Messages in this thread | | | Date | Sat, 12 Apr 2014 16:31:47 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels |
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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
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