Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 4 Nov 2010 15:33:22 +0100 | From | Marcus Meissner <> | Subject | Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of attacking |
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On Thu, Nov 04, 2010 at 03:11:04PM +0100, Ingo Molnar wrote: > > * 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.
Hiding the OS version is really quite hard I think.
I mean the kernel could hide it from uname, but lsb_release, /etc/redhat-release, /etc/SuSE-release etc still exist and then you can still use the fixed address list table inside your exploit. But an exploits needs to have such a list, making it harder to write.
If we avoid exploits being able to just do open("/boot/System.map") it would make it a useful step harder for exploit writers.
(This will end up a arms race between us and the exploit toolkit writers of course, but hopefully not a longer one than fixing all actual problems ;)
I also briefly thought about kernel ASLR, but my knowledge of the kernel loading is too limited whether this is even possible or at all useful.
Ciao, Marcus
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