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TUCoPS :: Linux :: General :: hack2739.htm

linux kernel Sbus PROM driver multiple integer overflows



linux kernel Sbus PROM driver multiple integer overflows

Sbus PROM driver multiple integer overflows
-------------------------------------------

Description:

    This character device driver allows user programs to access the PROM
    device tree. It is compatible with the SunOS /dev/openprom driver and the
    NetBSD /dev/openprom driver. The SunOS eeprom utility works without any
    modifications.

Source:
    
    This driver is distributed by default in all of the distros that I have.
    It's source can be found:
        /usr/src/linux/drivers/sbus/char/openprom.c

Impact:
    
    Local DOS, possible code execution depending on location of the overflowed
    buffer in the slab.

Bug:

    There exist multiple integer overflows in routines that handle copying in
    user supplied data.  In both routines, user supplied parameters are used
    to allocate buffers after being added to another number.  Passing certain
    values will result in integer overflows and small buffers being allocated.
    Then large amounts of user data are copied into these buffers.  Both of
    the functions in which the overflows occur are 'utility' functions called
    from multiple other functions.  An ioctl() call is the simplest way to
    access them.


Detail:

    The copyin() and the copyin_string() function are both vulnerable:


/* Copy in a whole string from userspace into kernelspace. */
static int copyin_string(char *user, size_t len, char **ptr)
{
        char *tmp;

    /* ME: it's not signed; if( len + 1 < len ) .. */
1]      if (len < 0 || len + 1 < 0)
                return -EINVAL;

2]      tmp = kmalloc(len + 1, GFP_KERNEL);
        if (!tmp)
                return -ENOMEM;

3]      if(copy_from_user(tmp, user, len)) {
                kfree(tmp);
                return -EFAULT;
        }
..... snip  .....

    In this function it appears they forgot that size_t is not signed.  The
    checks made at 1] will never be true, and by passing in ~0 for the len
    variable an integer overflow occurs at 2].  Then at 3] user supplied data
    is copied int this buffer.  In various ioctl() calls for both the bsd and
    sun versions the user has full control over the user and len variables.
    

static int copyin(struct openpromio *info, struct openpromio **opp_p)
{
        int bufsize;

        if (!info || !opp_p)
                return -EFAULT;

1]      if (get_user(bufsize, &info->oprom_size))
                return -EFAULT;

        if (bufsize == 0)
                return -EINVAL;

        /* If the bufsize is too large, just limit it.
         * Fix from Jason Rappleye.
 	 * ME: apparently not fixed enough
         */
2]      if (bufsize > OPROMMAXPARAM)
                bufsize = OPROMMAXPARAM;

3]      if (!(*opp_p = kmalloc(sizeof(int) + bufsize + 1, GFP_KERNEL)))
                return -ENOMEM;
        memset(*opp_p, 0, sizeof(int) + bufsize + 1);

4]      if (copy_from_user(&(*opp_p)->oprom_array,
                           &info->oprom_array, bufsize)) {
                kfree(*opp_p);
                return -EFAULT;
        }
        return bufsize;
}

    The bufsize integer variable obtained from the user at 1] is checked for
    being to large at 2], but it is not checked for being negative.  we then 
    have an overflow occuring at 3] when the buffer is allocated.  Then at 4] 
    this buffer is used to hold user data, resulting in memory being overwritten.

Vendor Status:
	
    The driver authors from the source listing were contacted and didn't reply.

-- 
-sean


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