[PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[Kuniyuki Iwashima]

Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().

  """
  We are seeing a use-after-free from a bpf prog attached to
  trace_tcp_retransmit_synack. The program passes the req->sk to the
  bpf_sk_storage_get_tracing kernel helper which does check for null
  before using it.
  """

The commit 83fccfc3940c ("inet: fix potential deadlock in
reqsk_queue_unlink()") added timer_pending() in reqsk_queue_unlink() not
to call del_timer_sync() from reqsk_timer_handler(), but it introduced a
small race window.

Before the timer is called, expire_timers() calls detach_timer(timer, true)
to clear timer->entry.pprev and marks it as not pending.

If reqsk_queue_unlink() checks timer_pending() just after expire_timers()
calls detach_timer(), TCP will miss del_timer_sync(); the reqsk timer will
continue running and send multiple SYN+ACKs until it expires.

The reported UAF could happen if req->sk is close()d earlier than the timer
expiration, which is 63s by default.

The scenario would be

  1. inet_csk_complete_hashdance() calls inet_csk_reqsk_queue_drop(),
     but del_timer_sync() is missed

  2. reqsk timer is executed and scheduled again

  3. req->sk is accept()ed and reqsk_put() decrements rsk_refcnt, but
     reqsk timer still has another one, and inet_csk_accept() does not
     clear req->sk for non-TFO sockets

  4. sk is close()d

  5. reqsk timer is executed again, and BPF touches req->sk

Let's not use timer_pending() by passing the caller context to
__inet_csk_reqsk_queue_drop().

Note that reqsk timer is pinned, so the issue does not happen in most
use cases. [1]

[0]
BUG: KFENCE: use-after-free read in bpf_sk_storage_get_tracing+0x2e/0x1b0

Use-after-free read at 0x00000000a891fb3a (in kfence-#1):
bpf_sk_storage_get_tracing+0x2e/0x1b0
bpf_prog_5ea3e95db6da0438_tcp_retransmit_synack+0x1d20/0x1dda
bpf_trace_run2+0x4c/0xc0
tcp_rtx_synack+0xf9/0x100
reqsk_timer_handler+0xda/0x3d0
run_timer_softirq+0x292/0x8a0
irq_exit_rcu+0xf5/0x320
sysvec_apic_timer_interrupt+0x6d/0x80
asm_sysvec_apic_timer_interrupt+0x16/0x20
intel_idle_irq+0x5a/0xa0
cpuidle_enter_state+0x94/0x273
cpu_startup_entry+0x15e/0x260
start_secondary+0x8a/0x90
secondary_startup_64_no_verify+0xfa/0xfb

kfence-#1: 0x00000000a72cc7b6-0x00000000d97616d9, size=2376, cache=TCPv6

allocated by task 0 on cpu 9 at 260507.901592s:
sk_prot_alloc+0x35/0x140
sk_clone_lock+0x1f/0x3f0
inet_csk_clone_lock+0x15/0x160
tcp_create_openreq_child+0x1f/0x410
tcp_v6_syn_recv_sock+0x1da/0x700
tcp_check_req+0x1fb/0x510
tcp_v6_rcv+0x98b/0x1420
ipv6_list_rcv+0x2258/0x26e0
napi_complete_done+0x5b1/0x2990
mlx5e_napi_poll+0x2ae/0x8d0
net_rx_action+0x13e/0x590
irq_exit_rcu+0xf5/0x320
common_interrupt+0x80/0x90
asm_common_interrupt+0x22/0x40
cpuidle_enter_state+0xfb/0x273
cpu_startup_entry+0x15e/0x260
start_secondary+0x8a/0x90
secondary_startup_64_no_verify+0xfa/0xfb

freed by task 0 on cpu 9 at 260507.927527s:
rcu_core_si+0x4ff/0xf10
irq_exit_rcu+0xf5/0x320
sysvec_apic_timer_interrupt+0x6d/0x80
asm_sysvec_apic_timer_interrupt+0x16/0x20
cpuidle_enter_state+0xfb/0x273
cpu_startup_entry+0x15e/0x260
start_secondary+0x8a/0x90
secondary_startup_64_no_verify+0xfa/0xfb

