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    SubjectRe: 2nd Linux kernel patch to remove stack exec
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    In message <199704131906.QAA06271@sun1.ideal.ru> Solar Designer writes:
    : About GCC trampolines -- yes, there is a problem, but in reality it turns
    : out to be quite easy to solve; also, nested functions, and especially
    : those which address gets passed somewhere else, are not common in real
    : world applications -- one of the reasons is that it's a GNU C extension.
    : Since most programs will never use the trampolines, it makes sense to run
    : them with non-executable stack, and enable stack execution permission for
    : those that really need it. This can be done automatically, by modifying
    : the GPF handler to switch back to the huge code segment (which covers the
    : stack) and re-executing the instruction, unless it was a RET. Since most
    : buffer overflows can only be exploited by overwriting the return address,
    : this will still make them unexploitable (RET has to be the instruction to
    : pass the control onto the stack), while C programs will normally only use
    : CALL, and it is extremely unlikely that some code will use RET for that
    : purpose (this can never happen for pure C programs compiled with GCC).
    : Note that such emulation won't make the things run any slower since only
    : one GPF per entire process life may get generated (after that the stack
    : remains executable for this entire process).

    There is a fatal flaw in this. All I have to do is send the process
    one signal, and then I can take advantage of a stack overflow. Or the
    process just has to generate one signal and keep going. This is a
    very common occurrance. For example, in ftpd I can cause it to do a
    signal by sending it urgent data. If there were then an exploitable
    buffer overflow down stream, I could now make use of that buffer
    overflow to get in.

    Warner

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