Date:   Thu, 12 Jan 2017 18:31:45 -0700
From:   Rob Gardner <rob.gardner@...cle.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, davem@...emloft.net,
        corbet@....net, arnd@...db.de, akpm@...ux-foundation.org
Cc:     hpa@...or.com, viro@...iv.linux.org.uk, nitin.m.gupta@...cle.com,
        chris.hyser@...cle.com, tushar.n.dave@...cle.com,
        sowmini.varadhan@...cle.com, mike.kravetz@...cle.com,
        adam.buchbinder@...il.com, minchan@...nel.org, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, keescook@...omium.org,
        allen.pais@...cle.com, aryabinin@...tuozzo.com,
        atish.patra@...cle.com, joe@...ches.com, pmladek@...e.com,
        jslaby@...e.cz, cmetcalf@...lanox.com,
        paul.gortmaker@...driver.com, mhocko@...e.com, jmarchan@...hat.com,
        lstoakes@...il.com, 0x7f454c46@...il.com, vbabka@...e.cz,
        tglx@...utronix.de, mingo@...hat.com, dan.j.williams@...el.com,
        iamjoonsoo.kim@....com, mgorman@...hsingularity.net,
        vdavydov.dev@...il.com, hannes@...xchg.org, namit@...are.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 0/4] Application Data Integrity feature introduced by
 SPARC M7

On 01/12/2017 05:22 PM, Khalid Aziz wrote:
> On 01/12/2017 10:53 AM, Dave Hansen wrote:
>> On 01/12/2017 08:50 AM, Khalid Aziz wrote:
>>> 2. Any shared page that has ADI protection enabled on it, must stay ADI
>>> protected across all processes sharing it.
>>
>> Is that true?
>>
>> What happens if a page with ADI tags set is accessed via a PTE without
>> the ADI enablement bit set?
>
> ADI protection applies across all processes in terms of all of them 
> must use the same tag to access the shared memory, but if a process 
> accesses a shared page with TTE.mcde bit cleared, access will be granted.
>
>>
>>> COW creates an intersection of the two. It creates a new copy of the
>>> shared data. It is a new data page and hence the process creating it
>>> must be the one responsible for enabling ADI protection on it.
>>
>> Do you mean that the application must be responsible?  Or the kernel
>> running in the context of the new process must be responsible?
>>
>>> It is also a copy of what was ADI protected data, so should it
>>> inherit the protection instead?
>>
>> I think the COW'd copy must inherit the VMA bit, the PTE bits, and the
>> tags on the cachelines.
>>
>>> I misspoke earlier. I had misinterpreted the results of test I ran.
>>> Changing the tag on shared memory is allowed by memory controller. The
>>> requirement is every one sharing the page must switch to the new tag or
>>> else they get SIGSEGV.
>>
>> I asked this in the last mail, but I guess I'll ask it again. Please
>> answer this directly.
>>
>> If we require that everyone coordinate their tags on the backing
>> physical memory, and we allow a lower-privileged program to access the
>> same data as a more-privileged one, then the lower-privilege app can
>> cause arbitrary crashes in the privileged application.
>>
>> For instance, say sudo mmap()'s /etc/passwd and uses ADI tags to protect
>> the mapping.  Couldn't any other app in the system prevent sudo from
>> working?
>>
>> How can we *EVER* allow tags to be set on non-writable mappings?

I don't think you can write a tag to memory if you don't have write 
access in the TTE. Writing a tag requires a store instruction, and if 
the machine is at all sane, this will fault if you don't have write access.

Rob



>
> I understand your quetion better now. That is a very valid concern. 
> Using ADI tags to prevent an unauthorized process from just reading 
> data in memory, say an in-memory copy of database, is one of the use 
> cases for ADI. This means there is a reasonable case to allow enabling 
> ADI and setting tags even on non-writable mappings. On the other hand, 
> if an unauthorized process manages to map the right memory pages in 
> its address space, it can read them any way by not setting TTE.mcd.
>
> Userspace app can set tag on any memory it has mapped in without 
> requiring assistance from kernel. Can this problem be solved by not 
> allowing setting TTE.mcd on non-writable mappings? Doesn't the same 
> problem occur on writable mappings? If a privileged process mmap()'s a 
> writable file with MAP_SHARED, enables ADI and sets tag on the mmap'd 
> memory region, then another lower privilege process mmap's the same 
> file writable (assuming file permissions allow it to), enables ADI and 
> sets a different tag on it, the privileged process would get SIGSEGV 
> when it tries to access the mmap'd file. Right?

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