On 03/07/2016 11:49 AM, Andy Lutomirski wrote:
On Mon, Mar 7, 2016 at 10:22 AM, Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
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?
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?
Thanks for asking these tough questions. These are very helpful in
refining my implementation and avoiding silly bugs.
--
Khalid
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