In linux-2.6.16, we have noticed a problem where the gs base value
returned from an arch_prtcl(ARCH_GET_GS, ...) call will be incorrect if:

   - the current/calling task has NOT set its own gs base yet to a
     non-zero value,

   - some other task that ran on the same processor previously set their
     own gs base to a non-zero value.

In this situation, the ARCH_GET_GS code will read and return the
MSR_KERNEL_GS_BASE msr register.

However, since the __switch_to() code does NOT load/zero the
MSR_KERNEL_GS_BASE register when the task that is switched IN has a zero
next->gs value, the caller of arch_prctl(ARCH_GET_GS, ...) will get back
the value of some previous tasks's gs base value instead of 0.

    Change the arch_prctl() ARCH_GET_GS code to only read and return
    the MSR_KERNEL_GS_BASE msr register if the 'gs' register of the calling
    task is non-zero.

    Side note: Since in addition to using arch_prctl(ARCH_SET_GS, ...),
    a task can also setup a gs base value by using modify_ldt() and write
    an index value into 'gs' from user space, the patch below reads
    'gs' instead of using thread.gs, since in the modify_ldt() case,
    the thread.gs value will be 0, and incorrect value would be returned
    (the task->thread.gs value).

    When the user has not set its own gs base value and the 'gs'
    register is zero, then the MSR_KERNEL_GS_BASE register will not be
    read and a value of zero will be returned by reading and returning
    'task->thread.gs'.

    The first patch shown below is an attempt at implementing this
    approach.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>