Date:   Fri, 13 Jan 2017 08:29:49 -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,
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        paul.gortmaker@...driver.com, mhocko@...e.com, jmarchan@...hat.com,
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        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 01/13/2017 07:48 AM, Khalid Aziz wrote:
> On 01/12/2017 06:31 PM, Rob Gardner wrote:
>> 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.
>>
>
> But could you have mmap'd the file writable, set the tags and then 
> changed the protection on memory to read-only? That would be the 
> logical way to ADI protect a memory being used to mmap a file. Right?


Sure, if you have write access to begin with, you can set memory 
versions, then remove write access to the page. But I think the point is 
that if a process doesn't have write access, and cannot get it, then it 
will not ever be able to change memory versions. So in the example of a 
non-root process opening /etc/passwd (read only), and mmaping it, the 
mapping would be read-only as well. Personally I don't really see a use 
case for ADI on memory mapped to a file. In an abstract sense, the 
"backing store" for a memory mapped file is the file itself on disk, not 
physical memory. And there is already a way to restrict access to files, 
so perhaps ADI should simply be disallowed for memory mapped to files, 
and this particular complication can be avoided. Thoughts?

Incidentally, I see ADI as primarily a way to protect memory from 
improper access within a process or group of cooperating processes. 
There is already a way to protect memory from unrelated processes, and 
if that is circumvented somehow, then ADI won't help at all. Perhaps we 
should stop talking about ADI as a "security" feature; It does add a 
layer of protection against buffer overflow attacks, but this attack 
only exists when there is a bug in the underlying application. If an 
attacker gains access to the virtual memory for a process, then nothing 
can help you.


Rob


>
> -- 
> Khalid
>
>> 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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