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

[James Morse]

Hi Will, Wei,

On 20/06/18 17:25, Wei Xu wrote:
> On 2018/6/20 23:54, James Morse wrote:
> 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?

> The log is as below:
>     [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x480fd010]
>     [    0.000000] Linux version 4.17.0-45865-g2b31fe7-dirty

Could you reproduce this with v4.17? This says there are ~45,000 extra patches,
and un-committed changes. None of the hashes so far have been commits in
mainline, so we have no idea what this tree is.


> (joyx@Turing-Arch-b) (gcc version 4.9.1 20140505 (prerelease) (crosstool-NG
> linaro-1.13.1-4.9-2014.05 - Linaro GCC 4.9-2014.05)) #10 SMP PREEMPT Wed Jun 20
> 23:59:05 CST 2018

>     [    0.000000] CPU0: using LPI pending table @0x000000007d860000
>     [    0.000000] GIC: PPI11 is secure or misconfigured
>     [    0.000000] arch_timer: WARNING: Invalid trigger for IRQ3, assuming level
> low
>     [    0.000000] arch_timer: WARNING: Please fix your firmware
>     [    0.000000] arch_timer: cp15 timer(s) running at 100.00MHz (virt).

(No idea what these mean, but I doubt they are relevant)


>     [    0.042421] Insufficient stack space to handle exception!
>     [    0.042423] ESR: 0x96000046 -- DABT (current EL)
>     [    0.043730] FAR: 0xffff0000093a80e0
>     [    0.044714] Task stack: [0xffff0000093a8000..0xffff0000093ac000]

This was a level 2 translation fault on a write, to an address that is within
the stack....


>     [    0.051113] IRQ stack: [0xffff000008000000..0xffff000008004000]
>     [    0.057610] Overflow stack: [0xffff80003efce2f0..0xffff80003efcf2f0]
>     [    0.064003] CPU: 0 PID: 12 Comm: migration/0 Not tainted
> 4.17.0-45865-g2b31fe7-dirty #10
>     [    0.072201] Hardware name: linux,dummy-virt (DT)

>     [    0.076797] pstate: 604003c5 (nZCv DAIF +PAN -UAO)
>     [    0.081727] pc : el1_sync+0x0/0xb0

... from the vectors.


>     [    0.085217] lr : kpti_install_ng_mappings+0x120/0x214

What I think is happening is: we come out of the kpti idmap with the stack
unmapped. Shortly after we access the stack, which faults. el1_sync faults as
well when it tries to push the registers to the stack, and we keep going until
we overflow the stack.

I can't reproduce this with kvmtool or qemu in the model.


Thanks,

James