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    SubjectRe: [patch] remove MNT_NOEXEC check for PROT_EXEC MAP_PRIVATE mmaps
    Ok, I've had a chance to look into this in a little more detail, and
    I have now come to a different view. Although I think Stas's proposal
    is well-motivated, I'm now more skeptical about some of the details of
    his proposal. Here is my current thinking on this issue:

    1) It is important that when has some way to ensure that executing
    a program off of a noexec partition will fail. The details of how it
    does that don't matter, from a security point of view, though they do
    matter from a maintenance perspective. If Stas's proposal requires
    changes to, Stas should implement and test those changes before
    any changes are made to the kernel.

    2) Currently, uses mmap(PROT_EXEC, MAP_PRIVATE) to load the text
    of the programs it executes. Consequently, if we want to avoid changing, then we need mmap(PROT_EXEC, MAP_PRIVATE) to fail with an error
    if the text lives on a noexec partition.

    3) I think it's a requirement that any change to Linux mmap() semantics
    will not break's noexec check.

    4) Everything else about the way the Linux kernel handles mmap(PROT_EXEC)
    is not security-critical. As long as will refuse to execute
    a program that lives on a noexec partition, you can choose the rest
    of mmap()'s semantics any way you want without fear of introducing a
    security loophole.

    5) There are already a million-and-one loopholes that allow one to
    indirectly execute code that lives on a noexec partition. For instance,
    one can run 'perl'. As another example, one
    can use mmap(PROT_EXEC, MAP_ANONYMOUS) to create an executable region
    of memory, then use read() to read the text in from a noexec partition
    and copy it into the executable region, and then jump to the executable
    region of memory. It would be trivial to write a replacement for's
    mmap(PROT_EXEC, MAP_PRIVATE) that has the same effect except that it
    ignores the noexec flag. There is no way to close all of these loopholes.

    6) In general, it's easy to see that as long as a program can create any
    region of memory that is both executable and writeable, there will always
    be a loophole. There's no way we are going to write a prohibition into
    the kernel saying that no program can ever have a region of memory that
    is simultaneously executable and writeable. Given that there are already
    zillions of loopholes, adding one more loophole just doesn't matter.
    Consequently, as long as we preserve the behavior of, the rest of
    the semantics of mmap() can be freely changed without fear of introducing
    a new security hole.

    7) For instance, you could imagine adding a MAP_IREALLYMEANIT flag which
    tells mmap() to ignore the noexec prohibition. This would be safe to add,
    from a security point of view (it wouldn't create any new vulnerabilities,
    as long as the user code knows what it is doing when it sets this flag).
    If Stas has a small set of userlevel programs (wine, UML, etc.) that need
    the ability to link in libraries that live on noexec partitions, then this
    would provide a safe way to meet this need in a backwards-compatible way.
    Of course, Stas would have to go patch those userlevel programs himself
    to add the MAP_IREALLYMEANIT flag, but that's his problem.

    8) The value of noexec is proportional to the number of partitions that
    you can mount with the noexec flag enabled. If the noexec semantics are
    tightened down so strictly that you can't enable noexec on any mount
    (because otherwise too many userlevel programs break), then the value
    of noexec is nil. If loosening the noexec restrictions slightly allows
    you to mark more partitions as noexec, then this is a net win (assuming
    that continues to reject programs that live on noexec partitions).

    9) Stas's request is a request for a change to Linux kernel semantics.
    The current mmap() semantics have been there for years. We're not talking
    about some recent change to mmap() semantics that have broken existing
    userspace tools. Rather, we have a longstanding set of semantics; Stas
    wants to be able to mark more partitions as noexec, and he is requesting
    a change to the semantics to do so. (I have to apologize; I see that
    this was obvious to everyone else, but I somehow failed to understand
    this part.) Stas's proposed change is well-motivated and I'm inclined
    to be sympathetic to the motivation behind it. Nonetheless, there is
    an philosophy that changes to existing kernel semantics have to be well
    justified, and in borderline cases, it is perfectly reasonable for the
    maintainers to reject such a patch. The burden is on the proposer of
    the change to make a compelling case for the change.

    10) At a minimum, if Stas wants to propose a change, he should test his
    proposed change to make sure that they don't break's noexec check.
    I can't tell whether Stas has done that. I don't see any evidence that
    he has. Based on my description of in point 2) above, I would
    expect Stas's patches to break If this is correct, then I think
    the current patch is unacceptable and needs to be re-thought.

    11) Given all of this, I've changed my mind. I think it may be premature
    to accept this patch, and the negative reactions from others seem entirely
    reasonable to me. I think Stas probably has some more work to do if he
    wants to push this patch.
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      Last update: 2006-09-29 03:45    [W:0.048 / U:60.620 seconds]
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