Re: [PATCH RFC] user-namespaced file capabilities - now with more magic

"Serge E. Hallyn" [off-list ref] writes:

Quoting Eric W. Biederman (ebiederm@xmission.com):

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Mimi Zohar [off-list ref] writes:

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On Thu, 2016-05-19 at 22:40 -0500, Serge E. Hallyn wrote:

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Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):

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On Wed, 2016-05-18 at 16:57 -0500, Serge E. Hallyn wrote:

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diff --git a/fs/xattr.c b/fs/xattr.c
index 4861322..5c0e7ae 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c

@@ -94,11 +94,26 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
 {
 	struct inode *inode = dentry->d_inode;
 	int error = -EOPNOTSUPP;
+	void *wvalue = NULL;
+	size_t wsize = 0;
 	int issec = !strncmp(name, XATTR_SECURITY_PREFIX,
 				   XATTR_SECURITY_PREFIX_LEN);

-	if (issec)
+	if (issec) {
 		inode->i_flags &= ~S_NOSEC;
+		/* if root in a non-init user_ns tries to set
+		 * security.capability, write a security.nscapability
+		 * in its place */
+		if (!strcmp(name, "security.capability") &&
+				current_user_ns() != &init_user_ns) {
+			cap_setxattr_make_nscap(dentry, value, size, &wvalue, &wsize);
+			if (!wvalue)
+				return -EPERM;
+			value = wvalue;
+			size = wsize;
+			name = "security.nscapability";
+		}

The call to capable_wrt_inode_uidgid() is hidden behind
cap_setxattr_make_nscap().  Does it make sense to call it here instead,
before the security.capability test?  This would lay the foundation for
doing something similar for IMA.

Might make sense to move that.  Though looking at it with fresh eyes I wonder
whether adding less code here at __vfs_setxattr_noperm(), i.e.

		if (!cap_setxattr_makenscap(dentry, &value, &size, &name))
			return -EPERM;

would be cleaner.

Yes, it would be cleaner,  but I'm suggesting you do all the hard work
making it generic.  Then the rest of us can follow your lead.  Its more
likely that you'll get it right.  At a high level, it might look like:

               /* Permit root in a non-init user_ns to modify the security
                 * namespace xattr equivalents (eg. nscapability, ns_ima, etc). 
                 */
                if ((current_user_ns() != &init_user_ns) &&
                        capable_wrt_inode_uidgid(inode, CAP_SETFCAP)) {

			if  security..capability
				call capability  /* set nscapability? */

			else if security.ima 
				call ima 	/* set ns_ima? */
		}

Hmm.  I am confused about this part of the strategy.

I don't understand the capability vs nscapability distinction.  It seems
to add complexity without benefit.

...  Well, yes, we could simply make a new version of security.capability
xattr, and make rootid == 0 mean it was written by the init_user_ns.  Is
that what you mean?

Yes.

That would seem to simplify the logic to ensure the policy we enforce is
consistent with what is on disk.

Eric