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    SubjectRe: Unexecutable Stack / Buffer Overflow Exploits...
    Theodore Y. Ts'o writes:
    > I will observe that by putting in some bounds checking code in strcpy,
    > strncpy, memcpy, bcopy, and a few other libc funtions, you gain most of
    > the advantages of Solar Designer's patches. It also protects against a
    > few things his patches don't --- such as simply installing a return
    > address to some location not on the stack. (Yes, it doesn't protect
    > against open-coded for loops that reference *cp++; the libc patch
    > approach isn't perfect. But then again, neither is the non-executable
    > stack hack.)

    Exactly how do you propose to make that work? Will the unbounded
    str*() and mem*() functions simply have an implicit limit on the
    number of characters they copy, or make complex checks to
    determine whether copy targets are on the stack and guess how to
    bound them to avoid overwriting important parts of the stack
    frame? How do you pick the implicit limit that prevents buffer
    overruns in an effective number of cases?

    I see a non-executable stack as being completely in line with all
    the other security partitioning and protection methods that are
    already in an operating system -- page protections and virtual
    memory partitioning so programs can't access or modify memory
    they aren't supposed to; permissions and user/group checks so
    programs can't access or modify resources they're not supposed to
    and users can't execute programs or modify files they're not
    supposed to, and so on. Many of these things could also be
    accomplished by careful coding in programs, but these protections
    are put into the operating system because programs cannot be
    universally trusted to play nice.

    The classic stack buffer overrun (smash the stack with code to be
    executed on the stack) is always going to be easier to construct
    than trickier buffer overruns that try to return into locations
    outside the stack, because the stack is mapped at a fixed easily
    determined location in all programs, while the location of data
    buffers and library routines vary greatly. And since it is
    easier to construct, _and_ if systems continue to permit it, then
    it will continue to be deployed.

    Come to think of it, the ability to have non-executable data
    pages would also be nice for high-security systems, since it
    would defeat even the more sophisticated stack overflow attacks.

    > So why aren't the folks who think that every marginal increase in
    > security, is worth any arbitrary amount of extra code complexity (after
    > all, *they're* not the ones who have to maintain it :-), lobbying the
    > glibc maintainers to put such hacks into libc? Just a thought.....
    >
    > - Ted

    My position is that on OSes and architectures that support it, a
    non-executable stack provides a much more than marginal increase
    in security for very little code complexity (as little as not
    setting the execute bit on page permissions for stack pages, if
    the architecture supports it).

    I understand that the i386 architecture doesn't have page-level
    execute permissions and that Solar Designer's patch goes through
    some fairly complex gyrations to obtain a (mostly) non-executable
    stack, so from an i386-centric point of view I can see how many
    would think that it's too much code complexity for what it buys.

    -
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