Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

From: Frederic Weisbecker <fweisbec@gmail.com>
To: dvlasenk@redhat.com, riel@redhat.com, bp@alien8.de,
	peterz@infradead.org, brgerst@gmail.com,
	vda.linux@googlemail.com, keescook@chromium.org,
	tglx@linutronix.de, oleg@redhat.com, luto@kernel.org,
	luto@amacapital.net, torvalds@linux-foundation.org,
	mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org
Cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code
Date: Wed, 12 Aug 2015 00:38:28 +0200	[thread overview]
Message-ID: <20150811223827.GB15639@lerouge> (raw)
In-Reply-To: <tip-02bc7768fe447ae305e924b931fa629073a4a1b9@git.kernel.org>

On Tue, Jul 07, 2015 at 03:53:29AM -0700, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  02bc7768fe447ae305e924b931fa629073a4a1b9
> Gitweb:     http://git.kernel.org/tip/02bc7768fe447ae305e924b931fa629073a4a1b9
> Author:     Andy Lutomirski <luto@kernel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:31 -0700
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 7 Jul 2015 10:59:08 +0200
> 
> x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Denys Vlasenko <vda.linux@googlemail.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: paulmck@linux.vnet.ibm.com
> Link: http://lkml.kernel.org/r/60e90901eee611e59e958bfdbbe39969b4f88fe5.1435952415.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/entry/entry_64.S        | 64 +++++++++++-----------------------------
>  arch/x86/entry/entry_64_compat.S |  5 ++++
>  2 files changed, 23 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 83eb63d..168ee26 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1088,7 +1055,12 @@ ENTRY(error_entry)
>  	SWAPGS
>  
>  .Lerror_entry_from_usermode_after_swapgs:
> +#ifdef CONFIG_CONTEXT_TRACKING
> +	call enter_from_user_mode
> +#endif

This makes me very nervous as well!

It means that instead of using the context tracking save/restore model that we had
with exception_enter/exception_exit(), now we rely on the CS register.

I don't think we can do that because our "context tracking" is a soft tracking whereas
CS is hard tracking and both are not atomically synchronized together.

Imagine this situation: we are running in userspace. Context tracking knows it, everything
is fine. Now we do a syscall, we enter in kernel entry code but we trigger an exception
(DEBUG for example) before we got a chance to call user_exit(), which means that the context
tracking code still thinks we are in userspace, so we look at CS from the exception entry code
and it says the exception happened in the kernel. Hence we don't call user_exit() before calling
the exception handler. There is the bug because the exception handler may use RCU which still
thinks we run in userspace.

In early context tracking days we have relied on CS. But I changed that because of such
issue. The only reliable source for soft context tracking is the soft context tracking itself.