CVE-2018-14634

An integer overflow flaw was found in the Linux kernel’s create_elf_tables() function. An unprivileged local user with access to SUID (or otherwise privileged) binary could use this flaw to escalate their privileges on the system.

Mitigation

To mitigate the issue:

Enable and install kernel-debuginfo packages as per <https://access.redhat.com/solutions/666123&gt;

1. On the host, save the following in a file with the ".stp" extension:

// CVE-2018-14634
//
// Theory of operations: adjust the thread's # rlimit-in-effect around
// calls to the vulnerable get_arg_page() function so as to encompass
// the newly required _STK_LIM / 4 * 3 maximum.

// Complication: the rlimit is stored in a current-> structure that
// is shared across the threads of the process. They may concurrently
// invoke this operation.

function clamp_stack_rlim_cur:long ()
%{
struct rlimit *rlim = current->signal->rlim;
unsigned long rlim_cur = READ_ONCE(rlim[RLIMIT_STACK].rlim_cur);

unsigned long limit = _STK_LIM / 4 * 3;
limit *= 4; // multiply it back up, to the scale used by rlim_cur

if (rlim_cur > limit) {
WRITE_ONCE(rlim[RLIMIT_STACK].rlim_cur, limit);
STAP_RETURN(limit);
} else
STAP_RETURN(0);
%}

probe kernel.function("copy_strings").call
{
l = clamp_stack_rlim_cur()
if (l)
printf("lowered process %s(%d) STACK rlim_cur to %p\n",
execname(), pid(), l)
}

probe begin {
printf("CVE-2018-14634 mitigation loaded\n")

}

probe end {
printf("CVE-2018-14634 mitigation unloaded\n")
}

2. Install the "systemtap" package and any required dependencies. Refer
   to the "2. Using SystemTap" chapter in the Red Hat Enterprise Linux
   "SystemTap Beginners Guide" document, available from docs.redhat.com,
   for information on installing the required -debuginfo and matching kernel-devel packages

3. Run the "stap -g [filename-from-step-1].stp" command as root.

If the host is rebooted, the changes will be lost and the script must be
run again.

Alternatively, build the systemtap script on a development system with
"stap -g -p 4 [filename-from-step-1].stp", distribute the resulting
kernel module to all affected systems, and run "staprun -L <module>" on those.
When using this approach only systemtap-runtime package is required on
the affected systems. Please notice that the kernel version must be the same
across all systems.

This may not be a suitable workaround if your application uses massive amounts of stack space. Please consider this if there are any adverse affects when running this mitigation.