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    SubjectRe: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space)
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    On Wed, Jun 3, 2009 at 11:38 AM, Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    >
    >
    > On Wed, 3 Jun 2009, Christoph Lameter wrote:
    >
    >> On Wed, 3 Jun 2009, Linus Torvalds wrote:
    >>
    >> > The point being that we do need to support mmap at zero. Not necessarily
    >> > universally, but it can't be some fixed "we don't allow that".
    >>
    >> Hmmm... Depend on some capability? CAP_SYS_PTRACE may be something
    >> remotely related?
    >
    > But as mentioned several times, we do have the system-wide setting in
    > 'mmap_min_addr' (that then can be overridden by CAP_SYS_RAWIO, so in that
    > sense a capability already exists).
    >
    > It defaults to 64kB in at least the x86 defconfig files, but to 0 in the
    > Kconfig defaults. Also, for some reason it has a "depends on SECURITY",
    > which means that if you just default to the old-style unix security you'll
    > lose it.
    >
    > So there are several ways to disable it by mistake. I don't know what
    > distros do.

    Fedora has it on.

    As I recall the only need for CONFIG_SECURITY is for the ability to
    override the check.

    I think I could probably pretty cleanly change it to use
    CAP_SYS_RAWIO/SELinux permissions if CONFIG_SECURITY and just allow it
    for uid=0 in the non-security case? Deny it for everyone in the
    non-security case and make them change the /proc tunable if they need
    it?

    -Eric


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