On Fri, 24 Jan 2025 17:35:24 +0300 Dan Carpenter wrote:

On Wed, Jan 22, 2025 at 06:24:27AM -0800, Jakub Kicinski wrote:

quoted

On Wed, 22 Jan 2025 16:49:17 +0300 Dan Carpenter wrote:  

quoted

The "payload" variable is type size_t, however the nlmsg_total_size()
function will a few bytes to it and then truncate the result to type
int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
function might allocate a buffer which is smaller than intended.  

Is there a bug, or is this theoretical?  

The rule here is that if we pass something very close to UINT_MAX to
nlmsg_new() the it leads to an integer overflow.  I'm not a networking
expert.  The caller that concerned me was:

*** 1 ***

net/netfilter/ipset/ip_set_core.c
  1762                  /* Error in restore/batch mode: send back lineno */
  1763                  struct nlmsghdr *rep, *nlh = nlmsg_hdr(skb);
  1764                  struct sk_buff *skb2;
  1765                  struct nlmsgerr *errmsg;
  1766                  size_t payload = min(SIZE_MAX,
  1767                                       sizeof(*errmsg) + nlmsg_len(nlh));

I don't know the limits of limits of nlmsg_len() here.

Practically speaking the limits are fairly small. The nlh comes from
user's request / sendmsg() call. So the user must have prepared 
a message of at least that len, and kernel must had been able to
kvmalloc() a linear buffer large enough to copy that message in.

The min(SIZE_MAX is what scared me.  That was added to silence a Smatch
warning.  :P  It should be fixed or removed.

Yeah, that ip_set code looks buggy. Mostly because we use @payload
for the nlmsg_put() call, but then raw nlh->nlmsg_len for memcpy() :S

  1768                  int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
  1769                  struct nlattr *cda[IPSET_ATTR_CMD_MAX + 1];
  1770                  struct nlattr *cmdattr;
  1771                  u32 *errline;
  1772  
  1773                  skb2 = nlmsg_new(payload, GFP_KERNEL);
  1774                  if (!skb2)
  1775                          return -ENOMEM;

*** 2 ***
There is similar code in netlink_ack() where the payload comes from
nlmsg_len(nlh).

This one is correct. Each piece of the message is nlmsg_put()
individually, which does bounds checking. So if the allocation 
of the skb was faulty and the skb is shorter than we expected 
we'll just error out on the put.

*** 3 ***

There is a potential issue in queue_userspace_packet() when we call:

	len = upcall_msg_size(upcall_info, hlen - cutlen, ...
                                           ^^^^^^^^^^^^^
	user_skb = genlmsg_new(len, GFP_ATOMIC);

It's possible that hlen is less than cutlen.  (That's a separate bug,
I'll send a fix for it).

Ack.

In general IMVHO the check in nlmsg_new() won't be too effective.
The callers can overflow their local message size calculation.
Not to mention that the size calculation is often inexact.
So using nla_put() and checking error codes is the best way
to prevent security issues..