Fixes: 83fccfc3940c ("inet: fix potential deadlock in reqsk_queue_unlink()")
Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
Closes: https://lore.kernel.org/netdev/eb6684d0-ffd9-4bdc-9196-33f690c25824@linux.dev/
Link: https://lore.kernel.org/netdev/b55e2ca0-42f2-4b7c-b445-6ffd87ca74a0@linux.dev/ [1]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
v3:
  * Stop timer before reqsk_queue_removed().

v2: https://lore.kernel.org/netdev/20241009174226.7738-1-kuniyu@amazon.com/
  * Added issue scenario in changelog
  * Correct reqsk for __inet_csk_reqsk_queue_drop()

v1: https://lore.kernel.org/netdev/20241007141557.14424-1-kuniyu@amazon.com/
---
 net/ipv4/inet_connection_sock.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 2c5632d4fddb..2b698f8419fe 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1045,21 +1045,31 @@ static bool reqsk_queue_unlink(struct request_sock *req)
 		found = __sk_nulls_del_node_init_rcu(sk);
 		spin_unlock(lock);
 	}
-	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
-		reqsk_put(req);
+
 	return found;
 }
 
-bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
+static bool __inet_csk_reqsk_queue_drop(struct sock *sk,
+					struct request_sock *req,
+					bool from_timer)
 {
 	bool unlinked = reqsk_queue_unlink(req);
 
+	if (!from_timer && timer_delete_sync(&req->rsk_timer))
+		reqsk_put(req);
+
 	if (unlinked) {
 		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
 		reqsk_put(req);
 	}
+
 	return unlinked;
 }
+
+bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
+{
+	return __inet_csk_reqsk_queue_drop(sk, req, false);
+}
 EXPORT_SYMBOL(inet_csk_reqsk_queue_drop);
 
 void inet_csk_reqsk_queue_drop_and_put(struct sock *sk, struct request_sock *req)
@@ -1152,7 +1162,7 @@ static void reqsk_timer_handler(struct timer_list *t)
 
 		if (!inet_ehash_insert(req_to_sk(nreq), req_to_sk(oreq), NULL)) {
 			/* delete timer */
-			inet_csk_reqsk_queue_drop(sk_listener, nreq);
+			__inet_csk_reqsk_queue_drop(sk_listener, nreq, true);
 			goto no_ownership;
 		}
 
@@ -1178,7 +1188,8 @@ static void reqsk_timer_handler(struct timer_list *t)
 	}
 
 drop:
-	inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
+	__inet_csk_reqsk_queue_drop(sk_listener, oreq, true);
+	reqsk_put(req);
 }
 
 static bool reqsk_queue_hash_req(struct request_sock *req,
-- 
2.39.5 (Apple Git-154)

Re: [PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[Eric Dumazet]

On Tue, Oct 15, 2024 at 12:33 AM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
>
> Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().
>
>   """
>   We are seeing a use-after-free from a bpf prog attached to
>   trace_tcp_retransmit_synack. The program passes the req->sk to the
>   bpf_sk_storage_get_tracing kernel helper which does check for null
>   before using it.
>   """
>
> The commit 83fccfc3940c ("inet: fix potential deadlock in
> reqsk_queue_unlink()") added timer_pending() in reqsk_queue_unlink() not
> to call del_timer_sync() from reqsk_timer_handler(), but it introduced a
> small race window.
>
> Before the timer is called, expire_timers() calls detach_timer(timer, true)
> to clear timer->entry.pprev and marks it as not pending.
>
> If reqsk_queue_unlink() checks timer_pending() just after expire_timers()
> calls detach_timer(), TCP will miss del_timer_sync(); the reqsk timer will
> continue running and send multiple SYN+ACKs until it expires.
>
> The reported UAF could happen if req->sk is close()d earlier than the timer
> expiration, which is 63s by default.
>
>
> Fixes: 83fccfc3940c ("inet: fix potential deadlock in reqsk_queue_unlink()")
> Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
> Closes: https://lore.kernel.org/netdev/eb6684d0-ffd9-4bdc-9196-33f690c25824@linux.dev/
> Link: https://lore.kernel.org/netdev/b55e2ca0-42f2-4b7c-b445-6ffd87ca74a0@linux.dev/ [1]
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[Martin KaFai Lau]

