Re: [PATCH] Lock down drivers that can have io ports, io mem, irqs and dma changed

From: Corey Minyard
Date: Mon Nov 28 2016 - 17:32:35 EST

On 11/21/2016 05:10 PM, David Howells wrote:
One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> wrote:

You need to filter or lock down kernel module options because a lot of
modules let you set the I/O port or similar (eg mmio) which means you can
hack the entire machine with say the 8250 driver just by using it with an
mmio of the right location to patch the secure state to zero just by
getting the ability to write to the modules conf file.
Is the attached patch the right sort of idea? [Note that I haven't actually
compiled most of these drivers to check my changes yet.]



diff --git a/drivers/char/ipmi/ipmi_si_intf.c b/drivers/char/ipmi/ipmi_si_intf.c
index a112c0146012..7fb9c299a183 100644
--- a/drivers/char/ipmi/ipmi_si_intf.c
+++ b/drivers/char/ipmi/ipmi_si_intf.c
@@ -3725,6 +3725,12 @@ static int init_ipmi_si(void)
struct smi_info *e;
enum ipmi_addr_src type = SI_INVALID;
+ if ((num_addrs || num_ports || num_irqs) &&
+ kernel_is_locked_down()) {
+ pr_err(PFX "Kernel is locked down\n");
+ return -EPERM;
+ }
if (initialized)
return 0;
initialized = 1;

This would prevent any IPMI interface from working if any address was given
on the kernel command line. I'm not sure what the best policy is, but that
sounds like a possible DOS to me.

Can you put this check in hardcode_find_bmc()? Thats the only place where
the hardcoded addresses are used, and a check there won't affect anything

Also, the error message sounds a little vague to me. If I was a sysadmin
and got this, I wouldn't be sure what was going on. Maybe something like:
The kernel is locked down, but hard-coded device addresses were given on
the driver command line. Ignoring these, but this is a possible security issue.

That's fairly wordy, but it gets the point across. You could also move the
pr_err() into kernel_is_locked_down() and pass in the prefix, since there is
basically the same pr_err() after every check.