bpf, sockmap: Reject unhashed UDP sockets on sockmap update

Overview

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

bpf, sockmap: Reject unhashed UDP sockets on sockmap update

UDP sockets get SOCK_RCU_FREE set when (auto-)bound. This means sk_is_refcounted(unbound) = true, while sk_is_refcounted(bound) = false.

Because sockmap accepts unbound UDP sockets, a BPF program can increment a socket's refcount via lookup. If the socket is subsequently bound, the transition from unbound to bound causes bpf_sk_release() to skip the decrement of the refcount, causing a memory leak.

unreferenced object 0xffff88810bc2eb40 (size 1984): comm "test_progs", pid 2451, jiffies 4295320596 hex dump (first 32 bytes): 7f 00 00 01 7f 00 00 01 d2 04 1b b7 04 d2 00 00 ................ 02 00 01 40 00 00 00 00 00 00 00 00 00 00 00 00 ...@............ backtrace (crc bdee079d): kmem_cache_alloc_noprof+0x557/0x660 sk_prot_alloc+0x69/0x240 sk_alloc+0x30/0x460 inet_create+0x2ce/0xf80 __sock_create+0x25b/0x5c0 __sys_socket+0x119/0x1d0 __x64_sys_socket+0x72/0xd0 do_syscall_64+0xa1/0x5f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Instead of special-casing for refcounted sockets, reject unhashed UDP sockets during sockmap updates, as there is no benefit to supporting those. This effectively reverts the commit under Fixes, with two exceptions:

1. sock_map_sk_state_allowed() maintains a fall-through return true.
2. In the spirit of commit b8b8315e39ff ("bpf, sockmap: Remove unhash handler for BPF sockmap usage"), the proto::unhash BPF handler is not reintroduced.

Historical note: this issue is related to commit 67312adc96b5 ("bpf: reject unhashed sockets in bpf_sk_assign").

Affected

• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 2271276ac5279d2d204be7739a1a28d4ef6cf608
• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 7ffe529e7127411806c8692fb1490f552c629dc2
• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 17b7ef6b86112a4e61cee1e9009a4b318e3225c5
• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 250474c69bc3fc48a5fc21d7c349f279caad947a
• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 8692655da369961128658cf8539334b6a960ecb0
• Linux >= 0c48eefae712c2fd91480346a07a1a9cd0f9470b < 66efd3368ae10d05e08fbe6425b50fdec7186ac7
• Linux 5.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.