On 10/14/24 3:33 PM, Kuniyuki Iwashima wrote:
> Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().
> 
>    """
>    We are seeing a use-after-free from a bpf prog attached to
>    trace_tcp_retransmit_synack. The program passes the req->sk to the
>    bpf_sk_storage_get_tracing kernel helper which does check for null
>    before using it.
>    """
> 
> The commit 83fccfc3940c ("inet: fix potential deadlock in
> reqsk_queue_unlink()") added timer_pending() in reqsk_queue_unlink() not
> to call del_timer_sync() from reqsk_timer_handler(), but it introduced a
> small race window.
> 
> Before the timer is called, expire_timers() calls detach_timer(timer, true)
> to clear timer->entry.pprev and marks it as not pending.
> 
> If reqsk_queue_unlink() checks timer_pending() just after expire_timers()
> calls detach_timer(), TCP will miss del_timer_sync(); the reqsk timer will
> continue running and send multiple SYN+ACKs until it expires.
> 
> The reported UAF could happen if req->sk is close()d earlier than the timer
> expiration, which is 63s by default.
> 
> The scenario would be
> 
>    1. inet_csk_complete_hashdance() calls inet_csk_reqsk_queue_drop(),
>       but del_timer_sync() is missed
> 
>    2. reqsk timer is executed and scheduled again
> 
>    3. req->sk is accept()ed and reqsk_put() decrements rsk_refcnt, but
>       reqsk timer still has another one, and inet_csk_accept() does not
>       clear req->sk for non-TFO sockets
> 
>    4. sk is close()d
> 
>    5. reqsk timer is executed again, and BPF touches req->sk
> 
> Let's not use timer_pending() by passing the caller context to
> __inet_csk_reqsk_queue_drop().
> 
> 
> Fixes: 83fccfc3940c ("inet: fix potential deadlock in reqsk_queue_unlink()")
> Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
> Closes: https://lore.kernel.org/netdev/eb6684d0-ffd9-4bdc-9196-33f690c25824@linux.dev/
> Link: https://lore.kernel.org/netdev/b55e2ca0-42f2-4b7c-b445-6ffd87ca74a0@linux.dev/ [1]
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Reviewed-by: Martin KaFai Lau <martin.lau@kernel.org>

Re: [PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 14 Oct 2024 15:33:12 -0700 you wrote:
> Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().
> 
>   """
>   We are seeing a use-after-free from a bpf prog attached to
>   trace_tcp_retransmit_synack. The program passes the req->sk to the
>   bpf_sk_storage_get_tracing kernel helper which does check for null
>   before using it.
>   """
> 
> [...]

