Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

H. Peter Anvin
2014-04-12T23:32:20Z

On 04/12/2014 04:26 PM, Alexander van Heukelum wrote:
>>>
>>> c. Trampoline in kernel space
>>>
>>> A trampoline in kernel space is not feasible since all ring transition
>>> instructions capable of returning to 16-bit mode require the use of the
>>> stack.
>
> "16 bit mode" -> "a mode with 16-bit stack"

Yes... I believe it is the SS.B bit that is relevant, not CS.B (although
I haven't confirmed that experimentally.) Not that that helps one iota,
as far as I can tell.

>>> d. Trampoline in user space
>>>
>>> A return to the vdso with values set up in registers r8-r15 would enable
>>> a trampoline in user space. Unfortunately there is no way
>>> to do a far JMP entirely with register state so this would require
>>> touching user space memory, possibly in an unsafe manner.
>
> d.2. trampoline in user space via long mode
>
> Return from the kernel to a user space trampoline via long mode.
> The kernel changes the stack frame just before executing the iret
> instruction. (the CS and RIP slots are set to run the trampoline code,
> where CS is a long mode segment.) The trampoline code in userspace
> is set up to this single instruction: a far jump to the final CS:EIP
> (compatibility mode).

This still requires user space memory that the kernel can write to.
Long mode is actually exactly identical to what I was suggesting above,
except that I would avoid using self-modifying code in favor of just
parameterization using the high registers.

-hpa