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    SubjectRe: [RFC] ext2 and ext3 block reservations can be bypassed
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    On Sun, May 12, 2002 at 02:15:12PM -0400, Alexander Viro wrote:
    >
    > On Sun, 12 May 2002, Elladan wrote:
    > >
    > > It just happens that the suid program wasn't the one who chose what file
    > > it was going to write stdout to - the shell did.
    > >
    > > Thus, the security violation.
    >
    > <shrug> relying on 5% in security-sensitive setup is *dumb*.
    > In that case you need properly set quota (better yet, no lusers with write
    > access anywhere on that fs)..
    >
    > There are worse holes problems 5% rule. E.g. you can create a
    > file with hole, mmap over that hole, dirty the pages and exit. Guess
    > who ends up writing them out? Right, kswapd. Which is run as root.
    > No suid applications required...

    Regardless of whether it's a good thing to depend on security-wise, it
    is a problem to have something that appears to be a security feature
    which doesn't actually work.

    Reading the documentation, I would expect that if I create an ext3
    filesystem, reserve 20% of it for root, and then run a cron job as root
    that uses 16% of that filesystem periodically, that cron job will not
    actually fail whenever some luser decides they want it to.

    I don't recall seeing it plastered in huge letters, "This space reserve
    is just a convenience feature, and can easily be circumvented by the
    user. You must never rely on it for anything."

    Having unsupported security features is typically a bad idea.

    -J
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