linux-kernel - Re: [PATCH] x86/mm: Remove broken vsyscall emulation code from the page fault code

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Date: Mon, 29 Apr 2024 16:29:54 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, Hillf Danton <hdanton@...a.com>, Peter Anvin <hpa@...or.com>, 
	Adrian Bunk <bunk@...nel.org>, 
	syzbot <syzbot+83e7f982ca045ab4405c@...kaller.appspotmail.com>, 
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, andrii@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] x86/mm: Remove broken vsyscall emulation code from the
 page fault code

On Mon, Apr 29, 2024 at 12:07 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, 29 Apr 2024 at 11:47, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > In particular, I think the page fault emulation code should be moved
> > from do_user_addr_fault() to do_kern_addr_fault(), and the horrible
> > hack that is fault_in_kernel_space() should be removed (it is what now
> > makes a vsyscall page fault be treated as a user address, and the only
> > _reason_ for that is that we do the vsyscall handling in the wrong
> > place).
>
> Final note: we should also remove the XONLY option entirely, and
> remove all the strange page table handling we currently do for it.
>
> It won't work anyway on future CPUs with LASS, and we *have* to
> emulate things (and not in the page fault path, I think LASS will
> cause a GP fault).

What strange page table handling do we do for XONLY?

EMULATE actually involves page tables.  XONLY is just in the "gate
area" (which is more or less just a procfs thing) and the page fault
code.

So I think we should remove EMULATE before removing XONLY.  We already
tried pretty hard to get everyone to stop using EMULATE.