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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:


    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.


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