Messages in this thread | | | Subject | Re: KVM guest sometimes failed to boot because of kernel stack overflow if KPTI is enabled on a hisilicon ARM64 platform. | From | Wei Xu <> | Date | Fri, 22 Jun 2018 09:33:04 +0100 |
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Hi Will,
On 2018/6/21 11:54, Will Deacon wrote: > Hi Wei, > > On Thu, Jun 21, 2018 at 11:14:28AM +0100, Wei Xu wrote: >> On 2018/6/21 10:18, Will Deacon wrote: >>> On Thu, Jun 21, 2018 at 09:38:53AM +0100, James Morse wrote: >>>> On 20/06/18 17:25, Wei Xu wrote: >>>>> [ 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. >>> >>> Hmm, one thing that occurs to me is that the kpti_install_ng_mappings() >>> code leaves the nG bit set in table entries, which is actually IGNORED in >>> the architecture. >>> >>> Wei -- does the diff below help at all? Make sure you disable CONFIG_KASAN, >>> otherwise your kernel will take an age to boot. >> >> Yes, amazing! This patch resolved the issue. > > Great... > >> I have tested 50 times and can not reproduce the issue any more. >> Could you please tell more why this patch works? > > You might need to ask your CPU design team ;) > > Without this patch, the code in idmap_kpti_install_ng_mappings() sets > bit 11 in table descriptors so that we can keep track of which parts of > the page table we've visited. With this patch, we don't bother tracking > and potentially rewalk parts of the page table (which takes a very long > time if KASAN is enabled).
Got it. Thanks!
> > The architecture documents I've looked at are clear that bit 11 is IGNORED > by the CPU, which: > > "Indicates that the architecture guarantees that the bit or field is not > interpreted or modified by hardware." > > Please can you double-check that your CPU is indeed ignoring bit 11 in > non-leaf (table) descriptors?
Do the non-leaf(table) descriptors mean the table descriptors of the section D4.3.1 "VMSAv8-64 translation table level 0, level 1, and level 2 descriptor formats" in the ARM Architecture Reference Manual ARMv8 for ARMv8-A(DDI0487C_a_armv8_arm.pdf)?
If yes, our hardware does ignore it(not interpret or modify).
Is there any other possible reason cause this? Thanks!
Best Regards, Wei
> > Thanks, > > Will > > . >
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