Messages in this thread Patch in this message | | | Subject | Re: [RFC][PATCH 4/5 v2] x86: Keep current stack in NMI breakpoints | From | Steven Rostedt <> | Date | Thu, 15 Dec 2011 14:15:35 -0500 |
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On Wed, 2011-12-14 at 08:43 -0500, Mathieu Desnoyers wrote:
> What happens to the following sequence ? > > - Hit a breakpoint. > - Execute an interrupt handler nesting over breakpoint handler (made > possible by preempt_conditional_sti(regs) in do_int3()). > (or take any kind of fault that switch the current stack) > - NMI fires, not detecting that it is nested over a breakpoint handler, > thus potentially corrupting the DEBUG stack. > > Instead of trying to detect if we nest on a stack to find out if we need > to change the IDT, I would recommend to unconditionally switch the int3 > IDT to use the current stack upon outermost NMI entry, and set it back > to its usual behavior upon outermost NMI exit.
There's also this simple patch to guarantee that NMIs know the debug stack is in use:
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index d748d1f..2fef5ba 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -402,6 +402,8 @@ DECLARE_PER_CPU(char *, irq_stack_ptr); DECLARE_PER_CPU(unsigned int, irq_count); extern unsigned long kernel_eflags; extern asmlinkage void ignore_sysret(void); +void inc_debug_stack_usage(void); +void dec_debug_stack_usage(void); int is_debug_stack(unsigned long addr); void zero_debug_stack(void); void reset_debug_stack(void); @@ -420,6 +422,8 @@ struct stack_canary { DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary); #endif static inline int is_debug_stack(unsigned long addr) { return 0; } +static inline void inc_debug_stack_usage(void) { } +static inline void dec_debug_stack_usage(void) { } static inline void zero_debug_stack(void) { } static inline void reset_debug_stack(void) { } #endif /* X86_64 */ diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 98faeff..f1ec612 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1093,11 +1093,23 @@ unsigned long kernel_eflags; DEFINE_PER_CPU(struct orig_ist, orig_ist); static DEFINE_PER_CPU(unsigned long, debug_stack_addr); +static DEFINE_PER_CPU(int, debug_stack_usage); + +void inc_debug_stack_usage(void) +{ + __get_cpu_var(debug_stack_usage)++; +} + +void dec_debug_stack_usage(void) +{ + __get_cpu_var(debug_stack_usage)--; +} int is_debug_stack(unsigned long addr) { - return addr <= __get_cpu_var(debug_stack_addr) && - addr > (__get_cpu_var(debug_stack_addr) - DEBUG_STKSZ); + return __get_cpu_var(debug_stack_usage) || + (addr <= __get_cpu_var(debug_stack_addr) && + addr > (__get_cpu_var(debug_stack_addr) - DEBUG_STKSZ)); } void zero_debug_stack(void) diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index a93c5ca..d2510e7 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -316,9 +316,15 @@ dotraplinkage void __kprobes do_int3(struct pt_regs *regs, long error_code) return; #endif + /* + * Let others (NMI) know that the debug stack is in use + * as we may switch to the interrupt stack. + */ + inc_debug_stack_usage(); preempt_conditional_sti(regs); do_trap(3, SIGTRAP, "int3", regs, error_code, NULL); preempt_conditional_cli(regs); + dec_debug_stack_usage(); } #ifdef CONFIG_X86_64 @@ -411,6 +417,12 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code) SIGTRAP) == NOTIFY_STOP) return; + /* + * Let others (NMI) know that the debug stack is in use + * as we may switch to the interrupt stack. + */ + inc_debug_stack_usage(); + /* It's safe to allow irq's after DR6 has been saved */ preempt_conditional_sti(regs); @@ -418,6 +430,7 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code) handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code, 1); preempt_conditional_cli(regs); + dec_debug_stack_usage(); return; } @@ -437,6 +450,7 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code) if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp) send_sigtrap(tsk, regs, error_code, si_code); preempt_conditional_cli(regs); + dec_debug_stack_usage(); return; }
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