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Linux Kernel ALSA snd_mem_proc_read Information Disclosure Vulnerability

09.25.07

BACKGROUND

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.

http://kernel.org/
http://www.alsa-project.org/main/index.php/Main_Page

DESCRIPTION

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.

ANALYSIS

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.

DETECTION

iDefense has confirmed the existence of this vulnerability in version 2.6.22.1 of the Linux Kernel as installed with Fedora CORE 7. It is suspected that other versions are also vulnerable.

WORKAROUND

The following workarounds will prevent exploitation of this vulnerability.

 * 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 

VENDOR RESPONSE

The Linux Kernel maintainers have addressed this vulnerability within version 2.6.22.8. More information can be found from the URLs shown below.

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=ccec6e2c4a74adf76ed4e2478091a311b1806212
http://kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.22.8

CVE INFORMATION

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.

DISCLOSURE TIMELINE

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

CREDIT

This vulnerability was reported to iDefense by Neil Kettle (mu-b) of www.digit-labs.org.

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LEGAL NOTICES

Copyright © 2007 Verisign, Inc.

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Disclaimer: The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.