ALSA: seq: Serialize UMP output teardown with event_input

ALSA: seq: Serialize UMP output teardown with event_input

seq_ump_process_event() borrows client->out_rfile.output without
synchronizing with the first-open and last-close transition in
seq_ump_client_open() and seq_ump_client_close().

The last output unuse can therefore drop opened[STR_OUT] to zero and
release the rawmidi file while an in-flight event_input callback is still
inside snd_rawmidi_kernel_write(). That leaves the rawmidi substream
runtime exposed to teardown before the write path has taken its own
buffer reference.

Add a per-client rwlock for the event_input-visible output file. Publish
a newly opened output file under the write side, and hold the read side
from the output lookup through snd_rawmidi_kernel_write(). The last
output close copies and clears the visible output file under the write
side, then drops the lock and releases the saved rawmidi file. Use
IRQ-safe rwlock guards because event_input can also be reached from
atomic sequencer delivery.

The buggy scenario involves two paths, with each column showing the
order within that path:

path A label: event_input path         path B label: last unuse path
1. seq_ump_process_event() reads       1. seq_ump_client_close()
   client->out_rfile.output.              drops opened[STR_OUT] to zero.
2. snd_rawmidi_kernel_write1()         2. snd_rawmidi_kernel_release()
   has not yet pinned runtime.            closes the output file.
3. The writer continues using          3. close_substream() frees
   the borrowed substream.                substream->runtime.

This keeps the output substream and runtime alive for the full
event_input write while keeping rawmidi release outside the rwlock.

KASAN reproduced this as a slab-use-after-free in
snd_rawmidi_kernel_write1(), with allocation through
seq_ump_use()/snd_seq_port_connect() and free through
seq_ump_unuse()/snd_seq_port_disconnect().

Suggested-by: Takashi Iwai <tiwai@suse.de>
Validation reproduced this kernel report:
KASAN slab-use-after-free in snd_rawmidi_kernel_write1+0x9d/0x400
RIP: 0033:0x7f5528af837f
Read of size 8
Call trace:
  dump_stack_lvl+0x73/0xb0 (?:?)
  print_report+0xd1/0x650 (?:?)
  srso_alias_return_thunk+0x5/0xfbef5 (?:?)
  __virt_addr_valid+0x1a7/0x340 (?:?)
  kasan_complete_mode_report_info+0x64/0x200 (?:?)
  kasan_report+0xf7/0x130 (?:?)
  snd_rawmidi_kernel_write1+0x9d/0x400 (?:?)
  __asan_load8+0x82/0xb0 (?:?)
  update_stack_state+0x1ef/0x2d0 (?:?)
  snd_rawmidi_kernel_write+0x1a/0x20 (?:?)
  seq_ump_process_event+0xd4/0x120 (sound/core/seq/seq_ump_client.c:82)
  __snd_seq_deliver_single_event+0x8a/0xe0 (?:?)
  snd_seq_deliver_from_ump+0x2b2/0xd60 (?:?)
  lock_acquire+0x14e/0x2e0 (?:?)
  find_held_lock+0x31/0x90 (?:?)
  snd_seq_port_use_ptr+0xa6/0xe0 (?:?)
  __kasan_check_write+0x18/0x20 (?:?)
  do_raw_read_unlock+0x32/0xa0 (?:?)
  _raw_read_unlock+0x26/0x50 (?:?)
  snd_seq_deliver_single_event+0x45c/0x4b0 (?:?)
  snd_seq_deliver_event+0x10d/0x1b0 (?:?)
  snd_seq_client_enqueue_event+0x192/0x240 (?:?)
  snd_seq_write+0x2cd/0x450 (?:?)
  apparmor_file_permission+0x20/0x30 (?:?)
  security_file_permission+0x51/0x60 (?:?)
  vfs_write+0x1ce/0x850 (?:?)
  __fget_files+0x12b/0x220 (?:?)
  lock_release+0xc8/0x2a0 (?:?)
  __rcu_read_unlock+0x74/0x2d0 (?:?)
  __fget_files+0x135/0x220 (?:?)
  ksys_write+0x15a/0x180 (?:?)
  rcu_is_watching+0x24/0x60 (?:?)
  __x64_sys_write+0x46/0x60 (?:?)
  x64_sys_call+0x7d/0x20d0 (?:?)
  do_syscall_64+0xc1/0x360 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)

Fixes: 81fd444aa371 ("ALSA: seq: Bind UMP device")
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Link: https://patch.msgid.link/20260520103249.3048345-1-rollkingzzc@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>