Fixing MIPS delay slot emulation weakness?

[Andy Lutomirski]

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@xxxxxxxxxx>
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.

--Andy