Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

[Borislav Petkov <bp@alien8.de>]

On Fri, Nov 21, 2014 at 01:26:08PM -0800, Andy Lutomirski wrote:
> We currently pretend that IST context is like standard exception
> context, but this is incorrect.  IST entries from userspace are like
> standard exceptions except that they use per-cpu stacks, so they are
> atomic.  IST entries from kernel space are like NMIs from RCU's
> perspective -- they are not quiescent states even if they
> interrupted the kernel during a quiescent state.
> 
> Add and use ist_enter and ist_exit to track IST context.  Even
> though x86_32 has no IST stacks, we track these interrupts the same
> way.
> 
> This fixes two issues:
> 
>  - Scheduling from an IST interrupt handler will now warn.  It would
>    previously appear to work as long as we got lucky and nothing
>    overwrote the stack frame.  (I don't know of any bugs in this
>    that would trigger the warning, but it's good to be on the safe
>    side.)
> 
>  - RCU handling in IST context was dangerous.  As far as I know,
>    only machine checks were likely to trigger this, but it's good to
>    be on the safe side.
> 
> Note that the machine check handlers appears to have been missing
> any context tracking at all before this patch.
> 
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Frédéric Weisbecker <fweisbec@gmail.com>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/include/asm/traps.h         |  4 +++
>  arch/x86/kernel/cpu/mcheck/mce.c     |  5 ++++
>  arch/x86/kernel/cpu/mcheck/p5.c      |  6 +++++
>  arch/x86/kernel/cpu/mcheck/winchip.c |  5 ++++
>  arch/x86/kernel/traps.c              | 49 ++++++++++++++++++++++++++++++------
>  5 files changed, 61 insertions(+), 8 deletions(-)

...

> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 0d0e922fafc1..f5c4b8813774 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -107,6 +107,39 @@ static inline void preempt_conditional_cli(struct pt_regs *regs)
>  	preempt_count_dec();
>  }
>  
> +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);
> +
> +	if (user_mode_vm(regs)) {
> +		/* Other than that, we're just an exception. */
> +		return exception_enter();
> +	} else {
> +		/*
> +		 * We might have interrupted pretty much anything.  In
> +		 * fact, if we're a machine check, we can even interrupt
> +		 * NMI processing.  We don't want in_nmi() to return true,
> +		 * but we need to notify RCU.
> +		 */
> +		rcu_nmi_enter();
> +		return IN_KERNEL;  /* the value is irrelevant. */
> +	}

I guess dropping the explicit else-branch could make it look a bit nicer
with the curly braces gone and all...

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);

        if (user_mode_vm(regs))
                /* Other than that, we're just an exception. */
                return exception_enter();

        /*
         * We might have interrupted pretty much anything.  In fact, if we're a
         * machine check, we can even interrupt NMI processing.  We don't want
         * in_nmi() to return true, but we need to notify RCU.
         */
        rcu_nmi_enter();
        return IN_KERNEL;  /* the value is irrelevant. */
} 

> +}
> +
> +void ist_exit(struct pt_regs *regs, enum ctx_state prev_state)
> +{
> +	preempt_count_sub(HARDIRQ_OFFSET);
> +
> +	if (user_mode_vm(regs))
> +		return exception_exit(prev_state);
> +	else
> +		rcu_nmi_exit();
> +}

Ditto here.

> +
>  static nokprobe_inline int
>  do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
>  		  struct pt_regs *regs,	long error_code)

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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