[PATCH net-next] tcp: fix null ptr deref in getsockopt(..., TCP_ULP, ...)

[Dave Watson]

If icsk_ulp_ops is unset, it dereferences a null ptr.
Add a null ptr check.

BUG: KASAN: null-ptr-deref in copy_to_user include/linux/uaccess.h:168 [inline]
BUG: KASAN: null-ptr-deref in do_tcp_getsockopt.isra.33+0x24f/0x1e30 
net/ipv4/tcp.c:3057
Read of size 4 at addr 0000000000000020 by task syz-executor1/15452

Signed-off-by: Dave Watson <davejwat...@fb.com>
Reported-by: "Levin, Alexander (Sasha Levin)" <alexander.le...@verizon.com>
---
 net/ipv4/tcp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 058f509..4c88d20 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3062,6 +3062,11 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
                if (get_user(len, optlen))
                        return -EFAULT;
                len = min_t(unsigned int, len, TCP_ULP_NAME_MAX);
+               if (!icsk->icsk_ulp_ops) {
+                       if (put_user(0, optlen))
+                               return -EFAULT;
+                       return 0;
+               }
                if (put_user(len, optlen))
                        return -EFAULT;
                if (copy_to_user(optval, icsk->icsk_ulp_ops->name, len))
-- 
2.9.3