CVE-2024-56664: bpf, sockmap: Fix race between element replace and close()
In the Linux kernel, the following vulnerability has been resolved:

bpf, sockmap: Fix race between element replace and close()

Element replace (with a socket different from the one stored) may race
with socket's close() link popping & unlinking. __sock_map_delete()
unconditionally unrefs the (wrong) element:

// set map[0] = s0
map_update_elem(map, 0, s0)

// drop fd of s0
close(s0)
  sock_map_close()
    lock_sock(sk)               (s0!)
    sock_map_remove_links(sk)
      link = sk_psock_link_pop()
      sock_map_unlink(sk, link)
        sock_map_delete_from_link
                                        // replace map[0] with s1
                                        map_update_elem(map, 0, s1)
                                          sock_map_update_elem
                                (s1!)       lock_sock(sk)
                                            sock_map_update_common
                                              psock = sk_psock(sk)
                                              spin_lock(&stab->lock)
                                              osk = stab->sks[idx]
                                              sock_map_add_link(..., &stab->sks[idx])
                                              sock_map_unref(osk, &stab->sks[idx])
                                                psock = sk_psock(osk)
                                                sk_psock_put(sk, psock)
                                                  if (refcount_dec_and_test(&psock))
                                                    sk_psock_drop(sk, psock)
                                              spin_unlock(&stab->lock)
                                            unlock_sock(sk)
          __sock_map_delete
            spin_lock(&stab->lock)
            sk = *psk                        // s1 replaced s0; sk == s1
            if (!sk_test || sk_test == sk)   // sk_test (s0) != sk (s1); no branch
              sk = xchg(psk, NULL)
            if (sk)
              sock_map_unref(sk, psk)        // unref s1; sks[idx] will dangle
                psock = sk_psock(sk)
                sk_psock_put(sk, psock)
                  if (refcount_dec_and_test())
                    sk_psock_drop(sk, psock)
            spin_unlock(&stab->lock)
    release_sock(sk)

Then close(map) enqueues bpf_map_free_deferred, which finally calls
sock_map_free(). This results in some refcount_t warnings along with
a KASAN splat [1].

Fix __sock_map_delete(), do not allow sock_map_unref() on elements that
may have been replaced.

[1]:
BUG: KASAN: slab-use-after-free in sock_map_free+0x10e/0x330
Write of size 4 at addr ffff88811f5b9100 by task kworker/u64:12/1063

CPU: 14 UID: 0 PID: 1063 Comm: kworker/u64:12 Not tainted 6.12.0+ #125
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
Workqueue: events_unbound bpf_map_free_deferred
Call Trace:
 <TASK>
 dump_stack_lvl+0x68/0x90
 print_report+0x174/0x4f6
 kasan_report+0xb9/0x190
 kasan_check_range+0x10f/0x1e0
 sock_map_free+0x10e/0x330
 bpf_map_free_deferred+0x173/0x320
 process_one_work+0x846/0x1420
 worker_thread+0x5b3/0xf80
 kthread+0x29e/0x360
 ret_from_fork+0x2d/0x70
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Allocated by task 1202:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x10/0x30
 __kasan_slab_alloc+0x85/0x90
 kmem_cache_alloc_noprof+0x131/0x450
 sk_prot_alloc+0x5b/0x220
 sk_alloc+0x2c/0x870
 unix_create1+0x88/0x8a0
 unix_create+0xc5/0x180
 __sock_create+0x241/0x650
 __sys_socketpair+0x1ce/0x420
 __x64_sys_socketpair+0x92/0x100
 do_syscall_64+0x93/0x180
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Freed by task 46:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x10/0x30
 kasan_save_free_info+0x37/0x60
 __kasan_slab_free+0x4b/0x70
 kmem_cache_free+0x1a1/0x590
 __sk_destruct+0x388/0x5a0
 sk_psock_destroy+0x73e/0xa50
 process_one_work+0x846/0x1420
 worker_thread+0x5b3/0xf80
 kthread+0x29e/0x360
 ret_from_fork+0x2d/0x70
 ret_from_fork_asm+0x1a/0x30

The bu
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CVSS v3.1: 7