Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

On Mon 2019-06-10 08:47:45, Yu-cheng Yu wrote:

On Sat, 2019-06-08 at 22:52 +0200, Pavel Machek wrote:

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Hi!

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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...
									Pavel

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
boot up.

Loading .so is already many syscalls, I'd not expect measurable
performance there. Are you sure?
								Pavel
-- 
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