Patch "socket, bpf: fix sk_filter use after free in sk_clone_lock" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

socket, bpf: fix sk_filter use after free in sk_clone_lock

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
socket-bpf-fix-sk_filter-use-after-free-in-sk_clone_lock.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Borkmann <[email protected]>
Date: Wed, 22 Mar 2017 13:08:08 +0100
Subject: socket, bpf: fix sk_filter use after free in sk_clone_lock

From: Daniel Borkmann <[email protected]>

[ Upstream commit a97e50cc4cb67e1e7bff56f6b41cda62ca832336 ]

In sk_clone_lock(), we create a new socket and inherit most of the parent's members via sock_copy() which memcpy()'s various sections. Now, in case the parent socket had a BPF socket filter attached, then newsk->sk_filter points to the same instance as the original sk->sk_filter.

sk_filter_charge() is then called on the newsk->sk_filter to take a reference and should that fail due to hitting max optmem, we bail out and release the newsk instance.

The issue is that commit 278571baca2a ("net: filter: simplify socket charging") wrongly combined the dismantle path with the failure path of xfrm_sk_clone_policy(). This means, even when charging failed, we call sk_free_unlock_clone() on the newsk, which then still points to the same sk_filter as the original sk.

Thus, sk_free_unlock_clone() calls into __sk_destruct() eventually where it tests for present sk_filter and calls sk_filter_uncharge() on it, which potentially lets sk_omem_alloc wrap around and releases the eBPF prog and sk_filter structure from the (still intact) parent.

Fix it by making sure that when sk_filter_charge() failed, we reset newsk->sk_filter back to NULL before passing to sk_free_unlock_clone(), so that we don't mess with the parents sk_filter.

Only if xfrm_sk_clone_policy() fails, we did reach the point where either the parent's filter was NULL and as a result newsk's as well or where we previously had a successful sk_filter_charge(), thus for that case, we do need sk_filter_uncharge() to release the prior taken reference on sk_filter.

---
 net/core/sock.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -1557,6 +1557,12 @@ struct sock *sk_clone_lock(const struct
 			is_charged = sk_filter_charge(newsk, filter);
 
 		if (unlikely(!is_charged || xfrm_sk_clone_policy(newsk, sk))) {
+			/* We need to make sure that we don't uncharge the new
+			 * socket if we couldn't charge it in the first place
+			 * as otherwise we uncharge the parent's filter.
+			 */
+			if (!is_charged)
+				RCU_INIT_POINTER(newsk->sk_filter, NULL);
 			/* It is still raw copy of parent, so invalidate
 			 * destructor and make plain sk_free() */
 			newsk->sk_destruct = NULL;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/socket-bpf-fix-sk_filter-use-after-free-in-sk_clone_lock.patch