Re: [PATCH net v3 1/2] tcp: fix use-after-free in do_tcp_getsockopt(TCP_CONGESTION)

[Breno Leitao <leitao@debian.org>]

On Thu, Aug 27, 2026 at 07:55:10PM -0400, Cen Zhang (Microsoft) wrote:
> From: "Cen Zhang (Microsoft Security FORGE Labs)" <blbllhy@gmail.com>
> 
> do_tcp_getsockopt() reads icsk->icsk_ca_ops->name without holding
> rcu_read_lock(). Since commit 0baf26b0fcd7 ("bpf: tcp: Support
> tcp_congestion_ops in bpf"), icsk_ca_ops can point to dynamically
> allocated BPF struct_ops memory that may be freed concurrently via
> setsockopt(TCP_CONGESTION), leading to a use-after-free.
> 
>   BUG: KASAN: slab-use-after-free in _copy_to_user+0x37/0x60
>   Read of size 16 at addr ffff888013505260 by task exploit/149
>    _copy_to_user+0x37/0x60
>    do_tcp_getsockopt+0x158a/0x2460 (net/ipv4/tcp.c:4585)
>    tcp_getsockopt+0x91/0xf0
>    __sys_getsockopt+0xf7/0x170
> 
> Fix this by holding rcu_read_lock() around the ca_ops->name access,
> using READ_ONCE() to load icsk_ca_ops, and copying the name to a
> stack buffer before releasing the lock. Also annotate the relevant
> icsk_ca_ops stores with WRITE_ONCE() to fix the accompanying KCSAN
> data-race issue.
> 
> Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
> Link: https://lore.kernel.org/all/20260821182449.79785-2-blbllhy@gmail.com/

Reviewed-by: Breno Leitao <leitao@debian.org>

> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4576,16 +4576,24 @@ int do_tcp_getsockopt(struct sock *sk, int level,
>  		val = !inet_csk_in_pingpong_mode(sk);
>  		break;
>  
> -	case TCP_CONGESTION:
> +	case TCP_CONGESTION: {
> +		char ca_name[TCP_CA_NAME_MAX] = {};

nit: Do you need to zero this string? It will be safely overwritten
below, no?