Re: Fixing MIPS delay slot emulation weakness?

From: Paul Burton
Date: Sun Dec 16 2018 - 03:35:20 EST

Hi Andy,

On Sat, Dec 15, 2018 at 11:19:37AM -0800, Andy Lutomirski wrote:
> Some security researchers pointed out that writing to the delay slot
> emulation page is a great exploit technique on MIPS. It was
> introduced in:
> commit 432c6bacbd0c16ec210c43da411ccc3855c4c010
> Author: Paul Burton <paul.burton@xxxxxxxxxx>
> Date: Fri Jul 8 11:06:19 2016 +0100
> MIPS: Use per-mm page to execute branch delay slot instructions

Are there any further details you can share? You'd still need to
persuade a program to both write to & jump to the page, right? We're
talking purely about this providing writable+executable memory?

For the record prior to this patch we had to keep the user's stack
executable & write instructions there, so this didn't make things any

> With my vDSO hat on, I hereby offer a couple of straightforward
> suggestions for fixing it. The offending code is:
> base = mmap_region(NULL, STACK_TOP, PAGE_SIZE,
> 0, NULL);
> VM_WRITE | VM_EXEC is a big no-no, especially at a fixed address.
> The really simple but possibly suboptimal fix is to get rid of
> VM_WRITE and to use get_user_pages(..., FOLL_FORCE) to write to it.
> A possibly nicer way to accomplish more or less the same thing would
> be to allocate the area with _install_special_mapping() and arrange to
> keep a reference to the struct page around.

Right, I can look into that.

> The really nice but less compatible fix would be to let processes or
> even the whole system opt out by promising not to put anything in FPU
> branch delay slots, of course.

The ultimate fix comes with a switch to the nanoMIPS ISA which has no
delay slots :)