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Re: [PATCH, RFC] MIPS: Implement the getcontext API

To: "David VomLehn (dvomlehn)" <dvomlehn@cisco.com>
Subject: Re: [PATCH, RFC] MIPS: Implement the getcontext API
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 2 Apr 2009 15:38:55 +0200
Cc: Brian Foster <brian.foster@innova-card.com>, David Daney <ddaney@caviumnetworks.com>, "Maciej W. Rozycki" <macro@codesourcery.com>, linux-mips@linux-mips.org, libc-ports@sourceware.org, "Maciej W. Rozycki" <macro@linux-mips.org>
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On Wed, Mar 04, 2009 at 05:25:35PM -0500, David VomLehn (dvomlehn) wrote:

> > >  it's more a matter of "when" rather than "if".
> > >  there is still an intention here to use XI (we
> > >  have SmartMIPS), which requires not using the
> > >  signal (or FP) trampoline on the stack.
> > > 
> > >  moving the signal trampoline to a vdso (which
> > >  is(? was?) called, maybe misleadingly, 'vsyscall',
> > >  on other architectures) is the obvious solution to
> > >  that part of the puzzle.  and yes, it is possible
> > >  to maintain the ABI; the signal trampoline is still
> > >  also put on the stack, and modulo XI, would work if
> > >  used - the trampoline-on-stack is simply not used
> > >  if there is a vdso with the signal trampoline.
> > 
> > We generally want to get rid of stack trampolines.  
> > Trampolines require
> > cacheflushing which especially on SMP systems can be a rather 
> > expensive
> > operation.
> 
> If I understand this correctly, using a vdso would allow a stack without
> execute permission on those processors that differentiate between read
> and execute permission. This defeats attaches that use buffer overrun to
> write code to be executed onto the stack, a nice thing for more secure
> systems.

The good news is that many of these stack buffer overruns don't work on
MIPS anyway due to the somewhat unusual stack frame.  Don't rely on that
too much for security though - like 10 years ago Phrack published an
article under the title "Smashing the stack for fun and profit" explaining
how to write exploits for IRIX 5 which als was using o32.

  Ralf

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