[VulnWatch] iDefense Security Advisory 09.25.07: Linux Kernel ALSA snd_mem_proc_read Information Disclosure Vulnerability

From: iDefense Labs (labs-no-reply@private)
Date: Tue Sep 25 2007 - 08:43:23 PDT

Linux Kernel ALSA snd_mem_proc_read Information Disclosure Vulnerability

iDefense Security Advisory 09.25.07
Sep 25, 2007


Linux is a clone of the UNIX operating system, written from scratch by
Linus Torvalds with assistance from a loosely-knit team of hackers
across the Internet. The Advanced Linux Sound Architecture (ALSA)
project provides audio device support for Linux systems. More
information can be found at the URLs shown below.




Local exploitation of an information disclosure vulnerability within the
ALSA driver included in the Linux Kernel allows attackers to obtain
sensitive information from kernel memory.

The problem lies within the handling of multiple reads from the
"/proc/driver/snd-page-alloc" file. The kernel side function that
handles the read system call, "snd_mem_proc_read", is defined in
sound/core/memalloc.c as shown below.

  484 static int snd_mem_proc_read(char *page, char **start, off_t off,
  485                              int count, int *eof, void *data)
  486 {
  487         int len = 0;
  494         len += snprintf(page + len, count - len,
  495                         "pages  : %li bytes (%li pages per %likB)\n",
  496                         pages * PAGE_SIZE, pages, PAGE_SIZE / 1024);
  508         return len;
  509 }

On line 494, snprintf is called to generate the output for the proc file
system entry. By supplying a count value of 1, snprintf will only write
a single byte to the destination buffer. However, the function will
return the number of bytes that would have been written if enough space
were available. The "*eof" value is never set, and the "*ppos" value is
never used.

This function is called from "proc_file_read" function, which is defined
in fs/proc/generic.c.

   51 static ssize_t
   52 proc_file_read(struct file *file, char __user *buf, size_t nbytes,
   53                loff_t *ppos)
   54 {
  136                         n = dp->read_proc(page, &start, *ppos,
  137                                           count, &eof, dp->data);
  155                         n -= *ppos;
  156                         if (n <= 0)
  157                                 break;
  158                         if (n > count)
  159                                 n = count;
  160                         start = page + *ppos;
  186                 n -= copy_to_user(buf, start < page ? page :
start, n);
  193                 *ppos += start < page ? (unsigned long)start : n;

The value "n" is returned from the call to the snd_proc_mem_read
function on line 136. Since the value returned, approximately 41 in
single device scenarios, is greater than the requested read size (1),
the value "n" is set to "count" on line 158. Later, "*ppos" is
incremented and "n" bytes are copied to user-land from "start" (which
is calculated as "page" + *ppos).

In subsequent user-land read operations, when "*ppos" is greater than
zero, the proc_file_read function will copy from beyond the part of the
page that snd_mem_proc_read wrote. This results in the disclosure of
kernel memory.


Exploitation of this vulnerability allows attackers to obtain sensitive
information from kernel memory.

In order to exploit this vulnerability, an attacker would need access to
open the /proc/driver/snd-page-alloc file. It is important to note that
this file does not exist unless an audio device is present.

Additionally, the Linux kernel must be built with ALSA support as well
as support for the proc file system. The kernels for the majority of
common Linux distributions are built with these options.

Since memory is only disclosed from the beginning of an uninitialized
page, it may not be possible to obtain certain types of information.
However, the ability to obtain the password hash for the root account
was confirmed during iDefense Labs testing.


iDefense has confirmed the existence of this vulnerability in version of the Linux Kernel as installed with Fedora CORE 7. It is
suspected that other versions are also vulnerable.


The following workarounds will prevent exploitation of this

  * If the ALSA drivers have been built as modules, unload the
snd_page_alloc module.
  * Restrict access to the /proc file system by modifying the mount
parameters within /etc/fstab


The Linux Kernel maintainers have addressed this vulnerability within
version More information can be found from the URLs shown




The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-4571 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.


09/12/2007  Initial vendor notification
09/12/2007  Initial vendor response
09/25/2007  Coordinated public disclosure


This vulnerability was reported to iDefense by Neil Kettle (mu-b) of

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Copyright  2007 iDefense, Inc.

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