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

    * Marcus Meissner <> 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
    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.

    - ( It will break a few tools that can be run as a plain user out of box - perf
    for example. "chmod a+r /proc/kallsyms" during bootup will work that around so
    it's not the end of the world. )

    - For self-built kernels it might make sense - but there's "chmod a-r
    /proc/kallsyms" during bootup one can do already.

    - There's the side-question of module symbols - those are dynamically allocated
    hence arguably per system. But module symbols make up only 1% on a typical
    booted up full distro box.

    So what does a distribution like Suse expect from this change alone? Those have
    public packages in rpms which can be downloaded by anyone, so it makes little sense
    to hide it - unless _all_ version information is hidden.

    So i'd like to see a _full_ version info sandboxing patch that thinks through all
    the angles and restricts uname -r kernel version info as well, and makes dmesg
    unaccessible to users - and closes a few other information holes as well that give
    away the exact kernel version - _that_ together will make it hard to blindly attack
    a very specific kernel version.

    But without actually declaring and achieving that sandboxing goal this security
    measure is just a feel-good thing really - and makes it harder to make more
    difficult steps down the road, like closing 'uname -r' ...

    I fully expect Linus to overrule me on this, but hey, i had to try it and lay out my
    arguments :-)



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