CVE-2026-68169

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

mptcp: pm: userspace: fix use-after-free in get_local_id

In mptcp_pm_userspace_get_local_id(), the address entry is looked up under
spinlock, but its id is read after dropping the lock. A concurrent deletion
can free the entry between the unlock and the read, leading to UAF.

The race window is narrow. It was reproduced only with a locally
constructed stress test that repeatedly overlaps an MP_JOIN SYN with a
MPTCP_PM_CMD_SUBFLOW_DESTROY request.

However, the KASAN report below confirms that the race is reachable:

  [  666.319376] BUG: KASAN: slab-use-after-free in mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
  [  666.319386] Read of size 1 at addr ffff888124845610 by task swapper/0/0
  ...
  [  666.319401] Call Trace:
  [  666.319405]  <IRQ>
  [  666.319408]  dump_stack_lvl+0x53/0x70
  [  666.319412]  print_address_description.constprop.0+0x2c/0x3b0
  [  666.319418]  print_report+0xbe/0x2b0
  [  666.319421]  ? mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
  [  666.319423]  kasan_report+0xce/0x100
  [  666.319426]  ? mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
  [  666.319429]  mptcp_userspace_pm_get_local_id+0x1dc/0x1f0
  [  666.319433]  mptcp_pm_get_local_id+0x371/0x440
  ...
  [  666.319821] Allocated by task 45539:
  [  666.319844]  kasan_save_stack+0x33/0x60
  [  666.319855]  kasan_save_track+0x14/0x30
  [  666.319858]  __kasan_kmalloc+0x8f/0xa0
  [  666.319863]  __kmalloc_noprof+0x1e7/0x520
  [  666.319867]  sock_kmalloc+0xdf/0x130
  [  666.319885]  sock_kmemdup+0x1b/0x40
  [  666.319888]  mptcp_userspace_pm_append_new_local_addr+0x261/0x500
  [  666.319910]  mptcp_pm_nl_announce_doit+0x16a/0x610
  ...
  [  666.319967] Freed by task 45560:
  [  666.319988]  kasan_save_stack+0x33/0x60
  [  666.319991]  kasan_save_track+0x14/0x30
  [  666.319994]  kasan_save_free_info+0x3b/0x60
  [  666.319998]  __kasan_slab_free+0x43/0x70
  [  666.320000]  kfree+0x166/0x440
  [  666.320003]  sock_kfree_s+0x1d/0x50
  [  666.320007]  mptcp_userspace_pm_delete_local_addr.isra.0+0x157/0x200
  [  666.320011]  mptcp_pm_nl_subflow_destroy_doit+0x51d/0xea0

Fix by copying the id into a local variable while still holding the lock,
and use -1 as a "not found" sentinel.