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

Amir Goldstein
2015-07-30T15:09:15Z

On Thu, Jul 30, 2015 at 4:57 PM, Serge Hallyn <serge.hallyn@ubuntu.com> wrote:
> Quoting Amir Goldstein (amir@cellrox.com):
>> On Tue, Jul 28, 2015 at 11:40 PM, Seth Forshee
>> <seth.forshee@canonical.com> wrote:
>> >
>> > On Wed, Jul 22, 2015 at 05:05:17PM -0700, Casey Schaufler wrote:
>> > > > 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.
>>
>> Seth,
>>
>> There were 2 main concerns discussed in this thread:
>> 1. trusting LSM labels outside the namespace
>> 2. trusting the content of the image file/loopdev
>>
>> While your approach addresses the first concern, I suspect it may be placing
>> an obstacle in a way for resolving the second concern.
>>
>> A viable security policy to mitigate the second concern could be:
>> - Allow only trusted programs (e.g. mkfs, fsck) to write to 'Loopback' images
>> - Allow mount only of 'Loopback' images
>>
>> This should allow the system as a whole to trust unprivileged mounts based on
>> the trust of the entities that had raw access the the fs layout.
>
> Just to be sure I understand right, you're looking for a way to let
> the host admin trust that the kernel's superblock parsers aren't being
> fed trash or an exploit?

Correct.
I do not believe in the direction of auditing file system code to
vulnerability free level
nor do I think that cryptographically signed file system metadata is
the only way
to ensure an exploit free unprivileged mount.

>
>> Alas, if you choose to propagate the backing dev label to contained files,
>> they would all share the designated 'Loopback' label and render the policy above
>> useless.
>>
>> Any thoughts on how to reconcile this conflict?
>>
>> Amir.
>>
>>
>> > > > 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.
>> > >
>> > > That's right. Accept correct data, reject anything that's not right.
>> > >
>> > > > 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.
>> > >
>> > > Yes.
>> > >
>> > > > 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.
>> > >
>> > > Yes, I think that skipping the smk_fetch(XATTR_NAME_SMACKEXEC, ...) in
>> > > smack_d_instantiate for unprivileged mounts would do the trick.
>> > >
>> > > > 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.
>> > >
>> > > In truth, there is no reason to expect that the "user" who did the
>> > > mount will ever have a Smack label that differs from the label of
>> > > the backing store. If what we've got here seems restrictive, it's
>> > > because you've got access from someone other than the "user".
>> > >
>> > > > Please let me know whether or not this matches up with what you are
>> > > > thinking, then I can procede with the implementation.
>> > >
>> > > My current mindset is that, if you're going to allow unprivileged
>> > > mounts of user defined backing stores, this is as safe as we can
>> > > make it.
>> >
>> > All right, I've got a patch which I think does this, and I've managed to
>> > do some testing to confirm that it behaves like I expect. How does this
>> > look?
>> >
>> > What's missing is getting the label from the block device inode; as
>> > Stephen discovered the inode that I thought we could get the label from
>> > turned out to be the wrong one. Afaict we would need a new hook in order
>> > to do that, so for now I'm using the label of the proccess calling
>> > mount.
>> >
>> > ---
>> >
>> > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>> > index a143328f75eb..8e631a66b03c 100644
>> > --- a/security/smack/smack_lsm.c
>> > +++ b/security/smack/smack_lsm.c
>> > @@ -662,6 +662,8 @@ static int smack_sb_kern_mount(struct super_block *sb, int flags, void *data)
>> > skp = smk_of_current();
>> > sp->smk_root = skp;
>> > sp->smk_default = skp;
>> > + if (sb_in_userns(sb))
>> > + transmute = 1;
>> > }
>> > /*
>> > * Initialize the root inode.
>> > @@ -1023,6 +1025,12 @@ static int smack_inode_permission(struct inode *inode, int mask)
>> > if (mask == 0)
>> > return 0;
>> >
>> > + if (sb_in_userns(inode->i_sb)) {
>> > + struct superblock_smack *sbsp = inode->i_sb->s_security;
>> > + if (smk_of_inode(inode) != sbsp->smk_root)
>> > + return -EACCES;
>> > + }
>> > +
>> > /* May be droppable after audit */
>> > if (no_block)
>> > return -ECHILD;
>> > @@ -3220,14 +3228,16 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
>> > if (rc >= 0)
>> > transflag = SMK_INODE_TRANSMUTE;
>> > }
>> > - /*
>> > - * Don't let the exec or mmap label be "*" or "@".
>> > - */
>> > - skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
>> > - if (IS_ERR(skp) || skp == &smack_known_star ||
>> > - skp == &smack_known_web)
>> > - skp = NULL;
>> > - isp->smk_task = skp;
>> > + if (!sb_in_userns(inode->i_sb)) {
>> > + /*
>> > + * Don't let the exec or mmap label be "*" or "@".
>> > + */
>> > + skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
>> > + if (IS_ERR(skp) || skp == &smack_known_star ||
>> > + skp == &smack_known_web)
>> > + skp = NULL;
>> > + isp->smk_task = skp;
>> > + }
>> >
>> > skp = smk_fetch(XATTR_NAME_SMACKMMAP, inode, dp);
>> > if (IS_ERR(skp) || skp == &smack_known_star ||
>> > --
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