute access. That should ensure that
> >> everything is labeled Rubble. If it isn't, someone has mucked
> >> with the metadata in a detectable way.
> > All right, that kind of makes sense, but I'm still missing some pieces.
> > Questions follow.
> >
> >>> diff --git a/include/linux/fs.h b/include/linux/fs.h
> >>> index 32f598db0b0d..4597420ab933 100644
> >>> --- a/include/linux/fs.h
> >>> +++ b/include/linux/fs.h
> >>> @@ -1486,6 +1486,10 @@ static inline void sb_start_intwrite(struct super_block *sb)
> >>>  	__sb_start_write(sb, SB_FREEZE_FS, true);
> >>>  }
> >>>  
> >>> +static inline bool sb_in_userns(struct super_block *sb)
> >>> +{
> >>> +	return sb->s_user_ns != &init_user_ns;
> >>> +}
> >>>  
> >>>  extern bool inode_owner_or_capable(const struct inode *inode);
> >>>  
> >>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> >>> index a143328f75eb..591fd19294e7 100644
> >>> --- a/security/smack/smack_lsm.c
> >>> +++ b/security/smack/smack_lsm.c
> >>> @@ -255,6 +255,10 @@ static struct smack_known *smk_fetch(const char *name, struct inode *ip,
> >>>  	char *buffer;
> >>>  	struct smack_known *skp = NULL;
> >>>  
> >>> +	/* Should never fetch xattrs from untrusted mounts */
> >>> +	if (WARN_ON(sb_in_userns(ip->i_sb)))
> >>> +		return ERR_PTR(-EPERM);
> >>> +
> >> Go ahead and fetch it, we'll check to make sure it's viable later.
> >>
> >>>  	if (ip->i_op->getxattr == NULL)
> >>>  		return ERR_PTR(-EOPNOTSUPP);
> >>>  
> >>> @@ -656,10 +660,14 @@ static int smack_sb_kern_mount(struct super_block *sb, int flags, void *data)
> >>>  		 */
> >>>  		if (specified)
> >>>  			return -EPERM;
> >>> +
> >>>  		/*
> >>> -		 * Unprivileged mounts get root and default from the caller.
> >>> +		 * User namespace mounts get root and default from the backing
> >>> +		 * store, if there is one. Other unprivileged mounts get them
> >>> +		 * from the caller.
> >>>  		 */
> >>> -		skp = smk_of_current();
> >>> +		skp = (sb_in_userns(sb) && sb->s_bdev) ?
> >>> +			smk_of_inode(sb->s_bdev->bd_inode) : smk_of_current();
> >>>  		sp->smk_root = skp;
> >>>  		sp->smk_default = skp;
> >> 			sp->smk_flags |= SMK_INODE_TRANSMUTE;
> > I assume that you meant skp and not sp here.
> 
> Actually, neither is correct. You want to set SMK_INODE_TRANSMUTE
> in the smk_flags field of the root inode. That's easy:
> 
> 			transmute = 1;
> 
> and the code after "Initialize the root inode" will take care of it.

Yeah, that's what I've actually done.

