Re: [PATCH] arm64: fix a concurrency issue in emulation_proc_handler()

[Ruan Jinjie]

On 2022/12/9 19:09, Mark Rutland wrote:
> On Fri, Dec 09, 2022 at 06:55:56PM +0800, ruanjinjie wrote:
>> In emulation_proc_handler(), read and write operations are performed on
>> insn->current_mode. In the concurrency scenario, mutex only protects
>> writing insn->current_mode, and not protects the read. Suppose there are
>> two concurrent tasks, task1 updates insn->current_mode to INSN_EMULATE
>> in the critical section, the prev_mode of task2 is still the old data
>> INSN_UNDEF of insn->current_mode. As a result, two tasks call
>> update_insn_emulation_mode twice with prev_mode = INSN_UNDEF and
>> current_mode = INSN_EMULATE, then call register_emulation_hooks twice,
>> resulting in a list_add double problem.
>>
>> Call trace:
>>  __list_add_valid+0xd8/0xe4
>>  register_undef_hook+0x94/0x13c
>>  update_insn_emulation_mode+0xd0/0x12c
>>  emulation_proc_handler+0xd8/0xf4
>>  proc_sys_call_handler+0x140/0x250
>>  proc_sys_write+0x1c/0x2c
>>  new_sync_write+0xec/0x18c
>>  vfs_write+0x214/0x2ac
>>  ksys_write+0x70/0xfc
>>  __arm64_sys_write+0x24/0x30
>>  el0_svc_common.constprop.0+0x7c/0x1bc
>>  do_el0_svc+0x2c/0x94
>>  el0_svc+0x20/0x30
>>  el0_sync_handler+0xb0/0xb4
>>  el0_sync+0x160/0x180
> 
> The version queued in the arm64 for-next/core branch no longer has the list
> manipulation, but we do need to fix this for stable, and there is a remaining
> race on reading insn->current_mode in emulation_proc_handler().
Hi Mark, Should I send this patch to linux-stable?
> 
>> Fixes: af483947d472 ("arm64: fix oops in concurrently setting insn_emulation sysctls")
>> Signed-off-by: ruanjinjie <ruanjinjie@xxxxxxxxxx>
>> ---
>>  arch/arm64/kernel/armv8_deprecated.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
>> index fb0e7c7b2e20..d33e5d9e6990 100644
>> --- a/arch/arm64/kernel/armv8_deprecated.c
>> +++ b/arch/arm64/kernel/armv8_deprecated.c
>> @@ -208,10 +208,12 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
>>  				  loff_t *ppos)
>>  {
>>  	int ret = 0;
>> -	struct insn_emulation *insn = container_of(table->data, struct insn_emulation, current_mode);
>> -	enum insn_emulation_mode prev_mode = insn->current_mode;
>> +	struct insn_emulation *insn;
>> +	enum insn_emulation_mode prev_mode;
>>  
>>  	mutex_lock(&insn_emulation_mutex);
>> +	insn = container_of(table->data, struct insn_emulation, current_mode);
>> +	prev_mode = insn->current_mode;
>>  	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> 
> We don't strictly need to move the container_of(), but it makes no odds either
> way, and this looks good to me:
> 
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Mark.
> 
>>  
>>  	if (ret || !write || prev_mode == insn->current_mode)
>> -- 
>> 2.25.1
>>
>