diff options
Diffstat (limited to 'arch/x86/kernel/nmi.c')
| -rw-r--r-- | arch/x86/kernel/nmi.c | 47 |
1 files changed, 33 insertions, 14 deletions
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index a0b2f84..f84f5c5 100644 --- a/arch/x86/kernel/nmi.c +++ b/arch/x86/kernel/nmi.c @@ -365,8 +365,9 @@ static __kprobes void default_do_nmi(struct pt_regs *regs) #ifdef CONFIG_X86_32 /* * For i386, NMIs use the same stack as the kernel, and we can - * add a workaround to the iret problem in C. Simply have 3 states - * the NMI can be in. + * add a workaround to the iret problem in C (preventing nested + * NMIs if an NMI takes a trap). Simply have 3 states the NMI + * can be in: * * 1) not running * 2) executing @@ -383,32 +384,50 @@ static __kprobes void default_do_nmi(struct pt_regs *regs) * If an NMI hits a breakpoint that executes an iret, another * NMI can preempt it. We do not want to allow this new NMI * to run, but we want to execute it when the first one finishes. - * We set the state to "latched", and the first NMI will perform - * an cmpxchg on the state, and if it doesn't successfully - * reset the state to "not running" it will restart the next - * NMI. + * We set the state to "latched", and the exit of the first NMI will + * perform a dec_return, if the result is zero (NOT_RUNNING), then + * it will simply exit the NMI handler. If not, the dec_return + * would have set the state to NMI_EXECUTING (what we want it to + * be when we are running). In this case, we simply jump back + * to rerun the NMI handler again, and restart the 'latched' NMI. + * + * No trap (breakpoint or page fault) should be hit before nmi_restart, + * thus there is no race between the first check of state for NOT_RUNNING + * and setting it to NMI_EXECUTING. The HW will prevent nested NMIs + * at this point. + * + * In case the NMI takes a page fault, we need to save off the CR2 + * because the NMI could have preempted another page fault and corrupt + * the CR2 that is about to be read. As nested NMIs must be restarted + * and they can not take breakpoints or page faults, the update of the + * CR2 must be done before converting the nmi state back to NOT_RUNNING. + * Otherwise, there would be a race of another nested NMI coming in + * after setting state to NOT_RUNNING but before updating the nmi_cr2. */ enum nmi_states { - NMI_NOT_RUNNING, + NMI_NOT_RUNNING = 0, NMI_EXECUTING, NMI_LATCHED, }; static DEFINE_PER_CPU(enum nmi_states, nmi_state); +static DEFINE_PER_CPU(unsigned long, nmi_cr2); #define nmi_nesting_preprocess(regs) \ do { \ - if (__get_cpu_var(nmi_state) != NMI_NOT_RUNNING) { \ - __get_cpu_var(nmi_state) = NMI_LATCHED; \ + if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) { \ + this_cpu_write(nmi_state, NMI_LATCHED); \ return; \ } \ - nmi_restart: \ - __get_cpu_var(nmi_state) = NMI_EXECUTING; \ - } while (0) + this_cpu_write(nmi_state, NMI_EXECUTING); \ + this_cpu_write(nmi_cr2, read_cr2()); \ + } while (0); \ + nmi_restart: #define nmi_nesting_postprocess() \ do { \ - if (cmpxchg(&__get_cpu_var(nmi_state), \ - NMI_EXECUTING, NMI_NOT_RUNNING) != NMI_EXECUTING) \ + if (unlikely(this_cpu_read(nmi_cr2) != read_cr2())) \ + write_cr2(this_cpu_read(nmi_cr2)); \ + if (this_cpu_dec_return(nmi_state)) \ goto nmi_restart; \ } while (0) #else /* x86_64 */ |