Re: Fixing MIPS delay slot emulation weakness?

[Rich Felker <dalias@libc.org>]

On Sat, Dec 15, 2018 at 11:19:37AM -0800, Andy Lutomirski wrote:
> Hi all-
> 
> 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@imgtec.com>
> Date:   Fri Jul 8 11:06:19 2016 +0100
> 
>     MIPS: Use per-mm page to execute branch delay slot instructions
> 
> 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,
>                            VM_READ|VM_WRITE|VM_EXEC|
>                            VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
>                            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.
> 
> 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.

As I noted on Twitter when Mudge brought this topic back up, there's a
much more compatible, elegant, and safe fix possible that does not
involve any W+X memory. Emulate the delay slot in kernel-space. This
is trivial to do safely for pretty much everything but loads/stores.
For loads/stores, where you want them to execute with user privilege
level, what you do is compute the effective address in kernel-space,
then return to a fixed instruction in the vdso page that performs a
generic load/store using the register the kernel put the effective
address result in, then restores registers off the stack and jumps to
the branch destination.

Rich