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

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().

Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
Reported-by: [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 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 7360ff718f1d..43ccd2d37f9c 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4562,9 +4562,11 @@ int do_tcp_getsockopt(struct sock *sk, int level,
 		if (copy_from_sockptr(&len, optlen, sizeof(int)))
 			return -EFAULT;
 
+		rcu_read_lock();
 		ca_ops = 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)))
-- 
2.55.0