Re: Executing from readablee, no-exec pages

On Fri, 2007-07-06 at 15:36 +0200, Segher Boessenkool wrote:

quoted

Umm, are you sure about this? The "prevent data exception in kernel
space" patch came from a test program I had that attempted to  
execute a
page with /no permissions/ at all!

I haven't looked at the code path in detail, but I believe Scott's
analysis is correct.  The kernel would merrily let a program run
code from a page without execute permission (so also from a page
without any permissions at all); not anymore, after my patch.

Programs relying on this behaviour are obviously buggy, but the
problem is that one of these broken programs is glibc, at least
some not-all-that-new but also not-all-that-old versions.

Ok I can see how your patch changes that if the page is readable but not
executable and hasn't been faulted in yet, then executing it will kill
the program. However, reading first (prefaulting) and then executing
code on the page will not kill the program.

Too many negatives, I don't see which way you're arguing :-)

Heh.

I think you're saying to treat read access as including execute
access?  I believe that would be too permissive here.

Yeah well what I was trying to say is that there's no point in having
read without execute if prefaulting the page can get you around that.

johannes