Patch "bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx" has been added to the 5.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx

to the 5.4-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:
     bpf-fix-bpf_fib_lookup-helper-mtu-check-for-skb-ctx.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit d08c1f1b25b4265072c2616a28211481416770a0
Author: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Date:   Tue Feb 9 14:38:14 2021 +0100

    bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx
    
    [ Upstream commit 2c0a10af688c02adcf127aad29e923e0056c6b69 ]
    
    BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
    bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
    by adjusting fib_params 'tot_len' with the packet length plus the expected
    encap size. (Just like the bpf_check_mtu helper supports). He discovered
    that for SKB ctx the param->tot_len was not used, instead skb->len was used
    (via MTU check in is_skb_forwardable() that checks against netdev MTU).
    
    Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
    zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
    check is done like XDP code-path, which checks against FIB-dst MTU.
    
    V16:
    - Revert V13 optimization, 2nd lookup is against egress/resulting netdev
    
    V13:
    - Only do ifindex lookup one time, calling dev_get_by_index_rcu().
    
    V10:
    - Use same method as XDP for 'tot_len' MTU check
    
    Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
    Reported-by: Carlo Carraro <colrack@xxxxxxxxx>
    Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
    Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/bpf/161287789444.790810.15247494756551413508.stgit@firesoul
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/core/filter.c b/net/core/filter.c
index 2fa10fdcf6b1d..524f3364f8a05 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4880,6 +4880,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 {
 	struct net *net = dev_net(skb->dev);
 	int rc = -EAFNOSUPPORT;
+	bool check_mtu = false;
 
 	if (plen < sizeof(*params))
 		return -EINVAL;
@@ -4887,22 +4888,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
 		return -EINVAL;
 
+	if (params->tot_len)
+		check_mtu = true;
+
 	switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
 	case AF_INET:
-		rc = bpf_ipv4_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 #if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
-		rc = bpf_ipv6_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 	}
 
-	if (!rc) {
+	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
 		struct net_device *dev;
 
+		/* When tot_len isn't provided by user, check skb
+		 * against MTU of FIB lookup resulting net_device
+		 */
 		dev = dev_get_by_index_rcu(net, params->ifindex);
 		if (!is_skb_forwardable(dev, skb))
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;