Re: [tip:x86/asm] x86/entry: Add enter_from_user_mode() and use it in syscalls

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jul 07, 2015 at 03:51:29AM -0700, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  feed36cde0a10adb957445a37e48f957f30b2273
> Gitweb:     http://git.kernel.org/tip/feed36cde0a10adb957445a37e48f957f30b2273
> Author:     Andy Lutomirski <luto@kernel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:25 -0700
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 7 Jul 2015 10:59:06 +0200
> 
> x86/entry: Add enter_from_user_mode() and use it in syscalls
> 
> Changing the x86 context tracking hooks is dangerous because
> there are no good checks that we track our context correctly.
> Add a helper to check that we're actually in CONTEXT_USER when
> we enter from user mode and wire it up for syscall entries.
> 
> Subsequent patches will wire this up for all non-NMI entries as
> well.  NMIs are their own special beast and cannot currently
> switch overall context tracking state.  Instead, they have their
> own special RCU hooks.
> 
> This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
> branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a
> layer of indirection).  Eventually, we should fix up the core
> context tracking code to supply a function that does what we
> want (and can be much simpler than user_exit), which will enable
> us to get rid of the extra call.
> 
> 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/853b42420066ec3fb856779cdc223a6dcb5d355b.1435952415.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/entry/common.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 917d0c3..9a327ee 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -28,6 +28,15 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/syscalls.h>
>  
> +#ifdef CONFIG_CONTEXT_TRACKING
> +/* Called on entry from user mode with IRQs off. */
> +__visible void enter_from_user_mode(void)
> +{
> +	CT_WARN_ON(ct_state() != CONTEXT_USER);
> +	user_exit();
> +}
> +#endif
> +
>  static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
>  {
>  #ifdef CONFIG_X86_64
> @@ -65,14 +74,16 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
>  	work = ACCESS_ONCE(current_thread_info()->flags) &
>  		_TIF_WORK_SYSCALL_ENTRY;
>  
> +#ifdef CONFIG_CONTEXT_TRACKING
>  	/*
>  	 * If TIF_NOHZ is set, we are required to call user_exit() before
>  	 * doing anything that could touch RCU.
>  	 */
>  	if (work & _TIF_NOHZ) {
> -		user_exit();
> +		enter_from_user_mode();
>  		work &= ~_TIF_NOHZ;

We should move the sanity check to user_exit/enter() and use user_exit/enter()
only when we actually enter/exit user. Here it's the case but syscall_trace_leave()
and do_notify_resume() are special case that should probably use exception_enter/exit()
unless your patchset have changed things such that there is only one call to user_exit()
once we completed everything before resuming userspace. I need to review the rest of
the patchset to discover that :-)

>  	}
> +#endif
>  
>  #ifdef CONFIG_SECCOMP
>  	/*