CVE-2025-38472: netfilter: nf_conntrack: fix crash due to removal of uninitialised entry

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

netfilter: nfconntrack: 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: nfctdeletefromlists+172] [..] #7 [ff539b5a2b043aa0] nfctdelete at ffffffffc124d421 [nfconntrack] #8 [ff539b5a2b043ad0] nfctgcexpired at ffffffffc124d999 [nfconntrack] #9 [ff539b5a2b043ae0] nfconntrackfindget at ffffffffc124efbc [nfconntrack] [..]

The nfconn 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 IPSCONFIRMED|IPSDYING, 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 IPSCONFIRMED, nfconntrackfindget() 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> nfctdelete() return E to rcu slab initconntrack 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.

nfconntrackconfirm 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> nfctexpired() -> yes (ct->timeout is 30s) confirmed bit set.

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

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 NFDROP <unblocks, then crashes on hlistnullsdelrcu 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: nfconntrackput -> nfconntrackdestroy -> nfctdestroy.

To resolve this, move the IPSCONFIRMED 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 setbit and beforeatomic 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 nfctshouldgc() 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 nfctshouldgc() to double-check step 1.

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