Re: [PATCH net v3] tcp: free saved_syn after an RCU grace period

[Kuniyuki Iwashima]

On Wed, 04 Nov 2026 12:32:19 +0000, Yingjie Wang wrote:
> tcp_reqsk_record_syn() allocates saved_syn separately from its request
> socket. bpf_sock_ops_get_syn() can load that pointer while processing a
> retransmitted SYN under RCU.
> 
> There are two ownership transfers which can let another CPU reclaim the
> allocation while such a reader is still using it.
> 
> First, tcp_move_syn() transfers req->saved_syn to a newly created child
> socket. The child can be closed and tcp_saved_syn_free() can reclaim the
> allocation before a reader of the old request finishes.
> 
> Second, inet_reqsk_clone() shallow-copies request_sock during request
> migration. After reqsk_migrate_reset() clears the old request pointer, the
> migrated clone owns it. However, a reader which already loaded the pointer
> from the old request can still be using it when the clone is released.
> 
> I found this through LLM-assisted concurrency fuzzing.
> 
> Mark the request-socket saved_syn pointer as RCU protected and use RCU
> accessors for its reads and updates. Keep the full-socket pointer plain
> because all of its accesses hold the socket lock. Embed an rcu_head in
> struct saved_syn and use kfree_rcu() in both teardown paths. This lets a
> request-side reader which loaded the object before an ownership transfer
> finish.
> 
> This follows commit e3f33b0a1d89 ("ipv4: free inet_opt and ireq_opt
> after an RCU grace period"), which fixed the same ownership-transfer
> pattern for IPv4 options.
> 
> The issue was confirmed with KASAN on v7.3-16007-g36aad21d50d1 using
> temporary kernel instrumentation to widen the reader and request-migration
> window. KASAN reported a slab-use-after-free read in
> bpf_sock_ops_get_syn(), with the allocation freed by __reqsk_free() from
> reqsk_timer_handler(). The same explicit migration test completed without
> a report with deferred reclamation. A patched kernel without the delay
> instrumentation also completed a successful migration and seven saved-SYN
> reads without a KASAN report.
> 
> Fixes: 0813a841566f ("bpf: tcp: Allow bpf prog to write and parse TCP header option")
> Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at retransmitting SYN+ACKs.")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yingjie Wang <x-professor@xxxxxxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>