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>

diff --git a/sound/core/seq/seq_ump_client.c b/sound/core/seq/seq_ump_client.c
index 9079ccfdc8666dddb8e8b1aa3359bbfae4578b26..ccd93599b493b73bf6141225c509d92f204d26d2 100644 (file)
--- a/sound/core/seq/seq_ump_client.c
+++ b/sound/core/seq/seq_ump_client.c
@@ -37,6 +37,7 @@ struct seq_ump_client {
        struct snd_ump_endpoint *ump;   /* assigned endpoint */
        int seq_client;                 /* sequencer client id */
        int opened[2];                  /* current opens for each direction */
+       rwlock_t output_lock;           /* protects out_rfile output access */
        struct snd_rawmidi_file out_rfile; /* rawmidi for output */
        struct seq_ump_input_buffer input; /* input parser context */
        void *ump_info[SNDRV_UMP_MAX_BLOCKS + 1]; /* shadow of seq client ump_info */
@@ -88,6 +89,7 @@ static int seq_ump_process_event(struct snd_seq_event *ev, int direct,
        unsigned char type;
        int len;
 
+       guard(read_lock_irqsave)(&client->output_lock);
        substream = client->out_rfile.output;
        if (!substream)
                return -ENODEV;
@@ -106,6 +108,7 @@ static int seq_ump_process_event(struct snd_seq_event *ev, int direct,
 static int seq_ump_client_open(struct seq_ump_client *client, int dir)
 {
        struct snd_ump_endpoint *ump = client->ump;
+       struct snd_rawmidi_file rfile = {};
        int err;
 
        guard(mutex)(&ump->open_mutex);
@@ -113,9 +116,11 @@ static int seq_ump_client_open(struct seq_ump_client *client, int dir)
                err = snd_rawmidi_kernel_open(&ump->core, 0,
                                              SNDRV_RAWMIDI_LFLG_OUTPUT |
                                              SNDRV_RAWMIDI_LFLG_APPEND,
-                                             &client->out_rfile);
+                                             &rfile);
                if (err < 0)
                        return err;
+               scoped_guard(write_lock_irqsave, &client->output_lock)
+                       client->out_rfile = rfile;
        }
        client->opened[dir]++;
        return 0;
@@ -125,11 +130,19 @@ static int seq_ump_client_open(struct seq_ump_client *client, int dir)
 static int seq_ump_client_close(struct seq_ump_client *client, int dir)
 {
        struct snd_ump_endpoint *ump = client->ump;
+       struct snd_rawmidi_file rfile = {};
 
        guard(mutex)(&ump->open_mutex);
-       if (!--client->opened[dir])
-               if (dir == STR_OUT)
-                       snd_rawmidi_kernel_release(&client->out_rfile);
+       if (!--client->opened[dir]) {
+               if (dir == STR_OUT) {
+                       scoped_guard(write_lock_irqsave, &client->output_lock) {
+                               rfile = client->out_rfile;
+                               client->out_rfile = (struct snd_rawmidi_file){};
+                       }
+                       if (rfile.rmidi)
+                               snd_rawmidi_kernel_release(&rfile);
+               }
+       }
        return 0;
 }
 
@@ -467,6 +480,7 @@ static int snd_seq_ump_probe(struct snd_seq_device *dev)
 
        INIT_WORK(&client->group_notify_work, handle_group_notify);
        client->ump = ump;
+       rwlock_init(&client->output_lock);
 
        client->seq_client =
                snd_seq_create_kernel_client(card, ump->core.device,