linux-kernel - [PATCH] macintosh/mac_hid: fix race condition in mac_hid_toggle

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Message-Id: <20250819091035.2263329-1-leo.lilong@huaweicloud.com>
Date: Tue, 19 Aug 2025 17:10:35 +0800
From: leo.lilong@...weicloud.com
To: linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Cc: leo.lilong@...wei.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	lonuxli.64@...il.com
Subject: [PATCH] macintosh/mac_hid: fix race condition in mac_hid_toggle_emumouse

From: Long Li <leo.lilong@...wei.com>

The following warning appears when running syzkaller, and this issue also
exists in the mainline code.

 ------------[ cut here ]------------
 list_add double add: new=ffffffffa57eee28, prev=ffffffffa57eee28, next=ffffffffa5e63100.
 WARNING: CPU: 0 PID: 1491 at lib/list_debug.c:35 __list_add_valid_or_report+0xf7/0x130
 Modules linked in:
 CPU: 0 PID: 1491 Comm: syz.1.28 Not tainted 6.6.0+ #3
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
 RIP: 0010:__list_add_valid_or_report+0xf7/0x130
 RSP: 0018:ff1100010dfb7b78 EFLAGS: 00010282
 RAX: 0000000000000000 RBX: ffffffffa57eee18 RCX: ffffffff97fc9817
 RDX: 0000000000040000 RSI: ffa0000002383000 RDI: 0000000000000001
 RBP: ffffffffa57eee28 R08: 0000000000000001 R09: ffe21c0021bf6f2c
 R10: 0000000000000001 R11: 6464615f7473696c R12: ffffffffa5e63100
 R13: ffffffffa57eee28 R14: ffffffffa57eee28 R15: ff1100010dfb7d48
 FS:  00007fb14398b640(0000) GS:ff11000119600000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 000000010d096005 CR4: 0000000000773ef0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 PKRU: 80000000
 Call Trace:
  <TASK>
  input_register_handler+0xb3/0x210
  mac_hid_start_emulation+0x1c5/0x290
  mac_hid_toggle_emumouse+0x20a/0x240
  proc_sys_call_handler+0x4c2/0x6e0
  new_sync_write+0x1b1/0x2d0
  vfs_write+0x709/0x950
  ksys_write+0x12a/0x250
  do_syscall_64+0x5a/0x110
  entry_SYSCALL_64_after_hwframe+0x78/0xe2

The WARNING occurs when two processes concurrently write to the mac-hid
emulation sysctl, causing a race condition in mac_hid_toggle_emumouse().
Both processes read old_val=0, then both try to register the input handler,
leading to a double list_add of the same handler.

  CPU0                             CPU1
  -------------------------        -------------------------
  vfs_write() //write 1            vfs_write()  //write 1
    proc_sys_write()                 proc_sys_write()
      mac_hid_toggle_emumouse()          mac_hid_toggle_emumouse()
        old_val = *valp // old_val=0
                                           old_val = *valp // old_val=0
                                           mutex_lock_killable()
                                           proc_dointvec() // *valp=1
                                           mac_hid_start_emulation()
                                             input_register_handler()
                                           mutex_unlock()
        mutex_lock_killable()
        proc_dointvec()
        mac_hid_start_emulation()
          input_register_handler() //Trigger Warning
        mutex_unlock()

Fix this by moving the old_val read inside the mutex lock region.

Fixes: 99b089c3c38a ("Input: Mac button emulation - implement as an input filter")
Signed-off-by: Long Li <leo.lilong@...wei.com>
---
 drivers/macintosh/mac_hid.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/macintosh/mac_hid.c b/drivers/macintosh/mac_hid.c
index 369d72f59b3c..06fd910b3fd1 100644
--- a/drivers/macintosh/mac_hid.c
+++ b/drivers/macintosh/mac_hid.c
@@ -187,13 +187,14 @@ static int mac_hid_toggle_emumouse(const struct ctl_table *table, int write,
 				   void *buffer, size_t *lenp, loff_t *ppos)
 {
 	int *valp = table->data;
-	int old_val = *valp;
+	int old_val;
 	int rc;
 
 	rc = mutex_lock_killable(&mac_hid_emumouse_mutex);
 	if (rc)
 		return rc;
 
+	old_val = *valp;
 	rc = proc_dointvec(table, write, buffer, lenp, ppos);
 
 	if (rc == 0 && write && *valp != old_val) {
-- 
2.39.2