Re: [PATCH] arm64: treat PF_IO_WORKER like PF_KTHREAD for mitigations

From: Jens Axboe <axboe@kernel.dk>
To: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: treat PF_IO_WORKER like PF_KTHREAD for mitigations
Date: Tue, 28 Mar 2023 10:47:18 -0600	[thread overview]
Message-ID: <5209a880-483d-bad0-1339-eb0e06da5413@kernel.dk> (raw)
In-Reply-To: <20230328141420.GC1333@willie-the-truck>

On 3/28/23 8:14?AM, Will Deacon wrote:
> Hey Jens,
> 
> On Thu, Jan 26, 2023 at 07:07:45AM -0700, Jens Axboe wrote:
>> 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@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.
> 
> I think this one slipped through the cracks, as I don't see any usage of
> PF_IO_WORKER in arch/arm64/. Do you plan to update the patch?
> 

Oops, yes looks like it did. Should look something like this, added a
comment as well. I'll send it out properly.

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 71d59b5abede..9ce614b2177e 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -445,9 +445,11 @@ static void ssbs_thread_switch(struct task_struct *next)
 {
 	/*
 	 * Nothing to do for kernel threads, but 'regs' may be junk
-	 * (e.g. idle task) so check the flags and bail early.
+	 * (e.g. idle task) so check the flags and bail early. Nothing to do
+	 * for IO worker threads either, as they never transition between
+	 * kernel and userspace.
 	 */
-	if (unlikely(next->flags & PF_KTHREAD))
+	if (unlikely(next->flags & (PF_KTHREAD | PF_IO_WORKER)))
 		return;
 
 	/*
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);
 
 	if (spectre_v4_mitigations_off())
 		ssbs = true;

-- 
Jens Axboe


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