Here is the summary with links:
  - [v3,net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().
    https://git.kernel.org/netdev/net/c/e8c526f2bdf1

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[liujian (CE)]

在 2024/10/15 6:33, Kuniyuki Iwashima 写道:
> Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().
> 
>    """
>    We are seeing a use-after-free from a bpf prog attached to
>    trace_tcp_retransmit_synack. The program passes the req->sk to the
>    bpf_sk_storage_get_tracing kernel helper which does check for null
>    before using it.
>    """
> 
> The commit 83fccfc3940c ("inet: fix potential deadlock in
> reqsk_queue_unlink()") added timer_pending() in reqsk_queue_unlink() not
> to call del_timer_sync() from reqsk_timer_handler(), but it introduced a
> small race window.
> 
> Before the timer is called, expire_timers() calls detach_timer(timer, true)
> to clear timer->entry.pprev and marks it as not pending.
> 
> If reqsk_queue_unlink() checks timer_pending() just after expire_timers()
> calls detach_timer(), TCP will miss del_timer_sync(); the reqsk timer will
> continue running and send multiple SYN+ACKs until it expires.
> 
> The reported UAF could happen if req->sk is close()d earlier than the timer
> expiration, which is 63s by default.
> 
> The scenario would be
> 
>    1. inet_csk_complete_hashdance() calls inet_csk_reqsk_queue_drop(),
>       but del_timer_sync() is missed
> 
>    2. reqsk timer is executed and scheduled again
> 
>    3. req->sk is accept()ed and reqsk_put() decrements rsk_refcnt, but
>       reqsk timer still has another one, and inet_csk_accept() does not
>       clear req->sk for non-TFO sockets
> 
>    4. sk is close()d
> 
>    5. reqsk timer is executed again, and BPF touches req->sk
> 
> Let's not use timer_pending() by passing the caller context to
> __inet_csk_reqsk_queue_drop().
> 
> Note that reqsk timer is pinned, so the issue does not happen in most
> use cases. [1]
> 
> [0]
> BUG: KFENCE: use-after-free read in bpf_sk_storage_get_tracing+0x2e/0x1b0
> 
> Use-after-free read at 0x00000000a891fb3a (in kfence-#1):
> bpf_sk_storage_get_tracing+0x2e/0x1b0
> bpf_prog_5ea3e95db6da0438_tcp_retransmit_synack+0x1d20/0x1dda
> bpf_trace_run2+0x4c/0xc0
> tcp_rtx_synack+0xf9/0x100
> reqsk_timer_handler+0xda/0x3d0
> run_timer_softirq+0x292/0x8a0
> irq_exit_rcu+0xf5/0x320
> sysvec_apic_timer_interrupt+0x6d/0x80
> asm_sysvec_apic_timer_interrupt+0x16/0x20
> intel_idle_irq+0x5a/0xa0
> cpuidle_enter_state+0x94/0x273
> cpu_startup_entry+0x15e/0x260
> start_secondary+0x8a/0x90
> secondary_startup_64_no_verify+0xfa/0xfb
> 
> kfence-#1: 0x00000000a72cc7b6-0x00000000d97616d9, size=2376, cache=TCPv6
> 
> allocated by task 0 on cpu 9 at 260507.901592s:
> sk_prot_alloc+0x35/0x140
> sk_clone_lock+0x1f/0x3f0
> inet_csk_clone_lock+0x15/0x160
> tcp_create_openreq_child+0x1f/0x410
> tcp_v6_syn_recv_sock+0x1da/0x700
> tcp_check_req+0x1fb/0x510
> tcp_v6_rcv+0x98b/0x1420
> ipv6_list_rcv+0x2258/0x26e0
> napi_complete_done+0x5b1/0x2990
> mlx5e_napi_poll+0x2ae/0x8d0
> net_rx_action+0x13e/0x590
> irq_exit_rcu+0xf5/0x320
> common_interrupt+0x80/0x90
> asm_common_interrupt+0x22/0x40
> cpuidle_enter_state+0xfb/0x273
> cpu_startup_entry+0x15e/0x260
> start_secondary+0x8a/0x90
> secondary_startup_64_no_verify+0xfa/0xfb
> 
> freed by task 0 on cpu 9 at 260507.927527s:
> rcu_core_si+0x4ff/0xf10
