[PATCH] ALSA: seq: Use atomic to prevent data races in total_elements

[Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>]

syzbot reported a data-race in snd_seq_poll / snd_seq_pool_init. [1]

Just use atomic_set/atomic_read for handling this case.

[1]
BUG: KCSAN: data-race in snd_seq_poll / snd_seq_pool_init

write to 0xffff888114535610 of 4 bytes by task 7006 on cpu 1:
 snd_seq_pool_init+0x1c1/0x200 sound/core/seq/seq_memory.c:469
 snd_seq_write+0x17f/0x500 sound/core/seq/seq_clientmgr.c:1022
 vfs_write+0x27d/0x920 fs/read_write.c:677
 ksys_write+0xe8/0x1b0 fs/read_write.c:731
 __do_sys_write fs/read_write.c:742 [inline]
 __se_sys_write fs/read_write.c:739 [inline]
 __x64_sys_write+0x42/0x50 fs/read_write.c:739
 x64_sys_call+0x287e/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888114535610 of 4 bytes by task 7005 on cpu 0:
 snd_seq_total_cells sound/core/seq/seq_memory.h:83 [inline]
 snd_seq_write_pool_allocated sound/core/seq/seq_clientmgr.c:95 [inline]
 snd_seq_poll+0x103/0x170 sound/core/seq/seq_clientmgr.c:1139
 vfs_poll include/linux/poll.h:82 [inline]
 __io_arm_poll_handler+0x1e5/0xd50 io_uring/poll.c:582
 io_arm_poll_handler+0x464/0x5b0 io_uring/poll.c:707
 io_queue_async+0x89/0x320 io_uring/io_uring.c:1925
 io_queue_sqe io_uring/io_uring.c:1954 [inline]
 io_req_task_submit+0xb9/0xc0 io_uring/io_uring.c:1373
 io_handle_tw_list+0x1b9/0x200 io_uring/io_uring.c:1059
 tctx_task_work_run+0x6e/0x1c0 io_uring/io_uring.c:1123
 tctx_task_work+0x40/0x80 io_uring/io_uring.c:1141
 task_work_run+0x13a/0x1a0 kernel/task_work.c:227
 get_signal+0xe78/0x1000 kernel/signal.c:2809
 arch_do_signal_or_restart+0x95/0x4b0 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
 syscall_exit_to_user_mode+0x62/0x120 kernel/entry/common.c:218
 do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000000 -> 0x000001f4

Reported-by: syzbot+2d373c9936c00d7e120c@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=2d373c9936c00d7e120c
Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
---
 sound/core/seq/seq_memory.c | 12 ++++++------
 sound/core/seq/seq_memory.h |  6 +++---
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/sound/core/seq/seq_memory.c b/sound/core/seq/seq_memory.c
index 20155e3e87c6..4a5c771ce8f9 100644
--- a/sound/core/seq/seq_memory.c
+++ b/sound/core/seq/seq_memory.c
@@ -21,7 +21,7 @@
 
 static inline int snd_seq_pool_available(struct snd_seq_pool *pool)
 {
-	return pool->total_elements - atomic_read(&pool->counter);
+	return atomic_read(&pool->total_elements) - atomic_read(&pool->counter);
 }
 
 static inline int snd_seq_output_ok(struct snd_seq_pool *pool)
@@ -353,7 +353,7 @@ int snd_seq_event_dup(struct snd_seq_pool *pool, struct snd_seq_event *event,
 		extlen = event->data.ext.len & ~SNDRV_SEQ_EXT_MASK;
 		ncells = DIV_ROUND_UP(extlen, sizeof(struct snd_seq_event));
 	}
-	if (ncells >= pool->total_elements)
+	if (ncells >= atomic_read(&pool->total_elements))
 		return -ENOMEM;
 
 	err = snd_seq_cell_alloc(pool, &cell, nonblock, file, mutexp);
@@ -466,7 +466,7 @@ int snd_seq_pool_init(struct snd_seq_pool *pool)
 
 	/* init statistics */
 	pool->max_used = 0;
-	pool->total_elements = pool->size;
+	atomic_set(&pool->total_elements, pool->size);
 	return 0;
 }
 
@@ -499,7 +499,7 @@ int snd_seq_pool_done(struct snd_seq_pool *pool)
 		ptr = pool->ptr;
 		pool->ptr = NULL;
 		pool->free = NULL;
-		pool->total_elements = 0;
+		atomic_set(&pool->total_elements, 0);
 	}
 
 	kvfree(ptr);
@@ -523,7 +523,7 @@ struct snd_seq_pool *snd_seq_pool_new(int poolsize)
 	spin_lock_init(&pool->lock);
 	pool->ptr = NULL;
 	pool->free = NULL;
-	pool->total_elements = 0;
+	atomic_set(&pool->total_elements, 0);
 	atomic_set(&pool->counter, 0);
 	pool->closing = 0;
 	init_waitqueue_head(&pool->output_sleep);
@@ -555,7 +555,7 @@ void snd_seq_info_pool(struct snd_info_buffer *buffer,
 {
 	if (pool == NULL)
 		return;
-	snd_iprintf(buffer, "%sPool size          : %d\n", space, pool->total_elements);
+	snd_iprintf(buffer, "%sPool size          : %d\n", space, atomic_read(&pool->total_elements));
 	snd_iprintf(buffer, "%sCells in use       : %d\n", space, atomic_read(&pool->counter));
 	snd_iprintf(buffer, "%sPeak cells in use  : %d\n", space, pool->max_used);
 	snd_iprintf(buffer, "%sAlloc success      : %d\n", space, pool->event_alloc_success);
diff --git a/sound/core/seq/seq_memory.h b/sound/core/seq/seq_memory.h
index 7f7a2c0b187d..812edbe8cee0 100644
--- a/sound/core/seq/seq_memory.h
+++ b/sound/core/seq/seq_memory.h
@@ -44,7 +44,7 @@ struct snd_seq_pool {
 	struct snd_seq_event_cell *ptr;	/* pointer to first event chunk */
 	struct snd_seq_event_cell *free;	/* pointer to the head of the free list */
 
-	int total_elements;	/* pool size actually allocated */
+	atomic_t total_elements;	/* pool size actually allocated */
 	atomic_t counter;	/* cells free */
 
 	int size;		/* pool size to be allocated */
@@ -74,13 +74,13 @@ int snd_seq_event_dup(struct snd_seq_pool *pool, struct snd_seq_event *event,
 /* return number of unused (free) cells */
 static inline int snd_seq_unused_cells(struct snd_seq_pool *pool)
 {
-	return pool ? pool->total_elements - atomic_read(&pool->counter) : 0;
+	return pool ? atomic_read(&pool->total_elements) - atomic_read(&pool->counter) : 0;
 }
 
 /* return total number of allocated cells */
 static inline int snd_seq_total_cells(struct snd_seq_pool *pool)
 {
-	return pool ? pool->total_elements : 0;
+	return pool ? atomic_read(&pool->total_elements) : 0;
 }
 
 /* init pool - allocate events */
-- 
2.43.0