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

Wed, 20 Jun 2018 09:34:15 -0700 

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

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