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Message-ID: <c2fff623-a9e2-dd36-b420-5ebb9e9a0d01@arm.com>
Date:   Fri, 14 Oct 2022 12:01:21 +0100
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Subject: Re: [RFC PATCH] iommu/vt-d: Add sanity check to
 iommu_sva_bind_device()
To:     Jerry Snitselaar <jsnitsel@redhat.com>,
        Baolu Lu <baolu.lu@linux.intel.com>
Cc:     iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Yu, Fenghua" <fenghua.yu@intel.com>
References: <20221013153355.2365865-1-jsnitsel@redhat.com>
 <89029bfb-bdf6-43e3-b24c-09c266277f0e@linux.intel.com>
 <20221014021043.4rpzufoleeg7z5gx@cantor>
 <ea7ea775-5b87-82d6-c30a-8e60f1462476@linux.intel.com>
 <20221014065259.5koqcoxiuegg6kzj@cantor>
Content-Language: en-GB
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <20221014065259.5koqcoxiuegg6kzj@cantor>
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On 2022-10-14 07:52, Jerry Snitselaar wrote:
> On Fri, Oct 14, 2022 at 10:22:21AM +0800, Baolu Lu wrote:
>> On 2022/10/14 10:10, Jerry Snitselaar wrote:
>>> On Fri, Oct 14, 2022 at 09:52:44AM +0800, Baolu Lu wrote:
>>>> On 2022/10/13 23:33, Jerry Snitselaar wrote:
>>>>> iommu_sva_bind_device() should only be called if
>>>>> iommu_dev_enable_feature() succeeded. There has been one case already
>>>>> where that hasn't been the case, which resulted in a null pointer
>>>>> deref in dev_iommu_ops(). To avoid that happening in the future if
>>>>> another driver makes that mistake, sanity check dev->iommu and
>>>>> dev->iommu->iommu_dev prior to calling dev_iommu_ops().
>>>>>
>>>>> Cc: Joerg Roedel<joro@8bytes.org>
>>>>> Cc: Will Deacon<will@kernel.org>
>>>>> Cc: Robin Murphy<robin.murphy@arm.com>
>>>>> Cc: Lu Baolu<baolu.lu@linux.intel.com>
>>>>> Signed-off-by: Jerry Snitselaar<jsnitsel@redhat.com>
>>>>> ---
>>>>> drivers/iommu/iommu.c | 10 +++++++++-
>>>>>     1 file changed, 9 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>>>> index 4893c2429ca5..20ec75667529 100644
>>>>> --- a/drivers/iommu/iommu.c
>>>>> +++ b/drivers/iommu/iommu.c
>>>>> @@ -2746,7 +2746,15 @@ iommu_sva_bind_device(struct device *dev, struct mm_struct *mm, void *drvdata)
>>>>>     {
>>>>>     	struct iommu_group *group;
>>>>>     	struct iommu_sva *handle = ERR_PTR(-EINVAL);
>>>>> -	const struct iommu_ops *ops = dev_iommu_ops(dev);
>>>>> +	const struct iommu_ops *ops;
>>>>> +
>>>>> +	if (!dev->iommu || !dev->iommu->iommu_dev) {
>>>>> +		dev_warn(dev, "%s called without checking succes of iommu_dev_enable_feature?\n",
>>>>> +			__func__);
>>>>> +		return ERR_PTR(-ENODEV);
>>>>> +	}
>>>> If that's the case, dev_iommu_ops() will warn a NULL pointer reference.
>>>> This kind of error will be discovered at the first place.
>>>>
>>>> Best regards,
>>>> baolu
>>>>
>>> It will warn this by crashing the system (example from back when idxd had the problem):
>>>
>>>       [   21.423729] BUG: kernel NULL pointer dereference, address: 0000000000000038
>>>       [   21.445108] #PF: supervisor read access in kernel mode
>>>       [   21.450912] #PF: error_code(0x0000) - not-present page
>>>       [   21.456706] PGD 0
>>>       [   21.459047] Oops: 0000 [#1] PREEMPT SMP NOPTI
>>>       [   21.464004] CPU: 0 PID: 1420 Comm: kworker/0:3 Not tainted 5.19.0-0.rc3.27.eln120.x86_64 #1
>>>       [   21.464011] Hardware name: Intel Corporation EAGLESTREAM/EAGLESTREAM, BIOS EGSDCRB1.SYS.0067.D12.2110190954 10/19/2021
>>>       [   21.464015] Workqueue: events work_for_cpu_fn
>>>       [   21.464030] RIP: 0010:iommu_sva_bind_device+0x1d/0xe0
>>>       [   21.464046] Code: c3 cc 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 57 41 56 49 89 d6 41 55 41 54 55 53 48 83 ec 08 48 8b 87 d8 02 00 00 <48> 8b 40 38 48 8b 50 10 48 83 7a 70 00 48 89 14 24 0f 84 91 00 00
>>>       [   21.464050] RSP: 0018:ff7245d9096b7db8 EFLAGS: 00010296
>>>       [   21.464054] RAX: 0000000000000000 RBX: ff1eadeec8a51000 RCX: 0000000000000000
