CVE-2023-52523

In the Linux kernel, the following vulnerability has been resolved:

bpf, sockmap: Reject sk_msg egress redirects to non-TCP sockets

With a SOCKMAP/SOCKHASH map and an sk_msg program user can steer messages

sent from one TCP socket (s1) to actually egress from another TCP

socket (s2):

tcp_bpf_sendmsg(s1) // = sk_prot->sendmsg

tcp_bpf_send_verdict(s1) // __SK_REDIRECT case

tcp_bpf_sendmsg_redir(s2)

tcp_bpf_push_locked(s2)

tcp_bpf_push(s2)

tcp_rate_check_app_limited(s2) // expects tcp_sock

tcp_sendmsg_locked(s2) // ditto

There is a hard-coded assumption in the call-chain, that the egress

socket (s2) is a TCP socket.

However in commit 122e6c79efe1 ("sock_map: Update sock type checks for

UDP") we have enabled redirects to non-TCP sockets. This was done for the

sake of BPF sk_skb programs. There was no indention to support sk_msg

send-to-egress use case.

As a result, attempts to send-to-egress through a non-TCP socket lead to a

crash due to invalid downcast from sock to tcp_sock:

BUG: kernel NULL pointer dereference, address: 000000000000002f

...

Call Trace:

<TASK>

? show_regs+0x60/0x70

? __die+0x1f/0x70

? page_fault_oops+0x80/0x160

? do_user_addr_fault+0x2d7/0x800

? rcu_is_watching+0x11/0x50

? exc_page_fault+0x70/0x1c0

? asm_exc_page_fault+0x27/0x30

? tcp_tso_segs+0x14/0xa0

tcp_write_xmit+0x67/0xce0

__tcp_push_pending_frames+0x32/0xf0

tcp_push+0x107/0x140

tcp_sendmsg_locked+0x99f/0xbb0

tcp_bpf_push+0x19d/0x3a0

tcp_bpf_sendmsg_redir+0x55/0xd0

tcp_bpf_send_verdict+0x407/0x550

tcp_bpf_sendmsg+0x1a1/0x390

inet_sendmsg+0x6a/0x70

sock_sendmsg+0x9d/0xc0

? sockfd_lookup_light+0x12/0x80

__sys_sendto+0x10e/0x160

? syscall_enter_from_user_mode+0x20/0x60

? __this_cpu_preempt_check+0x13/0x20

? lockdep_hardirqs_on+0x82/0x110

__x64_sys_sendto+0x1f/0x30

do_syscall_64+0x38/0x90

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Reject selecting a non-TCP sockets as redirect target from a BPF sk_msg

program to prevent the crash. When attempted, user will receive an EACCES

error from send/sendto/sendmsg() syscall.