svn commit: r328166 - in head/sys: amd64/amd64 x86/include x86/x86

[Dexuan-BSD Cui]

Hi,
Today I found the KPTI patch broke FreeBSD VM running on Hyper-V: the VM
can't boot due to:

vmbus0: cannot find free IDT vector

This is the related snippet:

dev/hyperv/vmbus/vmbus.c: vmbus_intr_setup() -> lapic_ipi_alloc() fails:

        /*
         * All Hyper-V ISR required resources are setup, now let's find a
         * free IDT vector for Hyper-V ISR and set it up.
         */
        sc->vmbus_idtvec = lapic_ipi_alloc(pti ? IDTVEC(vmbus_isr_pti) :
            IDTVEC(vmbus_isr));
        if (sc->vmbus_idtvec < 0) {
                device_printf(sc->vmbus_dev, "cannot find free IDT
vector\n");
                return ENXIO;
        }

Luckily for now I can work around this boot failure by adding vm.pmap.pti=0
into /boot/loader.conf.

Any suggestion?


Thanks!

-- Dexuan


On Fri, Jan 19, 2018 at 7:42 AM, Ed Maste <emaste at freebsd.org> wrote:

> Author: emaste
> Date: Fri Jan 19 15:42:34 2018
> New Revision: 328166
> URL: https://svnweb.freebsd.org/changeset/base/328166
>
> Log:
>   Enable KPTI by default on amd64 for non-AMD CPUs
>
>   Kernel Page Table Isolation (KPTI) was introduced in r328083 as a
>   mitigation for the 'Meltdown' vulnerability.  AMD CPUs are not affected,
>   per https://www.amd.com/en/corporate/speculative-execution:
>
>       We believe AMD processors are not susceptible due to our use of
>       privilege level protections within paging architecture and no
>       mitigation is required.
>
>   Thus default KPTI to off for AMD CPUs, and to on for others.  This may
>   be refined later as we obtain more specific information on the sets of
>   CPUs that are and are not affected.
>
>   Submitted by: Mitchell Horne
>   Reviewed by:  cem
>   Relnotes:     Yes
>   Security:     CVE-2017-5754
>   Sponsored by: The FreeBSD Foundation
>   Differential Revision:        https://reviews.freebsd.org/D13971
>
> Modified:
>   head/sys/amd64/amd64/machdep.c
>   head/sys/x86/include/x86_var.h
>   head/sys/x86/x86/identcpu.c
>
> Modified: head/sys/amd64/amd64/machdep.c
> ============================================================
> ==================
> --- head/sys/amd64/amd64/machdep.c      Fri Jan 19 15:32:27 2018
> (r328165)
> +++ head/sys/amd64/amd64/machdep.c      Fri Jan 19 15:42:34 2018
> (r328166)
> @@ -1621,6 +1621,7 @@ hammer_time(u_int64_t modulep, u_int64_t physfree)
>         mtx_init(&dt_lock, "descriptor tables", NULL, MTX_DEF);
>
>         /* exceptions */
> +       pti = pti_get_default();
>         TUNABLE_INT_FETCH("vm.pmap.pti", &pti);
>
>         for (x = 0; x < NIDT; x++)
>
> Modified: head/sys/x86/include/x86_var.h
> ============================================================
> ==================
> --- head/sys/x86/include/x86_var.h      Fri Jan 19 15:32:27 2018
> (r328165)
> +++ head/sys/x86/include/x86_var.h      Fri Jan 19 15:42:34 2018
> (r328166)
> @@ -136,6 +136,7 @@ void        nmi_call_kdb_smp(u_int type, struct
> trapframe *fr
>  void   nmi_handle_intr(u_int type, struct trapframe *frame);
>  void   pagecopy(void *from, void *to);
>  void   printcpuinfo(void);
> +int    pti_get_default(void);
>  int    user_dbreg_trap(void);
>  int    minidumpsys(struct dumperinfo *);
>  struct pcb *get_pcb_td(struct thread *td);
>
> Modified: head/sys/x86/x86/identcpu.c
> ============================================================
> ==================
> --- head/sys/x86/x86/identcpu.c Fri Jan 19 15:32:27 2018        (r328165)
> +++ head/sys/x86/x86/identcpu.c Fri Jan 19 15:42:34 2018        (r328166)
> @@ -1608,6 +1608,16 @@ finishidentcpu(void)
>  #endif
>  }
>
> +int
> +pti_get_default(void)
> +{
> +
> +       if (strcmp(cpu_vendor, AMD_VENDOR_ID) == 0)
> +               return (0);
> +
> +       return (1);
> +}
> +
>  static u_int
>  find_cpu_vendor_id(void)
>  {
>
>