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SubjectRe: [Announcement] "Exec Shield", new Linux security feature

On Sun, 4 May 2003, Ingo Molnar wrote:

> > IIRC, x86 ints have the high-order byte _last_ (ie the fourth byte).
> > What's to stop someone from, say, smashing a buffer (and consequently
> > return-address) on the stack using something like {0x01, 0x01, 0x01,
> > 0x00} which is really address '65793' in base-10. The above is a valid
> > ASCII string (3 1's followed by a NUL) which could conceivably end up on
> > the stack as the result of an errant strcpy() or gets() or whatever...
>
> you are right, it is possible to use the enclosing \0 to generate an
> address into the first 16MB, but how do you get any arguments passed to
> that function?

ie. if the binary anywhere has code that does:

system("/bin/sh")

then this address can be jumped to and an exploit becomes possible. Also,
in the case of non-ASCII overflows the attacker has a much higher degree
of freedom to create a proper stackframe.

wrt. address-space randomization, "prelink -R" already provides quite good
randomization of the shared library addresses, which should give some
statistical protection against remote attacks, i dont think we'll need
kernel support for that.

Ingo

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