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    SubjectRe: [announce] [patch] NX (No eXecute) support for x86, 2.6.7-rc2-bk2
    Ingo Molnar wrote:
    > * Linus Torvalds <> wrote:
    >>>If the NX feature is supported by the CPU then the patched kernel turns
    >>>on NX and it will enforce userspace executability constraints such as a
    >>>no-exec stack and no-exec mmap and data areas. This means less chance
    >>>for stack overflows and buffer-overflows to cause exploits.
    >>Just out of interest - how many legacy apps are broken by this? I
    >>assume it's a non-zero number, but wouldn't mind to be happily
    > in the full install of FC1 and FC2 the number is zero - and Fedora has
    > exec-shield which does a couple of things more: it makes the heap
    > non-executable as well [this broke X], it randomizes the address-space
    > layout and has a 4:4 VM [which broke the Sun JVM].
    >>And do we have some way of on a per-process basis say "avoid NX
    >>because this old version of Oracle/flash/whatever-binary-thing doesn't
    >>run with it"?
    > we have three mechanisms for this in Fedora:
    > 1) the PT_GNU_STACK flag itself - you can turn executability on/off
    > compile-time or even after the fact via the execstack(8) utility
    > Jakub wrote. This only affects the stack's executability - if an
    > application assumes a non-PROT_EXEC mmap() can be executed it might
    > still break with NX - but based on experience with Fedora Core i'd
    > say there's almost no such application.
    > this method works in 2.6 too, since it supports PT_GNU_STACK. gcc's
    > PT_GNU_STACK mechanism is very conservative - e.g. if an application
    > does an asm() then gcc assumes that it might rely on stack executability
    > and emits the X flag. [applications can then turn this off in the source
    > if stack executability is not required.] Likewise, if gcc emits
    > trampolines then the X flag is emitted too. (glibc knows about
    > PT_GNU_STACK all across - so e.g. if a nonexec stack application
    > dlopen()s a library that needs stack executability then glibc makes the
    > stack executable on the fly via PROT_GROWSDOWN/GROWSUP.)
    > 2) via a runtime method: via the i386 personality. So an application can
    > trigger the 'legacy' Linux VM layout by e.g doing 'i386 java
    > ./test.class'.
    > this is a hack in Fedora - we wanted to have a finegrained runtime
    > mechanism just in case. But it would be nice to have this upstream too -
    > e.g. via a PERSONALITY_3G?
    > 3) via a kernel boot parameter (exec-shield=0)
    > with the NX patch this becomes noexec=off [the same flag works on x86_64
    > too]. This method is the most inflexible one, and is a last-resort
    > thing. (Fedora also has a runtime global switch to turn off the VM
    > layout changes.)
    > here's a list of applications that we had to fix/work around in Fedora
    > when the VM layout changed:
    > - emacs _rebuild_. (it coredumps itself during build ... xemacs is OK.)
    > - some JDKs. Since they generate code and try to be as fast as possible
    > they tend to rely more on VM details than normal applications.
    > - X's module loader assumed that brk was executable. (fixed)
    > - Wine. (it implements another OS so it's by definition very sensitive
    > to layout changes.)

    Wine breaks because of the part of exec-shield that relocates shared
    libs to low addresses, where the (stripped) Windows binaries expect to
    be loaded at. NX stack doesn't affect it.

    Brian Gerst
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