Re: [PATCH v2 net 1/3] ipv4: igmp: Fix potential UAF in igmp_gq_start_timer()

[Ido Schimmel <idosch@nvidia.com>]

On Sun, Jul 05, 2026 at 06:17:54PM +0000, Eric Dumazet wrote:
> A race condition exists between device teardown (inetdev_destroy) and
> incoming IGMP query processing (igmp_rcv), leading to a Use-After-Free
> in the IGMP timer callback.
> 
> During device destruction, inetdev_destroy() drops the primary reference
> to in_device, which can drop its refcount to 0. The actual freeing of
> in_device memory is deferred via RCU (using call_rcu()).
> 
> Concurrently, igmp_rcv() runs under RCU read lock and obtains the
> in_device pointer. Because the memory is RCU-protected, CPU-0 can safely
> dereference in_device even if its refcount has hit 0.
> 
> However, if CPU-0 calls igmp_gq_start_timer() and re-arms the timer, it
> attempts to acquire a reference using in_dev_hold(). This increments the
> refcount from 0 to 1, triggering a "refcount_t: addition on 0" warning.
> Since the in_device memory is still scheduled to be freed after the RCU
> grace period (as the free callback does not check the refcount again),
> the device is freed while the timer is still armed. When the timer
> expires, it accesses the freed memory, causing a kernel panic.
> 
> Fix this by using refcount_inc_not_zero() (via a new helper
> in_dev_hold_safe()) to prevent acquiring a reference if the device is
> already being destroyed. If the refcount is 0, we do not arm the timer.
> 
> A similar issue in IPv6 MLD is fixed in a subsequent patch.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: Zero Day Initiative <zdi-disclosures@trendmicro.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>