Re: [PATCH v4 0/4] Application Data Integrity feature introduced by SPARC M7

[Rob Gardner]

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?