[PATCH] arm64: treat PF_IO_WORKER like PF_KTHREAD for mitigations

[Jens Axboe]

On 1/26/23 7:00 AM, Will Deacon wrote:
> On Wed, Jan 25, 2023 at 09:43:34AM -0700, Jens Axboe wrote:
>> Like PF_KTHREAD, PF_IO_WORKER never exit to userspace. They exist
>> entirely within the kernel, and hence don't need any task mitigations
>> applied.
>>
>> Signed-off-by: Jens Axboe <axboe at kernel.dk>
>>
>> ---
>>
>> diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
>> index fca9cc6f5581..25a21c3d446c 100644
>> --- a/arch/arm64/kernel/proton-pack.c
>> +++ b/arch/arm64/kernel/proton-pack.c
>> @@ -654,7 +654,7 @@ static void __update_pstate_ssbs(struct pt_regs *regs, bool state)
>>  void spectre_v4_enable_task_mitigation(struct task_struct *tsk)
>>  {
>>  	struct pt_regs *regs = task_pt_regs(tsk);
>> -	bool ssbs = false, kthread = tsk->flags & PF_KTHREAD;
>> +	bool ssbs = false, kthread = tsk->flags & (PF_KTHREAD | PF_IO_WORKER);
> 
> Hmm, the other two uses of PF_KTHREAD in arch/arm64 also look pretty
> suspect in light of this proposal. Should we also update
> ssbs_thread_switch() and access_ok()? If not, then a comment would be
> handy to say why PF_KTHREAD is sufficient there.

The uaccess one looks like, PF_IO_WORKER threads are just normal userspace
threads. The only difference is that they never exit to userspace, they
remain in the kernel. But everything else is just like a thread.

But yes, the ssbs_thread_switch() should have this check too. I'll send
out an updated patch.

-- 
Jens Axboe