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

[Kuniyuki Iwashima]

On Mon, 02 Nov 2026 10:55:58 +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.  The migrated clone becomes the owner after
> reqsk_migrate_reset() clears the old request's pointer, but a reader which
> already loaded the pointer from the old request can still be using it when
> the clone is released.
> 
> Both request and full TCP sockets currently free saved_syn with plain
> kfree().  Embed an rcu_head in struct saved_syn and use kfree_rcu() for
> both teardown paths.

Please add rcu_dereference() in bpf_sock_ops_get_syn()
and RCU_INIT_POINTER() on the writer side.



> 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 this change.  A patched kernel without the delay
> instrumentation also completed a successful migration and seven saved-SYN
> reads without a KASAN report.
> 
> Full allmodconfig and allyesconfig builds completed with W=1.  Existing
> warnings outside the modified files were made non-fatal with
> KCFLAGS=-Wno-error; the modified files emitted none.

I'd drop this paragraph or move after --- since it does not
add any value after review.

---
pw-bot: cr