Revert 95ebabde ("capabilities: Don't allow writing ambiguous v3 file capabilities")

commit 3b0c2d3e upstream.

It turns out that there are in fact userspace implementations that
care and this recent change caused a regression.

https://github.com/containers/buildah/issues/3071

As the motivation for the original change was future development,
and the impact is existing real world code just revert this change
and allow the ambiguity in v3 file caps.

Cc: stable@vger.kernel.org
Fixes: 95ebabde ("capabilities: Don't allow writing ambiguous v3 file capabilities")
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

security/commoncap.c

@@ -506,8 +506,7 @@ int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)
 	__u32 magic, nsmagic;
 	struct inode *inode = d_backing_inode(dentry);
 	struct user_namespace *task_ns = current_user_ns(),
-		*fs_ns = inode->i_sb->s_user_ns,
-		*ancestor;
+		*fs_ns = inode->i_sb->s_user_ns;
 	kuid_t rootid;
 	size_t newsize;
 
@@ -530,15 +529,6 @@ int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)
 	if (nsrootid == -1)
 		return -EINVAL;
 
-	/*
-	 * Do not allow allow adding a v3 filesystem capability xattr
-	 * if the rootid field is ambiguous.
-	 */
-	for (ancestor = task_ns->parent; ancestor; ancestor = ancestor->parent) {
-		if (from_kuid(ancestor, rootid) == 0)
-			return -EINVAL;
-	}
-
 	newsize = sizeof(struct vfs_ns_cap_data);
 	nscap = kmalloc(newsize, GFP_ATOMIC);
 	if (!nscap)