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

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 and
copying the name to a stack buffer before releasing the lock.

Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
Reported-by: [email protected]
Reported-by: Xiang Mei (Microsoft) <[email protected]>
Reported-by: Cen Zhang (Microsoft) <[email protected]>
Cc: [email protected]
Signed-off-by: Cen Zhang (Microsoft) <[email protected]>
---
The unsynchronized icsk_ca_ops load also constitutes a data race.
READ_ONCE()/WRITE_ONCE() annotations are intentionally left to a
separate change; related TCP annotation work is available at:
https://lore.kernel.org/all/[email protected]/

 net/ipv4/tcp.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b4237d0e994d..7360ff718f1d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4577,16 +4577,23 @@ 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] = {};
+
 		if (copy_from_sockptr(&len, optlen, sizeof(int)))
 			return -EFAULT;
 		len = min_t(unsigned int, len, TCP_CA_NAME_MAX);
 		if (copy_to_sockptr(optlen, &len, sizeof(int)))
 			return -EFAULT;
-		if (copy_to_sockptr(optval, icsk->icsk_ca_ops->name, len))
+
+		rcu_read_lock();
+		memcpy(ca_name, icsk->icsk_ca_ops->name, sizeof(ca_name));
+		rcu_read_unlock();
+
+		if (copy_to_sockptr(optval, ca_name, len))
 			return -EFAULT;
 		return 0;
-
+	}
 	case TCP_ULP:
 		if (copy_from_sockptr(&len, optlen, sizeof(int)))
 			return -EFAULT;
-- 
2.55.0