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TUCoPS :: Linux :: General :: hack0692.htm

Linux Kernel i2c Integer Overflow Vulnerability



Linux Kernel i2c Integer Overflow Vulnerability

_________________________________________________________

Product:      Linux Kernel
              i2c driver
Versions:     2.4.x

Bug:          Integer Overflow
Impact:       Kernel Panic
              Code execution with kernel privs
Risk:         High/Medium
Date:         June 16, 2004
Author:       shaun2k2
              http://www.nettwerked.co.uk 
_________________________________________________________




Introduction
#############

The Linux Kernel is the core of the Linux Operating
System, and provides the usual features of a modern
multi-user kernel.  Drivers to support many different
devices are available packaged with the Linux Kernel,
including the 'i2c' driver, which provides support for
the 2-wire I2C bus.

In the i2c driver, there is an integer overflow
vulnerability during the allocation of memory,
potentially opening any systems using the i2c driver
up to a security hole.



Details
########

The integer overflow problem becomes present when
allocating memory, which is allowed to occur because
of a lack of sanity checks.  Below is the vulnerable
code, which is part of the i2cproc_bus_read() routine,
in the i2c-core.c component of the driver.

--- vuln code ---
ssize_t i2cproc_bus_read(struct file * file, char *
buf,size_t count, 
                          loff_t *ppos)
 {
         struct inode * inode =
file->f_dentry->d_inode;
         char *kbuf;
         struct i2c_client *client;
         int i,j,k,order_nr,len=0;
         size_t len_total;
         int order[I2C_CLIENT_MAX];
 
         if (count > 4000)
                 return -EINVAL; 
         len_total = file->f_pos + count;
         /* Too bad if this gets longer (unlikely) */
         if (len_total > 4000)
                 len_total = 4000;
         for (i = 0; i < I2C_ADAP_MAX; i++)
                 if (adapters[i]->inode ==
inode->i_ino) {
                 /* We need a bit of slack in the
kernel buffer; this makes the
                    sprintf safe. */
                         if (! (kbuf = kmalloc(count +
80,GFP_KERNEL)))
                                 return -ENOMEM;

[...]

--- end snippet ---

Although a quick check is made to ensure that the
user-supplied variable 'count' does not exceed 4000,
sanity checks do not occur to check for negative
integers in 'count'.  Since negative integers simply
become _very_ large integers when represented as
unsigned, a negative count argument to kmalloc() would
cause unexpected behavior:

---
 if (! (kbuf = kmalloc(count + 80,GFP_KERNEL)))
---

For example, if '-1' was passed to the routine as the
'count' argument, the above kmalloc() call would be
equivalent to below:

---
 if (! (kbuf = kmalloc(0xffffffff + 80,GFP_KERNEL)))
---

This would cause an integer overflow during the
kmalloc() call when 80 is added to count, resulting in
a very small amount of memory being allocated.

As in the comment just above the vulnerable kmalloc()
call (/* We need a bit of slack in the kernel buffer;
this makes the sprintf safe. */), the purpose of
incrementing the 'count' argument by 80 is to stop the
chance of a buffer overflow, but by supplying a
suitable negative integer as 'count' (i.e -1), this
allows an integer overflow, causing the kmalloc()
argument to wrap back round to a small/negative value.

In the sprintf() calls following the kmalloc() call,
there is quite a possibility of overflowing the bounds
of the newly allocated very small chunk of memory. 
This might result in kernel panic, corruption of
kernel memory, or maybe even elevation of privileges
(*very* unlikely).

i2cproc_bus_read() is implemented as a read() hook in
the driver, as below:

---
static struct file_operations i2cproc_operations = {
         read:           i2cproc_bus_read,
 };
---

This might allow unprivileged users to exploit the
issue.

Please take note that this potential security hole
only affects those using the i2c driver -- if this
driver (it can be installed as either a module or
built into the kernel) is not installed on your
system, you're not vulnerable.  The issue is present
in all 2.4 kernels, including the latest release.



Solution
#########

The following sanity check can be added to the
beginning of the i2cproc_bus_read() in the i2c-core.c
file:

---
if(count < 0)
        return -EINVAL;
---

Then rebuild the kernel, and the issue should be
resolved.

A possible workaround would be to perhaps disable the
module or remove the driver if it's not needed on your
system.



Disclaimer
###########

The information contained within this advisory was
believed to be accurate at the time of it's
publishing.  However, it might be inaccurate at times,
so don't consider any information contained within
definitely correct.

Direct flames to /dev/null.  Don't bothering wasting
your time and mine with any crap about any disclosure
policies I may or may not have followed -- I'm not
interested, so I'll just ignore you if you don't
phrase things nicely.



Thank you for your time.
Shaun.


	
	
		
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