>>>       [   21.464058] RDX: ff7245d9096b7e24 RSI: 0000000000000000 RDI: ff1eadeec8a510d0
>>>       [   21.464060] RBP: ff1eadeec8a51000 R08: ffffffffb1a12300 R09: ff1eadffbfce25b4
>>>       [   21.464062] R10: ffffffffffffffff R11: 0000000000000038 R12: ffffffffc09f8000
>>>       [   21.464065] R13: ff1eadeec8a510d0 R14: ff7245d9096b7e24 R15: ff1eaddf54429000
>>>       [   21.464067] FS:  0000000000000000(0000) GS:ff1eadee7f600000(0000) knlGS:0000000000000000
>>>       [   21.464070] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>       [   21.464072] CR2: 0000000000000038 CR3: 00000008c0e10006 CR4: 0000000000771ef0
>>>       [   21.464074] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>       [   21.464076] DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
>>>       [   21.464078] PKRU: 55555554
>>>       [   21.464079] Call Trace:
>>>       [   21.464083]  <TASK>
>>>       [   21.464092]  idxd_pci_probe+0x259/0x1070 [idxd]
>>>       [   21.464121]  local_pci_probe+0x3e/0x80
>>>       [   21.464132]  work_for_cpu_fn+0x13/0x20
>>>       [   21.464136]  process_one_work+0x1c4/0x380
>>>       [   21.464143]  worker_thread+0x1ab/0x380
>>>       [   21.464147]  ? _raw_spin_lock_irqsave+0x23/0x50
>>>       [   21.464158]  ? process_one_work+0x380/0x380
>>>       [   21.464161]  kthread+0xe6/0x110
>>>       [   21.464168]  ? kthread_complete_and_exit+0x20/0x20
>>>       [   21.464172]  ret_from_fork+0x1f/0x30
>>>
>>>
>>> It was doing that to SPR systems that didn't boot with
>>> intel_iommu=on. They had to either enable the iommu, or blacklist the
>>> idxd driver until the idxd driver had a fix. The idea here is to
>>> avoid taking the system down, and just have the driver get an error back.
>>
>> If IOMMU is disabled, the iommu_dev_enable_feat(SVA) will return an
>> error, the idxd driver should not call the sva_bind() interfaces
>> anymore. If the driver doesn't do like this, why not fixing it in the
>> driver itself?
>>
>> Best regards,
>> baolu
> 
> The idxd case was found, and fixed by me back in June. I just was
> showing the stack trace to show that it crashes the system, not just
> puts out a warning.
> 
> Why would this stop someone fixing the problem in a driver that is
> calling sva_bind() interface incorrectly?
> 
> Nothing else in the system cares that some driver forgot to call
> iommu_dev_enable_feat(), or forgot to check the return value. Nothing
> else would be impacted by it, except that the system is being allowed
> to crash. If the idea is to get noticed more quickly, couldn't a
> WARN_ON() + returning an error solve that without resorting to
> crashing the system?

The point is really that this is the kind of obvious bug that should be 
found during development; it's not the IOMMU API's responsibility if 
some driver patch gets merged and gets as far as a distro release 
without ever being fully tested. If someone doesn't use an API properly 
it can go wrong in any number of ways, so the value of maintaining code 
to mitigate just one of those ways is questionable. In this case, 
consider if a driver *is* present but failed iommu_dev_enable_feat() for 
some other reason, then its ->sva_bind goes and dereferences some 
internal data that it expects the previous ->dev_enable_feat to have 
allocated. Boom, it just crashes somewhere else instead.

If you really think it's worth the effort to maintain code that only 
serves to give lazy developers a free pass (and I freely admit to being 
a lazy developer most of the time) then a robust approach would be some 
generic tracking in dev->iommu for which features have been successfully 
enabled. Certainly I'm not convinced this patch as-is is worthwhile.

However, if a bad driver isn't handling the return from 
iommu_dev_enable_feature(), who's to say it's actually handling the 
return from iommu_sva_bind_device() either, and wouldn't still end up 
crashing some other way? Ultimately it's a game you can't win.

Thanks,
Robin.