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

[Alexander van Heukelum]

On Sun, Apr 13, 2014, at 1:31, H. Peter Anvin wrote:
> >>> 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.

No self modifying code... The far jump must be in the indirect form
anyhow. The CS:EIP must be accessible from user mode, but not
necessarily from compatibility mode. So the trampoline (the jump)
and data (CS:EIP) can live pretty much anywhere in virtual memory.
But indeed, I see what you meant now.

Greetings,
   Alexander

> 
> 	-hpa
> 
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