bpf, sockmap: Fix race between element replace and close()
CVE: CVE-2024-56664
Component: kernel
Severity: Moderate
CVSS v3: 6.7/AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
Release date: 2024-12-27T00:00:00+00:00

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
---truncated---

Statement: The bug happens for the bpf() system call and as result of complex race condition. For the Red Hat Enterprise Linux the BPF accessible only for admin user. The security impact is limited.

Remediation:
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.