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

Borislav Petkov
2014-11-22T17:20:31Z

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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