FAILED: patch "[PATCH] batman-adv: tt: fix TOCTOU race for reported vlans" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 94d27005016be15ffc638b2ecbc4d58805ad7b48
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026052810-narrow-feast-bb41@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 94d27005016be15ffc638b2ecbc4d58805ad7b48 Mon Sep 17 00:00:00 2001
From: Sven Eckelmann <sven@narfation.org>
Date: Sat, 2 May 2026 19:47:11 +0200
Subject: [PATCH] batman-adv: tt: fix TOCTOU race for reported vlans

The local TT based TVLV is generated by first checking the number of VLANs
which have at least one TT entry. A new buffer with the correct size for
the VLANs is then allocated. Only then, the list of VLANs s used to fill
the VLAN entries in the buffer. During this time, the meshif_vlan_list_lock
is held. But the actual number of TT entries of each VLAN can still
increase during this time - just not the number of VLANs in the list.

But the prefilter used in the buffer size calculation might still cause an
increase of the number of VLANs which need to be stored. Simply because a
VLAN might now suddenly have at least one entry when it had none in the
pre-alloc check - and then needs to occupy space which was not allocated.

It is better to overestimate the buffer size at the beginning and then fill
the buffer only with the VLANs which are not empty.

Cc: stable@kernel.org
Fixes: 16116dac2339 ("batman-adv: prevent TT request storms by not sending inconsistent TT TLVLs")
Signed-off-by: Sven Eckelmann <sven@narfation.org>

diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c
index 06548dae1039..f009cbf8a276 100644
--- a/net/batman-adv/translation-table.c
+++ b/net/batman-adv/translation-table.c
@@ -887,11 +887,8 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 	spin_lock_bh(&bat_priv->meshif_vlan_list_lock);
 	hlist_for_each_entry(vlan, &bat_priv->meshif_vlan_list, list) {
 		vlan_entries = atomic_read(&vlan->tt.num_entries);
-		if (vlan_entries < 1)
-			continue;
-
-		num_vlan++;
 		total_entries += vlan_entries;
+		num_vlan++;
 	}
 
 	change_offset = struct_size(*tt_data, vlan_data, num_vlan);
@@ -916,6 +913,7 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 	(*tt_data)->num_vlan = htons(num_vlan);
 
 	tt_vlan = (*tt_data)->vlan_data;
+	num_vlan = 0;
 	hlist_for_each_entry(vlan, &bat_priv->meshif_vlan_list, list) {
 		vlan_entries = atomic_read(&vlan->tt.num_entries);
 		if (vlan_entries < 1)
@@ -926,8 +924,15 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 		tt_vlan->reserved = 0;
 
 		tt_vlan++;
+		num_vlan++;
 	}
 
+	/* recalculate in case number of VLANs reduced */
+	change_offset = struct_size(*tt_data, vlan_data, num_vlan);
+	tvlv_len = *tt_len + change_offset;
+
+	(*tt_data)->num_vlan = htons(num_vlan);
+
 	tt_change_ptr = (u8 *)*tt_data + change_offset;
 	*tt_change = (struct batadv_tvlv_tt_change *)tt_change_ptr;