[ubuntu-hardened] Re: Collecting NX information

John Richard Moser nigelenki at comcast.net
Tue Mar 29 02:45:22 CST 2005

Hash: SHA1

John Richard Moser wrote:
> Arjan van de Ven wrote:

Three more notes, then I'll sleep.  These notes won't include the two
paragraph long explaination of falling back to PT_GNU_STACK if
PT_PAX_FLAGS isn't there; compatibility has been touched what, 5 times?

1.  I don't want to continue using PT_GNU_STACK for three reasons.  The
first being that PaX uses a tristate in PT_PAX_FLAGS; the second being
that PT_GNU_STACK is a whole ELF field and I'm inclined to take the more
space-efficient method; and the third being that PT_GNU_STACK is not a

The last is particularly an important consideration to me:  a tristate
would allow for a compatibility/soft mode, but changing PT_GNU_STACK's
logic would change the current expected behavior and thus could be
unpredictable (break things).  I have no interest in breaking Fedora
horribly, nor wasting space with a full field where sharing with the
other parts of PT_PAX_FLAGS would do just fine.

2.  Although binutils can emit PT_GNU_STACK, the paxctl utility could
also be modified to detect PT_GNU_STACK in a binary without PT_PAX_FLAGS
and change it to PT_PAX_FLAGS, then nuke it.  This would allow the flags
to be changed without relinking (remember PT_GNU_STACK is to be ignored
if PT_PAX_FLAGS exists at all).  This is only of interest to
distributions which will use PT_PAX_FLAGS.

Note also that execstack would probably be wisely modified to set
PF_PAGEEXEC and PT_GNU_STACK both, just for future compatibility.  This
is of course a lot of work (I tried to make paxctl hack EI_PAX too, and
. . .well, it didn't work).

3.  PaX won't pay any attention to markings on libraries.  Exec Shield
and Mainline may, though I have no idea how.  If it can be done with
PT_GNU_STACK, it can be done with PT_PAX_FLAGS.  Such behavior is
acceptable, though libraries should be coded with the utmost care to
avoid this simply because the weakening of security around a library
weakens any and all programs using that library.

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