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

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Nov 21, 2014 at 1:26 PM, Andy Lutomirski <luto@amacapital.net> 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.

I should add:

I have no idea why RCU read-side critical sections are safe inside
__do_page_fault today.  It's guarded by exception_enter(), but that
doesn't do anything if context tracking is off, and context tracking
is usually off. What am I missing here?

--Andy

>
> 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/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index bc8352e7010a..eb16a61bfd06 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -1,6 +1,7 @@
>  #ifndef _ASM_X86_TRAPS_H
>  #define _ASM_X86_TRAPS_H
>
> +#include <linux/context_tracking_state.h>
>  #include <linux/kprobes.h>
>
>  #include <asm/debugreg.h>
> @@ -109,6 +110,9 @@ asmlinkage void smp_thermal_interrupt(void);
>  asmlinkage void mce_threshold_interrupt(void);
>  #endif
>
> +extern enum ctx_state ist_enter(struct pt_regs *regs);
> +extern void ist_exit(struct pt_regs *regs, enum ctx_state prev_state);
> +
>  /* Interrupts/Exceptions */
>  enum {
>         X86_TRAP_DE = 0,        /*  0, Divide-by-zero */
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index 61a9668cebfd..b72509d77337 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -43,6 +43,7 @@
>  #include <linux/export.h>
>
>  #include <asm/processor.h>
> +#include <asm/traps.h>
>  #include <asm/mce.h>
>  #include <asm/msr.h>
>
> @@ -1016,6 +1017,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>  {
>         struct mca_config *cfg = &mca_cfg;
>         struct mce m, *final;
> +       enum ctx_state prev_state;
>         int i;
>         int worst = 0;
>         int severity;
> @@ -1038,6 +1040,8 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>         DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
>         char *msg = "Unknown";
>
> +       prev_state = ist_enter(regs);
> +
>         this_cpu_inc(mce_exception_count);
>
>         if (!cfg->banks)
> @@ -1168,6 +1172,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>         mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
>  out:
>         sync_core();
> +       ist_exit(regs, prev_state);
>  }
>  EXPORT_SYMBOL_GPL(do_machine_check);
>
> diff --git a/arch/x86/kernel/cpu/mcheck/p5.c b/arch/x86/kernel/cpu/mcheck/p5.c
> index a3042989398c..ec2663a708e4 100644
> --- a/arch/x86/kernel/cpu/mcheck/p5.c
> +++ b/arch/x86/kernel/cpu/mcheck/p5.c
> @@ -8,6 +8,7 @@
>  #include <linux/smp.h>
>
>  #include <asm/processor.h>
> +#include <asm/traps.h>
>  #include <asm/mce.h>
>  #include <asm/msr.h>
>
> @@ -17,8 +18,11 @@ int mce_p5_enabled __read_mostly;
>  /* Machine check handler for Pentium class Intel CPUs: */
>  static void pentium_machine_check(struct pt_regs *regs, long error_code)
>  {
> +       enum ctx_state prev_state;
>         u32 loaddr, hi, lotype;
>
> +       prev_state = ist_enter(regs);
> +
>         rdmsr(MSR_IA32_P5_MC_ADDR, loaddr, hi);
>         rdmsr(MSR_IA32_P5_MC_TYPE, lotype, hi);
>
> @@ -33,6 +37,8 @@ static void pentium_machine_check(struct pt_regs *regs, long error_code)
>         }
>
>         add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
> +
> +       ist_exit(regs, prev_state);
>  }
>
>  /* Set up machine check reporting for processors with Intel style MCE: */
> diff --git a/arch/x86/kernel/cpu/mcheck/winchip.c b/arch/x86/kernel/cpu/mcheck/winchip.c
> index 7dc5564d0cdf..bd5d46a32210 100644
> --- a/arch/x86/kernel/cpu/mcheck/winchip.c
> +++ b/arch/x86/kernel/cpu/mcheck/winchip.c
> @@ -7,14 +7,19 @@
>  #include <linux/types.h>
>
>  #include <asm/processor.h>
> +#include <asm/traps.h>
>  #include <asm/mce.h>
>  #include <asm/msr.h>
>
>  /* Machine check handler for WinChip C6: */
>  static void winchip_machine_check(struct pt_regs *regs, long error_code)
>  {
> +       enum ctx_state prev_state = ist_enter(regs);
> +
>         printk(KERN_EMERG "CPU0: Machine Check Exception.\n");
>         add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
> +
> +       ist_exit(regs, prev_state);
>  }
>
>  /* Set up machine check reporting on the Winchip C6 series */
> 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. */
> +       }
> +}
> +
> +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();
> +}
> +
>  static nokprobe_inline int
>  do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
>                   struct pt_regs *regs, long error_code)
> @@ -244,14 +277,14 @@ dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
>  {
>         enum ctx_state prev_state;
>
> -       prev_state = exception_enter();
> +       prev_state = ist_enter(regs);
>         if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
>                        X86_TRAP_SS, SIGBUS) != NOTIFY_STOP) {
>                 preempt_conditional_sti(regs);
>                 do_trap(X86_TRAP_SS, SIGBUS, "stack segment", regs, error_code, NULL);
>                 preempt_conditional_cli(regs);
>         }
> -       exception_exit(prev_state);
> +       ist_exit(regs, prev_state);
>  }
>
>  dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> @@ -259,8 +292,8 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>         static const char str[] = "double fault";
>         struct task_struct *tsk = current;
>
> -       exception_enter();
> -       /* Return not checked because double check cannot be ignored */
> +       ist_enter(regs);
> +       /* Return not checked because double fault cannot be ignored */
>         notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);
>
>         tsk->thread.error_code = error_code;
> @@ -343,7 +376,7 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
>         if (poke_int3_handler(regs))
>                 return;
>
> -       prev_state = exception_enter();
> +       prev_state = ist_enter(regs);
>  #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
>         if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
>                                 SIGTRAP) == NOTIFY_STOP)
> @@ -369,7 +402,7 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
>         preempt_conditional_cli(regs);
>         debug_stack_usage_dec();
>  exit:
> -       exception_exit(prev_state);
> +       ist_exit(regs, prev_state);
>  }
>  NOKPROBE_SYMBOL(do_int3);
>
> @@ -433,7 +466,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
>         unsigned long dr6;
>         int si_code;
>
> -       prev_state = exception_enter();
> +       prev_state = ist_enter(regs);
>
>         get_debugreg(dr6, 6);
>
> @@ -508,7 +541,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
>         debug_stack_usage_dec();
>
>  exit:
> -       exception_exit(prev_state);
> +       ist_exit(regs, prev_state);
>  }
>  NOKPROBE_SYMBOL(do_debug);
>
> --
> 1.9.3
>



-- 
Andy Lutomirski
AMA Capital Management, LLC