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

Casey Schaufler
2015-07-30T16:18:24Z

On 7/28/2015 1:40 PM, Seth Forshee 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.
>>> 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.

That will be OK if the mount processing checks for write access to
the backing store. I haven't looked to see if it does. If it doesn't
the problems should be pretty obvious.

>
> ---
>
> 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 ||
>