Messages in this thread | | | Date | Sat, 12 Apr 2014 17:03:24 -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 04:49 PM, Alexander van Heukelum wrote: > 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. >
This is, in fact, exactly then what I was suggesting, except that data is passed directly in memory rather than in a register and letting user space sort it out (this could be in the vdso, but the vdso may be > 4 GB so it has to be in 64-bit mode until the last instruction.) The difference isn't huge; mostly an implementation detail.
A signal arriving while in the user space trampoline could seriously complicate life.
-hpa
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