Bug 2521405 (CVE-2026-74726)

Summary: CVE-2026-74726 kernel: bonding: alb: re-check primary_is_promisc under RTNL in bond_alb_monitor
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's bonding module, specifically within the Active-Backup Load Balancing (ALB) monitor. This vulnerability arises from a race condition where the system makes decisions based on outdated network interface status. This can lead to a system crash, causing a Denial of Service (DoS). Additionally, it may result in a network interface becoming stuck in promiscuous mode, which could lead to unexpected network behavior.
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Description OSIDB Bzimport 2026-08-22 15:43:57 UTC 
In the Linux kernel, the following vulnerability has been resolved:

bonding: alb: re-check primary_is_promisc under RTNL in bond_alb_monitor

bond_alb_monitor() reads primary_is_promisc under RCU, then drops RCU and
takes RTNL via rtnl_trylock() before undoing the promiscuity it set on the
active slave. In that window the active slave can change under RTNL
(RTM_DELLINK -> __bond_release_one() -> bond_alb_handle_active_change()),
which already drops the promiscuity and clears primary_is_promisc. The
monitor still acts on the stale decision: if the slave was removed with no
failover, curr_active_slave is now NULL and the deref faults; if it failed
over, the stale dev_set_promiscuity(-1) underflows the new slave's
promiscuity counter and pins it in IFF_PROMISC.

  Oops: general protection fault, probably for non-canonical address ...
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  Workqueue: b42 bond_alb_monitor
  RIP: 0010:bond_alb_monitor (drivers/net/bonding/bond_alb.c:1600)
   process_one_work (kernel/workqueue.c:3322)
   worker_thread (kernel/workqueue.c:3486)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
  Kernel panic - not syncing: Fatal exception

Re-check primary_is_promisc (and curr_active_slave) after taking RTNL so
the monitor only undoes an increment it still owns. The other bonding
monitors already re-read state under RTNL in their commit phase
(bond_miimon_commit/bond_ab_arp_commit); bond_alb_monitor() was the only
one acting on the pre-trylock decision.
Comment 1 Mauro Matteo Cascella 2026-08-24 11:41:07 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026082241-CVE-2026-74726-8141@gregkh/T