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

[Cen Zhang (Microsoft)]

From: "Cen Zhang (Microsoft Security FORGE Labs)" <blbllhy@xxxxxxxxx>

do_tcp_getsockopt() reads icsk->icsk_ca_ops and dereferences the
get_info function pointer without rcu_read_lock(). With BPF struct_ops
congestion control, ca_ops can point to dynamically allocated memory
that is freed concurrently, resulting in a use-after-free when the
kernel dereferences or calls through the stale pointer.

  BUG: KASAN: slab-use-after-free in do_tcp_getsockopt+0x2037/0x23e0
  Read of size 8 at addr ffff888013701258 by task exploit/149
   do_tcp_getsockopt+0x2037/0x23e0 (net/ipv4/tcp.c:4564)
   tcp_getsockopt+0x91/0xf0
   __sys_getsockopt+0xf7/0x170

Fix this by wrapping the ca_ops load and get_info call within
rcu_read_lock()/rcu_read_unlock(), and using READ_ONCE() to load
the icsk_ca_ops pointer.

Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: AutonomousCodeSecurity@xxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: GitHub-Copilot:claude-opus-4.6
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <blbllhy@xxxxxxxxx>
---
Link: https://lore.kernel.org/all/20260821182449.79785-3-blbllhy@xxxxxxxxx/ (v1)
Link: https://lore.kernel.org/all/20260826171344.4133-3-blbllhy@xxxxxxxxx/ (v2)
Changes in v3:
- Annotate dctcp_get_info()'s icsk_ca_ops comparison with READ_ONCE().
Changes in v2:
- Add READ_ONCE() for the icsk_ca_ops load.
---
 net/ipv4/tcp.c       | 4 +++-
 net/ipv4/tcp_dctcp.c | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 55ca74f60..71d4c5e6b 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4561,9 +4561,11 @@ int do_tcp_getsockopt(struct sock *sk, int level,
 		if (copy_from_sockptr(&len, optlen, sizeof(int)))
 			return -EFAULT;
 
-		ca_ops = icsk->icsk_ca_ops;
+		rcu_read_lock();
+		ca_ops = READ_ONCE(icsk->icsk_ca_ops);
 		if (ca_ops && ca_ops->get_info)
 			sz = ca_ops->get_info(sk, ~0U, &attr, &info);
+		rcu_read_unlock();
 
 		len = min_t(unsigned int, len, sz);
 		if (copy_to_sockptr(optlen, &len, sizeof(int)))
diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 99f68c299..5b457f68a 100644
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -228,7 +228,7 @@ static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 	if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
 	    ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 		memset(&info->dctcp, 0, sizeof(info->dctcp));
-		if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
+		if (READ_ONCE(inet_csk(sk)->icsk_ca_ops) != &dctcp_reno) {
 			info->dctcp.dctcp_enabled = 1;
 			info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
 			info->dctcp.dctcp_alpha = ca->dctcp_alpha;
-- 
2.43.0