Thread (70 messages) 70 messages, 7 authors, 2019-06-15

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

From: Pavel Machek <hidden>
Date: 2019-06-11 10:33:20
Also in: linux-arch, linux-doc, linux-mm, lkml

On Mon 2019-06-10 08:47:45, Yu-cheng Yu wrote:
On Sat, 2019-06-08 at 22:52 +0200, Pavel Machek wrote:
quoted
Hi!
quoted
quoted
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
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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