Re: [PATCH 5.10 28/43] xfrm: espintcp: fix UAF during close

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Sep 04, 2026 at 05:51:58PM +0800, D. Wythe wrote:
> On Mon, Aug 31, 2026 at 03:35:36PM +0200, Greg Kroah-Hartman wrote:
> > 5.10-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Sabrina Dubroca <sd@queasysnail.net>
> > 
> > commit deb232e884877bf10b4ce2580909eedec986c284 upstream.
> > 
> > ZDI reported and analyzed a race condition during close for espintcp
> > sockets:
> > 
> >     espintcp_close() frees emsg->skb via kfree_skb() without holding
> >     any socket lock. Concurrently, the xfrm_trans_reinject work queue
> >     invokes esp_output_tcp_finish() -> espintcp_push_skb() ->
> >     espintcp_push_msgs() -> skb_send_sock_locked(), which reads the
> >     same skb as a data source.
> > 
> > Fix this by adding a synchronize_rcu() call after resetting sk_prot,
> > since esp_output_tcp_finish() runs under RCU and won't use a socket
> > with sk_prot == &tcp_prot.  Simply taking the socket lock in
> > espintcp_close() could lead to leaks, if esp_output_tcp_finish()
> > re-adds an skb in the slot we just freed. After this, the existing
> > barrier() is no longer needed.
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: e27cca96cd68 ("xfrm: add espintcp (RFC 8229)")
> > Reported-by: zdi-disclosures@trendmicro.com
> > Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
> > Reviewed-by: Breno Leitao <leitao@debian.org>
> > Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > ---
> >  net/xfrm/espintcp.c |    3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > --- a/net/xfrm/espintcp.c
> > +++ b/net/xfrm/espintcp.c
> > @@ -506,7 +506,8 @@ static void espintcp_close(struct sock *
> >  	strp_stop(&ctx->strp);
> >  
> >  	sk->sk_prot = &tcp_prot;
> > -	barrier();
> > +
> > +	synchronize_rcu();
> 
> 
> This backport depends on upstream
> 028363685bd0 ("espintcp: remove encap socket caching to avoid reference leak"),
> which is not in 5.15.y. Without it, esp_find_tcp_sk() still
> returns the cached x->encap_sk based only on sk_state == TCP_ESTABLISHED
> (no tcp_is_ulp_esp() check), so new readers keep racing with
> espintcp_close()'s kfree_skb on ctx->partial.emsg->skb — the UAF issue
> this patch is supposed to fix.
> 
> Reproduced on 5.15.219+ (with this patch applied), KASAN trimmed:
> 
>   BUG: KASAN: use-after-free in __skb_send_sock+0x6cf/0x7b0
>   Workqueue: events xfrm_trans_reinject
> 
>   Reader:
>     __skb_send_sock <- espintcp_push_msgs <- espintcp_push_skb <-
>     esp_output_tcp_encap_cb <- xfrm_trans_reinject
> 
>   Free:
>     espintcp_close <- inet_release <- __sock_release <- sock_close <-
>     __fput
> 
>   Object: skbuff_head_cache (size 224), i.e. struct sk_buff.
> 
> Provenance: the static analysis (missing-prerequisite chain, code-path
> derivation) and the KASAN reproducer above were both produced by our
> internal AI-assisted backport triage pipeline. I have personally
> reviewed the reasoning line-by-line against the 5.15.y source, re-ran
> the reproducer end-to-end, and confirm the finding — sending it upstream
> on that basis.

That's LLM output, what do you mean to say here to fix this?  Please
send a patch...

thanks,

greg k-h