Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
From: Yu-cheng Yu
Date: Mon Jun 10 2019 - 12:00:17 EST
On Sat, 2019-06-08 at 22:52 +0200, Pavel Machek wrote:
> > > I've no idea what the kernel should do; since you failed to answer the
> > > question what happens when you point this to garbage.
> > >
> > > Does it then fault or what?
> > Yeah, I think you'll fault with a rather mysterious CR2 value since
> > you'll go look at the instruction that faulted and not see any
> > references to the CR2 value.
> > I think this new MSR probably needs to get included in oops output when
> > CET is enabled.
> > Why don't we require that a VMA be in place for the entire bitmap?
> > Don't we need a "get" prctl function too in case something like a JIT is
> > running and needs to find the location of this bitmap to set bits itself?
> > Or, do we just go whole-hog and have the kernel manage the bitmap
> > itself. Our interface here could be:
> > prctl(PR_MARK_CODE_AS_LEGACY, start, size);
> > and then have the kernel allocate and set the bitmap for those code
> > locations.
> For the record, that sounds like a better interface than userspace knowing
> about the bitmap formats...
Initially we implemented the bitmap that way. To manage the bitmap, every time
the application issues a syscall for a .so it loads, and the kernel does
copy_from_user() & copy_to_user() (or similar things). If a system has a few
legacy .so files and every application does the same, it can take a long time to