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    SubjectRe: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of attacking

    * Ingo Molnar <mingo@elte.hu> wrote:

    > * Marcus Meissner <meissner@suse.de> wrote:
    >
    > > On Thu, Nov 04, 2010 at 12:46:48PM +0100, Ingo Molnar wrote:
    > > >
    > > > * Marcus Meissner <meissner@suse.de> wrote:
    > > >
    > > > > Hi,
    > > > >
    > > > > Making /proc/kallsyms readable only for root makes it harder for attackers to
    > > > > write generic kernel exploits by removing one source of knowledge where things are
    > > > > in the kernel.
    > > >
    > > > Cc:-ed Linus - i think he argued in favor of such a patch in the past.
    > > >
    > > > I generally agree with such patches (i have written some myself), but there's a few
    > > > questions with this one, which make this limited change ineffective and which make
    > > > it harder to implement a fuller patch that makes it truly harder to figure out the
    > > > precise kernel build:
    > > >
    > > > - The real security obstruction effect is very small from this measure alone: the
    > > > overwhelming majority of our users are running distro kernels, so the Symbol.map
    > > > file (and hence 99% of /proc/kallsyms content) is well-known - unless we also
    > > > restrict 'uname -r' from nonprivileged users-ace. Hiding that might make sense -
    > > > but the two should be in one patch really.
    > >
    > > Of course. System.map and others also need to turn to mode 400.
    >
    > That is not what I meant, at all.
    >
    > It's not the System.map _on the system_.
    >
    > It's the SuSE or Fedora kernel rpm package with a System.map in it, which package
    > the attacker can download from a hundred mirrors on the internet, based on 'uname
    > -r' output.

    For example, on a Fedora testbox i have this version info:

    $ uname -r
    2.6.35.6-48.fc14.x86_64

    Any attacker can download that rpm from:

    http://download.fedora.redhat.com/pub/fedora/linux/updates/14/x86_64/kernel-2.6.35.6-48.fc14.x86_64.rpm

    And can extract the System.map from it, using rpm2cpio and cpio -i -d. That will
    include all the symbol addresses - without the attacker having any access to the
    System.map or /proc/kallsyms on this particular box.

    I.e. on distro kernel installations (which comprise the _vast_ majority of our
    userbase) your patch brings little security benefits.

    What i suggested in later parts of my mail might provide more security: to sandbox
    kernel version information from unprivileged user-space - if we decide that we want
    to sandbox kernel version information ...

    That is a big if, because it takes a considerable amount of work. Would be worth
    trying it - but feel-good non-solutions that do not bring much improvement to the
    majority of users IMHO hinder such efforts.

    Thanks,

    Ingo


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