On 07/11/2017 11:45 PM, Serge E. Hallyn wrote:
Quoting Stefan Berger (Stefan Bergerstefanb@xxxxxxxxxxxxxxxxxx):+/* + * xattr_list_userns_rewrite - Rewrite list of xattr names for user namespaces + * or determine needed size for attribute list + * in case size == 0 + * + * In a user namespace we do not present all extended attributes to the + * user. We filter out those that are in the list of userns supported xattr. + * Besides that we filter out those with @uid=<uid> when there is no mapping + * for that uid in the current user namespace. + * + * @list: list of 0-byte separated xattr names + * @size: the size of the list; may be 0 to determine needed list size + * @list_maxlen: allocated buffer size of list + */ +static ssize_t +xattr_list_userns_rewrite(char *list, ssize_t size, size_t list_maxlen) +{ + char *nlist = NULL; + size_t s_off, len, nlen; + ssize_t d_off; + char *name, *newname; + + if (!list || size < 0 || current_user_ns() == &init_user_ns) + return size; + + if (size) { + nlist = kmalloc(list_maxlen, GFP_KERNEL); + if (!nlist) + return -ENOMEM; + } + + s_off = d_off = 0; + while (s_off < size || size == 0) { + name = &list[s_off]; + + len = strlen(name); + if (!len) + break; + + if (xattr_is_userns_supported(name, false) >= 0) + newname = name; + else { + newname = xattr_rewrite_userns_xattr(name);Why are you doing this here? If we get here it means that xattr_is_userns_supported() returned < 0, meaning name is not userns-supported. So xattr_rewrite_userns_xattr() will just return name. Am I missing something?
xattr_is_userns_support(name, false) does a _full string match_ rather than a prefix match and will only return >= 0 for security.capability. This case handles the hosts's security.capability which 'shines through' for read and needs to be listed. Only in this case we set newname=name.
In the else branch we handle security.capability@uid=1000 and rewrite that to security.capability for root mapping to uid=1000.
+ if (IS_ERR(newname)) { + d_off = PTR_ERR(newname); + goto out_free; + } + } + if (newname && !xattr_list_contains(nlist, d_off, newname)) {Now here, if name was recalculated to @newname, and @newname is found in the nlist, that should raise an error right? Something fishy is going on?
If security.capability is set on a file but the container doesn't have security.capability@uid=1000, we still need to list the former here. However, we end up with duplicates if security.capability is there and security.capability@uid=1000 is also there and root is mapped to uid=1000. Both would be shown as security.capability inside the container. In this case we need to filter.
I think the code is correct. More problematic is a memory leak in the error case. Will fix that.
+ nlen = strlen(newname); + + if (nlist) { + if (nlen + 1 > list_maxlen)d_off needs to be set to -ERANGE here.
Fixed.
+ break; + strcpy(&nlist[d_off], newname); + } + + d_off += nlen + 1; + if (newname != name) + kfree(newname); + } + s_off += len + 1; + } + if (nlist) + memcpy(list, nlist, d_off); +out_free: + kfree(nlist); + + return d_off; +}
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