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


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