Re: KVM guest sometimes failed to boot because of kernel stack overflow if KPTI is enabled on a hisilicon ARM64 platform.

[Wei Xu]

Hi Will,

On 2018/6/21 0:28, Will Deacon wrote:
> On Thu, Jun 21, 2018 at 12:25:05AM +0800, Wei Xu wrote:
> > Hi James,
> >
> > On 2018/6/20 23:54, James Morse wrote:
> > > Hi Wei,
> > >
> > > On 20/06/18 16:52, Wei Xu wrote:
> > > > On 2018/6/20 22:42, Will Deacon wrote:
> > > > > Hmm, I wonder if this is at all related to RAS, since we've just enabled
> > > > > that and if we take a fault whilst rewriting swapper then we're going to
> > > > > get stuck. What happens if you set CONFIG_ARM64_RAS_EXTN=n in the guest?
> > > > I will try it now.
> > > It's not just the Kconfig symbol, could you also revert:
> > >
> > > f751daa4f9d3 ("arm64: Unconditionally enable IESB on exception entry/return for
> > > firmware-first")
> > >
> > >
> > > (reverts and build cleanly on 4.17)
> > Thanks to point out this!
> > I have disabled CONFIG_ARM64_RAS_EXTN and reverted that commit.
> > But I still got the stack overflow issue sometimes.
> > Do you have more hint?
> [...]
>
> >      [    0.076797] pstate: 604003c5 (nZCv DAIF +PAN -UAO)
> >      [    0.081727] pc : el1_sync+0x0/0xb0
> >      [    0.085217] lr : kpti_install_ng_mappings+0x120/0x214
> Please run:
>
> $ ./scripts/faddr2line vmlinux kpti_install_ng_mappings+0x120/0x214

Thanks for your kindly guide :)
The output is as below:

    joyx@Turing-Arch-b:~/plinth-kernel-v200$ ./scripts/faddr2line 
../kernel-dev.build/vmlinux kpti_install_ng_mappings+0x120/0x214
    kpti_install_ng_mappings+0x120/0x214:
    cpu_set_reserved_ttbr0 at arch/arm64/include/asm/mmu_context.h:52
    47      /*
    48       * Set TTBR0 to empty_zero_page. No translations will be 
possible via TTBR0.
    49       */
    50      static inline void cpu_set_reserved_ttbr0(void)
    51      {
    52              unsigned long ttbr = 
phys_to_ttbr(__pa_symbol(empty_zero_page));
    53
    54              write_sysreg(ttbr, ttbr0_el1);
    55              isb();
    56      }
    57
    (inlined by) cpu_uninstall_idmap at 
arch/arm64/include/asm/mmu_context.h:123
    118      */
    119     static inline void cpu_uninstall_idmap(void)
    120     {
    121             struct mm_struct *mm = current->active_mm;
    122
    123             cpu_set_reserved_ttbr0();
    124             local_flush_tlb_all();
    125             cpu_set_default_tcr_t0sz();
    126
    127             if (mm != &init_mm && !system_uses_ttbr0_pan())
    128                     cpu_switch_mm(mm->pgd, mm);
    (inlined by) kpti_install_ng_mappings at 
arch/arm64/kernel/cpufeature.c:922
    917
    918             remap_fn = (void 
*)__pa_symbol(idmap_kpti_install_ng_mappings);
    919
    920             cpu_install_idmap();
    921             remap_fn(cpu, num_online_cpus(), 
__pa_symbol(swapper_pg_dir));
    922             cpu_uninstall_idmap();
    923
    924             if (!cpu)
    925                     kpti_applied = true;
    926
    927             return;

Thanks!

Best Regards,
Wei

> as the GDB output wasn't helpful (it only showed local variable
> declarations?!).
>
> Will
>
> .