> irq_exit_rcu+0xf5/0x320
> sysvec_apic_timer_interrupt+0x6d/0x80
> asm_sysvec_apic_timer_interrupt+0x16/0x20
> cpuidle_enter_state+0xfb/0x273
> cpu_startup_entry+0x15e/0x260
> start_secondary+0x8a/0x90
> secondary_startup_64_no_verify+0xfa/0xfb
> 
> Fixes: 83fccfc3940c ("inet: fix potential deadlock in reqsk_queue_unlink()")
> Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
> Closes: https://lore.kernel.org/netdev/eb6684d0-ffd9-4bdc-9196-33f690c25824@linux.dev/
> Link: https://lore.kernel.org/netdev/b55e2ca0-42f2-4b7c-b445-6ffd87ca74a0@linux.dev/ [1]
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> ---
> v3:
>    * Stop timer before reqsk_queue_removed().
> 
> v2: https://lore.kernel.org/netdev/20241009174226.7738-1-kuniyu@amazon.com/
>    * Added issue scenario in changelog
>    * Correct reqsk for __inet_csk_reqsk_queue_drop()
> 
> v1: https://lore.kernel.org/netdev/20241007141557.14424-1-kuniyu@amazon.com/
> ---
>   net/ipv4/inet_connection_sock.c | 21 ++++++++++++++++-----
>   1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 2c5632d4fddb..2b698f8419fe 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -1045,21 +1045,31 @@ static bool reqsk_queue_unlink(struct request_sock *req)
>   		found = __sk_nulls_del_node_init_rcu(sk);
>   		spin_unlock(lock);
>   	}
> -	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
> -		reqsk_put(req);
> +
>   	return found;
>   }
>   
> -bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
> +static bool __inet_csk_reqsk_queue_drop(struct sock *sk,
> +					struct request_sock *req,
> +					bool from_timer)
>   {
>   	bool unlinked = reqsk_queue_unlink(req);
>   
> +	if (!from_timer && timer_delete_sync(&req->rsk_timer))
> +		reqsk_put(req);
> +
>   	if (unlinked) {
>   		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
>   		reqsk_put(req);
>   	}
> +
>   	return unlinked;
>   }
> +
> +bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
> +{
> +	return __inet_csk_reqsk_queue_drop(sk, req, false);
> +}
>   EXPORT_SYMBOL(inet_csk_reqsk_queue_drop);
>   
>   void inet_csk_reqsk_queue_drop_and_put(struct sock *sk, struct request_sock *req)
> @@ -1152,7 +1162,7 @@ static void reqsk_timer_handler(struct timer_list *t)
>   
>   		if (!inet_ehash_insert(req_to_sk(nreq), req_to_sk(oreq), NULL)) {
>   			/* delete timer */
> -			inet_csk_reqsk_queue_drop(sk_listener, nreq);
> +			__inet_csk_reqsk_queue_drop(sk_listener, nreq, true);
>   			goto no_ownership;
>   		}
>   
> @@ -1178,7 +1188,8 @@ static void reqsk_timer_handler(struct timer_list *t)
>   	}
>   
>   drop:
> -	inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
> +	__inet_csk_reqsk_queue_drop(sk_listener, oreq, true);
> +	reqsk_put(req);
Excuse me, why is "req" used here instead of "oreq"?
Typo?
>   }
>   
>   static bool reqsk_queue_hash_req(struct request_sock *req,

Re: [PATCH v3 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

[Kuniyuki Iwashima]

From: "liujian (CE)" <liujian56@huawei.com>
Date: Sat, 23 Nov 2024 21:24:39 +0800
> > @@ -1178,7 +1188,8 @@ static void reqsk_timer_handler(struct timer_list *t)
> >   	}
> >   
> >   drop:
> > -	inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
> > +	__inet_csk_reqsk_queue_drop(sk_listener, oreq, true);
> > +	reqsk_put(req);
> Excuse me, why is "req" used here instead of "oreq"?
> Typo?

Oops, will post a followup.

Thanks!