[PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

[PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

[Jakub Sitnicki]

Call to bpf_prog_put(), with help of call_rcu(), queues an RCU-callback to
free the program once a grace period has elapsed. The callback can run
together with new RCU readers that started after the last grace period.
New RCU readers can potentially see the "old" to-be-freed or already-freed
pointer to the program object before the RCU update-side NULLs it.

Reorder the operations so that the RCU update-side resets the protected
pointer before the end of the grace period after which the program will be
freed.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/core/flow_dissector.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 3e6fedb57bc1..2470b4b404e6 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -142,8 +142,8 @@ int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
 		mutex_unlock(&flow_dissector_mutex);
 		return -ENOENT;
 	}
-	bpf_prog_put(attached);
 	RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
+	bpf_prog_put(attached);
 	mutex_unlock(&flow_dissector_mutex);
 	return 0;
 }
-- 
2.20.1

Re: [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

[Petar Penkov]

This makes sense, thanks!

Acked-by: Petar Penkov <ppenkov@google.com>

On Wed, Aug 21, 2019 at 5:19 AM Jakub Sitnicki <jakub@cloudflare.com> wrote:
>
> Call to bpf_prog_put(), with help of call_rcu(), queues an RCU-callback to
> free the program once a grace period has elapsed. The callback can run
> together with new RCU readers that started after the last grace period.
> New RCU readers can potentially see the "old" to-be-freed or already-freed
> pointer to the program object before the RCU update-side NULLs it.
>
> Reorder the operations so that the RCU update-side resets the protected
> pointer before the end of the grace period after which the program will be
> freed.
>
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Reported-by: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
>  net/core/flow_dissector.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 3e6fedb57bc1..2470b4b404e6 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -142,8 +142,8 @@ int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
>                 mutex_unlock(&flow_dissector_mutex);
>                 return -ENOENT;
>         }
> -       bpf_prog_put(attached);
>         RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
> +       bpf_prog_put(attached);
>         mutex_unlock(&flow_dissector_mutex);
>         return 0;
>  }
> --
> 2.20.1
>

Re: [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

[Daniel Borkmann]

On 8/21/19 2:17 PM, Jakub Sitnicki wrote:
> Call to bpf_prog_put(), with help of call_rcu(), queues an RCU-callback to
> free the program once a grace period has elapsed. The callback can run
> together with new RCU readers that started after the last grace period.
> New RCU readers can potentially see the "old" to-be-freed or already-freed
> pointer to the program object before the RCU update-side NULLs it.
> 
> Reorder the operations so that the RCU update-side resets the protected
> pointer before the end of the grace period after which the program will be
> freed.
> 
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Reported-by: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>

Applied, thanks!