6.12-stable review patch. If anyone has any objections, please let me know.
------------------
From: Long Li leo.lilong@huawei.com
[ Upstream commit 1e4b207ffe54cf33a4b7a2912c4110f89c73bf3f ]
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@huawei.com Signed-off-by: Madhavan Srinivasan maddy@linux.ibm.com Link: https://patch.msgid.link/20250819091035.2263329-1-leo.lilong@huaweicloud.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 b461b1bed25b2..6247dbe493dea 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) {