Re: [PATCH] x86, traps: Fix ist_enter from userspace

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Jan 31, 2015 at 05:01:34AM -0800, Andy Lutomirski wrote:
> context_tracking_user_exit() has no effect if in_interrupt() returns true,
> so ist_enter() didn't work.  Fix it by calling exception_enter(), and thus
> context_tracking_user_exit(), before incrementing the preempt count.
> 
> This also adds an assertion that will catch the problem reliably if
> CONFIG_PROVE_RCU=y to help prevent the bug from being reintroduced.
> 
> Fixes: 959274753857 x86, traps: Track entry into and exit from IST context
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/kernel/traps.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 7176f84f95a4..c74f2f5652da 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -110,15 +110,11 @@ static inline void preempt_conditional_cli(struct pt_regs *regs)
> 
>  enum ctx_state ist_enter(struct pt_regs *regs)
>  {
> -	/*
> -	 * We are atomic because we're on the IST stack (or we're on x86_32,
> -	 * in which case we still shouldn't schedule.
> -	 */
> -	preempt_count_add(HARDIRQ_OFFSET);
> +	enum ctx_state prev_state;
> 
>  	if (user_mode_vm(regs)) {
>  		/* Other than that, we're just an exception. */
> -		return exception_enter();
> +		prev_state = exception_enter();
>  	} else {
>  		/*
>  		 * We might have interrupted pretty much anything.  In
> @@ -127,12 +123,27 @@ enum ctx_state ist_enter(struct pt_regs *regs)
>  		 * but we need to notify RCU.
>  		 */
>  		rcu_nmi_enter();
> -		return IN_KERNEL;  /* the value is irrelevant. */
> +		prev_state = IN_KERNEL;  /* the value is irrelevant. */
>  	}
> +
> +	/*
> +	 * We are atomic because we're on the IST stack (or we're on x86_32,
> +	 * in which case we still shouldn't schedule).
> +	 *
> +	 * This must be after exception_enter(), because exception_enter()
> +	 * won't do anything if in_interrupt() returns true.
> +	 */
> +	preempt_count_add(HARDIRQ_OFFSET);

So the reason we need this here is that ist_exit() does not have
access to the bits tested by user_mode_vm()?  Or is the idea to
avoid an additional test in ist_exit()?  (Wondering why this isn't
in the above "else" clause.)

							Thanx, Paul

> +
> +	/* This code is a bit fragile.  Test it. */
> +	rcu_lockdep_assert(rcu_is_watching(), "ist_enter didn't work");
> +
> +	return prev_state;
>  }
> 
>  void ist_exit(struct pt_regs *regs, enum ctx_state prev_state)
>  {
> +	/* Must be before exception_exit. */
>  	preempt_count_sub(HARDIRQ_OFFSET);
> 
>  	if (user_mode_vm(regs))
> -- 
> 2.1.0
>