> >>>  	}
> >>> @@ -792,7 +800,12 @@ static int smack_bprm_secureexec(struct linux_binprm *bprm)
> >>>   */
> >>>  static int smack_inode_alloc_security(struct inode *inode)
> >>>  {
> >>> -	struct smack_known *skp = smk_of_current();
> >>> +	struct smack_known *skp;
> >>> +
> >>> +	if (sb_in_userns(inode->i_sb))
> >>> +		skp = ((struct superblock_smack *)(inode->i_sb->s_security))->smk_default;
> >>> +	else
> >>> +		skp = smk_of_current();
> >> This should be left alone.
> >> smack_inode_init_security is where you could disallow access that doesn't
> >> legitimately result in a Rubble label on the file. It's something like
> >>
> >> 	... after the call may = smk_access_entry(...)
> >> 	if (sb_in_userns(inode->i_sb))
> >> 		if (skp != dsp && (may & MAY_TRANSMUTE) == 0)
> >> 			return -EACCES; 
> > I'm not getting how this covers all cases.
> >
> > So we've set the transmute flag on the root inode. Files and directories
> > created in the root directory get the same label, and directories also
> > get the transmute attribute. That's all fine.
> >
> > What about an existing directory in the filesystem that already has a
> > Slate label? I'm not getting what happens with this directory, or for
> > new files created in this directory, which also relates to my other
> > questions below.
> >
> > Also an aside - smk_access_entry looks weird. may is initialized to
> > -ENOENT, and then rule_list is searched for a rule which matches the
> > object and subject labels. Presumably it's possible that no rule could
> > be found, otherwise the prior initialization of may is pointless. If
> > this happens the following code treats it as though it always contains
> > access flags even though it might contain -ENOENT. Nothing bad actually
> > happens with a two's compliement representation of -ENOENT since it will
> > just set a bit that's already set, but it still seems like it should
> > have a may > 0 condition, for clarity if for no other reason.
> 
> My suggested code is just wrong. I wasn't looking at the whole code,
> only the patch, and got myself confused. Apologies.
> 
> If we want to go straight for the jugular how about this? I'm assuming
> that inode->i_sb->s_bdev->bd_inode is the inode of the backing store.

Yes.

> static int smack_inode_permission(struct inode *inode, int mask)
> {
> 	struct smk_audit_info ad;
> 	int no_block = mask & MAY_NOT_BLOCK;
> 	int rc;
> 
> 	mask &= (MAY_READ|MAY_WRITE|MAY_EXEC|MAY_APPEND);
> 	/*
> 	 * No permission to check. Existence test. Yup, it's there.
> 	 */
> 	if (mask == 0)
> 		return 0;
> 
> +	if (sb_in_userns(inode->i_sb)) &&
> +	    smk_of_inode(inode) != smk_of_inode(inode->i_sb->s_bdev->bd_inode))
> +		return -EACCES;
> +
> 	/* May be droppable after audit */
> 	if (no_block)
> 		return -ECHILD;
> 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_INODE);
> 	smk_ad_setfield_u_fs_inode(&ad, inode);
> 	rc = smk_curacc(smk_of_inode(inode), mask, &ad);
> 	rc = smk_bu_inode(inode, mask, rc);
> 	return rc;
> }

Hmm, okay. I think I've been a little confused all this time about how
you want to handle these unprivileged mounts.

Originally I thought you wanted all objects in the filesystem to get the
same label as the backing store. That's what I tried to implement
originally, i.e. smk_root=smk_default=smk_of_inode(...->bd_inode), then
assign every object (new and existing) smk_default and completely ignore
the labels on disk.

This is what I currently think you want for user ns mounts:

 1. smk_root and smk_default are assigned the label of the backing
    device.
 2. s_root is assigned the transmute property.
 3. For existing files:
    a. Files with the same label as the backing device are accessible.
    b. Files with any other label are not accessible.

If this is right, there are a couple lingering questions in my mind.

First, what happens with files created in directories with the same
label as the backing device but without the transmute property set? The
inode for the new file will initially be labeled with smk_of_current(),
but then during d_instantiate it will get smk_default and thus end up
with the label we want. So that seems okay.

The second is whether files with the SMACK64EXEC attribute is still a
problem. It seems it is, for files with the same label as the backing
store at least. I think we can simply skip the code that reads out this
xattr and sets smk_task for user ns mounts, or else skip assigning the
label to the new task in bprm_set_creds. The latter seems more
consistent with the approach you've suggested for dealing with labels
from disk.

So I guess all of that seems okay, though perhaps a bit restrictive
given that the user who mounted the filesystem already has full access
to the backing store.

Please let me know whether or not this matches up with what you are
thinking, then I can procede with the implementation.

Thanks,
Seth