CVE-2026-45913: net: bridge: mcast: always update mdb_n_entries for vlan contexts
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mcast: always update mdbnentries for vlan contexts
syzbot triggered a warning[1] about the number of mdb entries in a context. It turned out that there are multiple ways to trigger that warning today (some got added during the years), the root cause of the problem is that the increase is done conditionally, and over the years these different conditions increased so there were new ways to trigger the warning, that is to do a decrease which wasn't paired with a previous increase.
For example one way to trigger it is with flush: $ ip l add br0 up type bridge vlanfiltering 1 mcastsnooping 1 $ ip l add dumdum up master br0 type dummy $ bridge mdb add dev br0 port dumdum grp 239.0.0.1 permanent vid 1 $ ip link set dev br0 down $ ip link set dev br0 type bridge mcastvlansnooping 1 ^^^^ this will enable snooping, but will not update mdbnentries because in brmulticastenableportctx() we check !netifrunning $ bridge mdb flush dev br0 ^^^ this will trigger the warning because it will delete the pg which we added above, which will try to decrease mdbnentries
Fix the problem by removing the conditional increase and always keep the count up-to-date while the vlan exists. In order to do that we have to first initialize it on port-vlan context creation, and then always increase or decrease the value regardless of mcast options. To keep the current behaviour we have to enforce the mdb limit only if the context is port's or if the port-vlan's mcast snooping is enabled.
[1] ------------[ cut here ]------------ n == 0 WARNING: net/bridge/brmulticast.c:718 at brmulticastportngroupsdecone net/bridge/brmulticast.c:718 [inline], CPU#0: syz.4.4607/22043 WARNING: net/bridge/brmulticast.c:718 at brmulticastportngroupsdec net/bridge/brmulticast.c:771 [inline], CPU#0: syz.4.4607/22043 WARNING: net/bridge/brmulticast.c:718 at brmulticastdelpg+0x1bbe/0x1e20 net/bridge/brmulticast.c:825, CPU#0: syz.4.4607/22043 Modules linked in: CPU: 0 UID: 0 PID: 22043 Comm: syz.4.4607 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026 RIP: 0010:brmulticastportngroupsdecone net/bridge/brmulticast.c:718 [inline] RIP: 0010:brmulticastportngroupsdec net/bridge/brmulticast.c:771 [inline] RIP: 0010:brmulticastdelpg+0x1bbe/0x1e20 net/bridge/brmulticast.c:825 Code: 41 5f 5d e9 04 7a 48 f7 e8 3f 73 5c f7 90 0f 0b 90 e9 cf fd ff ff e8 31 73 5c f7 90 0f 0b 90 e9 16 fd ff ff e8 23 73 5c f7 90 <0f> 0b 90 e9 60 fd ff ff e8 15 73 5c f7 eb 05 e8 0e 73 5c f7 48 8b RSP: 0018:ffffc9000c207220 EFLAGS: 00010293 RAX: ffffffff8a68042d RBX: ffff88807c6f1800 RCX: ffff888066e90000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: 0000000000000000 R08: ffff888066e90000 R09: 000000000000000c R10: 000000000000000c R11: 0000000000000000 R12: ffff8880303ef800 R13: dffffc0000000000 R14: ffff888050eb11c4 R15: 1ffff1100a1d6238 FS: 00007fa45921b6c0(0000) GS:ffff8881256f5000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fa4591f9ff8 CR3: 0000000081df2000 CR4: 00000000003526f0 Call Trace: <TASK> brmdbflushpgs net/bridge/brmdb.c:1525 [inline] brmdbflush net/bridge/brmdb.c:1544 [inline] brmdbdelbulk+0x5e2/0xb20 net/bridge/brmdb.c:1561 rtnlmdbdel+0x48a/0x640 net/core/rtnetlink.c:-1 rtnetlinkrcvmsg+0x77e/0xbe0 net/core/rtnetlink.c:6967 netlinkrcvskb+0x232/0x4b0 net/netlink/afnetlink.c:2550 netlinkunicastkernel net/netlink/afnetlink.c:1318 [inline] netlinkunicast+0x80f/0x9b0 net/netlink/afnetlink.c:1344 netlinksendmsg+0x813/0xb40 net/netlink/afnetlink.c:1894 socksendmsgnosec net/socket.c:727 [inline] socksendmsg net/socket.c:742 [inline] syssendmsg+0xa68/0xad0 net/socket.c:2592 syssendmsg+0x2a5/0x360 net/socke ---truncated---
Affected Software
Remediation
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Event History
Frequently Asked Questions
What conditions are needed to trigger the warning?
The affected bridge must use VLAN filtering and multicast snooping, have a multicast database entry associated with a VLAN context, and undergo a state sequence where snooping is enabled while the bridge is not running. Flushing the bridge MDB afterward can delete an entry whose prior accounting increment was skipped.
What is the practical impact?
The reported impact is a warning caused by inconsistent multicast database entry accounting. The supplied severity vector rates availability impact as high, while confidentiality and integrity impacts are listed as none.
Does exploiting this require network access or elevated privileges?
The severity vector identifies local attack access, low attack complexity, low privileges required, and no user interaction. The example uses commands that create and configure a bridge, attach an interface, add MDB entries, change bridge state, and flush the MDB.
How is the issue fixed?
The resolution is to always update mdb_n_entries for VLAN contexts, removing the conditional behavior that could allow a decrement without a corresponding earlier increment. The referenced upstream Linux kernel commit is 8b769e311a86bb9d15c5658ad283b86fc8f080a2.