On Mon, Mar 7, 2016 at 11:44 AM, Khalid Aziz [off-list ref] wrote:

On 03/07/2016 11:49 AM, Andy Lutomirski wrote:

quoted

On Mon, Mar 7, 2016 at 10:22 AM, Khalid Aziz [off-list ref]
wrote:

quoted

No, it changes the tag associated with the virtual address for the
caller.
Physical page backing this virtual address is unaffected. Tag checking is
done for virtual addresses. The one restriction where physical address is
relevant is when two processes map the same physical page, they both have
to
use the same tag for the virtual addresses that map on to the shared
physical pages.

Slow down, please.  *Why* do the tags for two different VAs that map
to the same PA have to match?  What goes wrong if they don't, and why
is requiring them to be the same a good idea?

Consider this scenario:

1. Process A creates a shm and attaches to it.
2. Process A fills shm with data it wants to share with only known
processes. It enables ADI and sets tags on the shm.
3. Hacker triggers something like stack overflow on process A, exec's a new
rogue binary and manages to attach to this shm. MMU knows tags were set on
the virtual address mapping to the physical pages hosting the shm. If MMU
does not require the rogue process to set the exact same tags on its mapping
of the same shm, rogue process has defeated the ADI protection easily.

Does this make sense?

This makes sense, but I still think the design is poor.  If the hacker
gets code execution, then they can trivially brute force the ADI bits.

Also, if this is the use case in mind, shouldn't the ADI bits bet set
on the file, not the mapping?  E.g. have an ioctl on the shmfs file
that sets its ADI bits?

quoted

quoted

quoted

I sense DoS issues in your future.

Are you concerned about DoS even if the tag is associated with virtual
address, not physical address?

Yes, absolutely.

fd = open("/lib/ld.so");
mmap(fd)
stxa to write the tag

*boom*, presumably, because the tags apparently have to match for all
mappings.

A process can not just write version tags and make the file inaccessible to
others. It takes three steps to enable ADI:

1. Set PSTATE.mcde for the process.
2. Set TTE.mcd on all PTEs for the virtual addresses ADI is being enabled
on.
3. Set version tags.

Unless all three steps are taken, tag checking will not be done. stxa will
fail unless step 2 is completed. In your example, the step of setting
TTE.mcd will force sharing to stop for the process through
change_protection(), right?

OK, that makes some sense.

Can a shared page ever have TTE.mcd set?  How does one share a page,
even deliberately, between two processes with cmd set?

--Andy