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    SubjectRe: [announce] [patch] NX (No eXecute) support for x86, 2.6.7-rc2-bk2

    * 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
    > surprised.

    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

    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.)

    most of the breakages were unclean x86-only code that would have broken
    if ported over to 64-bit anyway.

    old, legacy applications dont have the PT_GNU_STACK flag so they all
    work fine.

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