[TUHS] The evolution of Unix facilities and architecture

Noel Chiappa jnc at mercury.lcs.mit.edu
Sat May 13 11:25:30 AEST 2017


    > From: Random832

    > It seems to me that this check is central to being able to (or not)
    > modify the in-core image of any process at all other than the one being
    > traced (say, by attaching to a SUID program that has already dropped
    > privileges, and making changes that will affect the next time it is
    > run).

Right, good catch: if you have a program that was _both_ sticky and SUID, when
the system is idle (so the text copy in the swap area won't get recycled),
call up a copy under the debugger, patch it, exit (leaving the patched copy),
and then re-run it without the debugger.

I'd have to check the handling of patched sticky pure texts - to see if they
are retained or not.

{Checks code.}

Well, the code to do with pure texts is _very_ different between V6 and
PWB1.

The exact approach above might not work in V6, because the modified (in-core)
copy of pure texts are simply deleted when the last user exits them. But it
might be possible for a slight variant to work; leave the copy under the
debugger (which will prevent the in-core copy from being discarded), and then
run it again without the debugger. That might do it.

Under PWB1, I'm not sure if any variant would work (very complicated, and I'm
fading). There's an extra flag bit, XWRIT, which is set when a pure text is
written into; when the last user stops using the in-code pure text, the
modified text is written to swap.  (It lools like the in-core copy is always
discarded when the last user stops using it.) But the check for sticky would
probably stop a sticky pure-text being modified? But maybe the approach that
seems like it would work under V6 (leave the patched, debugger copy running,
and start a new instance) looks like it should work here too.

So maybe the sticky thing is irrelevant? On both V6 and PWB1, it just needs a
pure text which is SETUID: start under the debugger, patch, leave running, and
start a _new_ copy, which will run the patched version as the SUID user.

      Noel



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