linux-kernel - Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for

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Message-ID: <80f362fd-8e69-1dcd-ba7a-abfc55894d61@arm.com>
Date:   Tue, 31 May 2022 17:42:36 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in
 debugfs

On 2022-05-31 16:59, Jason Gunthorpe wrote:
> On Tue, May 31, 2022 at 02:52:28PM +0100, Robin Murphy wrote:
> 
>>> +                break;
>>> +            pgtable_walk_level(m, phys_to_virt(phys_addr),
>>
>> Also, obligatory reminder that pfn_valid() only means that pfn_to_page()
>> gets you a valid struct page. Whether that page is direct-mapped kernel
>> memory or not is a different matter.
> 
> Even though this is debugfs, if the operation is sketchy like that and
> can theortically crash the kernel the driver should test capabilities,
> CAP_SYS_RAWIO or something may be appropriate. I don't think we have a
> better cap for 'userspace may crash the kernel'

It shouldn't be insurmountable to make this safe, it just needs a bit 
more than pfn_valid(), which can still return true off the ends of the 
memory map if they're not perfectly section-aligned, and for random 
reserved holes in the middle.

Robin.