Race Condition Affecting kernel-debug-core package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-debug-core package and not the kernel-debug-core package as distributed by RHEL. See How to fix? for RHEL:9 relevant fixed versions and status.

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_conntrack: fix crash due to removal of uninitialised entry

A crash in conntrack was reported while trying to unlink the conntrack entry from the hash bucket list: [exception RIP: __nf_ct_delete_from_lists+172] [..] #7 [ff539b5a2b043aa0] nf_ct_delete at ffffffffc124d421 [nf_conntrack] #8 [ff539b5a2b043ad0] nf_ct_gc_expired at ffffffffc124d999 [nf_conntrack] #9 [ff539b5a2b043ae0] __nf_conntrack_find_get at ffffffffc124efbc [nf_conntrack] [..]

The nf_conn struct is marked as allocated from slab but appears to be in a partially initialised state:

ct hlist pointer is garbage; looks like the ct hash value (hence crash). ct->status is equal to IPS_CONFIRMED|IPS_DYING, which is expected ct->timeout is 30000 (=30s), which is unexpected.

Everything else looks like normal udp conntrack entry. If we ignore ct->status and pretend its 0, the entry matches those that are newly allocated but not yet inserted into the hash:

• ct hlist pointers are overloaded and store/cache the raw tuple hash
• ct->timeout matches the relative time expected for a new udp flow rather than the absolute 'jiffies' value.

If it were not for the presence of IPS_CONFIRMED, __nf_conntrack_find_get() would have skipped the entry.

Theory is that we did hit following race:

cpu x cpu y cpu z found entry E found entry E E is expired <preemption> nf_ct_delete() return E to rcu slab init_conntrack E is re-inited, ct->status set to 0 reply tuplehash hnnode.pprev stores hash value.

cpu y found E right before it was deleted on cpu x. E is now re-inited on cpu z. cpu y was preempted before checking for expiry and/or confirm bit.

                ->refcnt set to 1
                E now owned by skb
                ->timeout set to 30000

If cpu y were to resume now, it would observe E as expired but would skip E due to missing CONFIRMED bit.

                nf_conntrack_confirm gets called
                sets: ct->status |= CONFIRMED
                This is wrong: E is not yet added
                to hashtable.

cpu y resumes, it observes E as expired but CONFIRMED: <resumes> nf_ct_expired() -> yes (ct->timeout is 30s) confirmed bit set.

cpu y will try to delete E from the hashtable: nf_ct_delete() -> set DYING bit __nf_ct_delete_from_lists

Even this scenario doesn't guarantee a crash: cpu z still holds the table bucket lock(s) so y blocks:

        wait for spinlock held by z

            CONFIRMED is set but there is no
            guarantee ct will be added to hash:
            "chaintoolong" or "clash resolution"
            logic both skip the insert step.
            reply hnnode.pprev still stores the
            hash value.

            unlocks spinlock
            return NF_DROP
    <unblocks, then
     crashes on hlist_nulls_del_rcu pprev>

In case CPU z does insert the entry into the hashtable, cpu y will unlink E again right away but no crash occurs.

Without 'cpu y' race, 'garbage' hlist is of no consequence: ct refcnt remains at 1, eventually skb will be free'd and E gets destroyed via: nf_conntrack_put -> nf_conntrack_destroy -> nf_ct_destroy.

To resolve this, move the IPS_CONFIRMED assignment after the table insertion but before the unlock.

Pablo points out that the confirm-bit-store could be reordered to happen before hlist add resp. the timeout fixup, so switch to set_bit and before_atomic memory barrier to prevent this.

It doesn't matter if other CPUs can observe a newly inserted entry right before the CONFIRMED bit was set:

Such event cannot be distinguished from above "E is the old incarnation" case: the entry will be skipped.

Also change nf_ct_should_gc() to first check the confirmed bit.

The gc sequence is:

1. Check if entry has expired, if not skip to next entry
2. Obtain a reference to the expired entry.
3. Call nf_ct_should_gc() to double-check step 1.

nf_ct_should_gc() is thus called only for entries that already failed an expiry check. After this patch, once the confirmed bit check pas ---truncated---

• https://access.redhat.com/security/cve/CVE-2025-38472
• https://git.kernel.org/stable/c/2d72afb340657f03f7261e9243b44457a9228ac7
• https://git.kernel.org/stable/c/76179961c423cd698080b5e4d5583cf7f4fcdde9
• https://git.kernel.org/stable/c/938ce0e8422d3793fe30df2ed0e37f6bc0598379
• https://git.kernel.org/stable/c/a47ef874189d47f934d0809ae738886307c0ea22
• https://git.kernel.org/stable/c/fc38c249c622ff5e3011b8845fd49dbfd9289afc