Re: __schedule #DF splat

From: Gleb Natapov
Date: Sun Jun 29 2014 - 09:43:01 EST

On Sun, Jun 29, 2014 at 03:14:43PM +0200, Borislav Petkov wrote:
> On Sun, Jun 29, 2014 at 02:22:35PM +0200, Jan Kiszka wrote:
> > OK, looks like I won ;):
>
> I gladly let you win. :-P
>
> > The issue was apparently introduced with "KVM: x86: get CPL from
> > SS.DPL" (ae9fedc793). Maybe we are not properly saving or restoring
> > this state on SVM since then.
>
> I wonder if this change in the CPL saving would have anything to do with
> the fact that we're doing a CR3 write right before we fail pagetable
> walk and end up walking a user page table. It could be unrelated though,
> as in the previous dump I had a get_user right before the #DF. Hmmm.
>
> I better go and revert that one and check whether it fixes things.
Please do so and let us know.

>
> > Need a break, will look into details later.
>
> Ok, some more info from my side, see relevant snippet below. We're
> basically not finding the pte at level 3 during the page walk for
> 7fff0b0f8908.
>
> However, why we're even page walking this userspace address at that
> point I have no idea.
>
> And the CR3 write right before this happens is there so I'm pretty much
> sure by now that this is related...
>
> qemu-system-x86-5007 [007] ...1 346.126204: vcpu_match_mmio: gva 0xffffffffff5fd0b0 gpa 0xfee000b0 Write GVA
> qemu-system-x86-5007 [007] ...1 346.126204: kvm_mmio: mmio write len 4 gpa 0xfee000b0 val 0x0
> qemu-system-x86-5007 [007] ...1 346.126205: kvm_apic: apic_write APIC_EOI = 0x0
> qemu-system-x86-5007 [007] ...1 346.126205: kvm_eoi: apicid 0 vector 253
> qemu-system-x86-5007 [007] d..2 346.126206: kvm_entry: vcpu 0
> qemu-system-x86-5007 [007] d..2 346.126211: kvm_exit: reason write_cr3 rip 0xffffffff816113a0 info 8000000000000000 0
> qemu-system-x86-5007 [007] ...2 346.126214: kvm_mmu_get_page: sp gen 25 gfn 7b2b1 4 pae q0 wux !nxe root 0 sync existing
> qemu-system-x86-5007 [007] d..2 346.126215: kvm_entry: vcpu 0
> qemu-system-x86-5007 [007] d..2 346.126216: kvm_exit: reason PF excp rip 0xffffffff816113df info 2 7fff0b0f8908
> qemu-system-x86-5007 [007] ...1 346.126217: kvm_page_fault: address 7fff0b0f8908 error_code 2
VCPU faults on 7fff0b0f8908.

> qemu-system-x86-5007 [007] ...1 346.126218: kvm_mmu_pagetable_walk: addr 7fff0b0f8908 pferr 2 W
> qemu-system-x86-5007 [007] ...1 346.126219: kvm_mmu_paging_element: pte 7b2b6067 level 4
> qemu-system-x86-5007 [007] ...1 346.126220: kvm_mmu_paging_element: pte 0 level 3
> qemu-system-x86-5007 [007] ...1 346.126220: kvm_mmu_walker_error: pferr 2 W
Address is not mapped by the page tables.

> qemu-system-x86-5007 [007] ...1 346.126221: kvm_multiple_exception: nr: 14, prev: 255, has_error: 1, error_code: 0x2, reinj: 0
> qemu-system-x86-5007 [007] ...1 346.126221: kvm_inj_exception: #PF (0x2)
KVM injects #PF.

> qemu-system-x86-5007 [007] d..2 346.126222: kvm_entry: vcpu 0
> qemu-system-x86-5007 [007] d..2 346.126223: kvm_exit: reason PF excp rip 0xffffffff816113df info 2 7fff0b0f8908
> qemu-system-x86-5007 [007] ...1 346.126224: kvm_multiple_exception: nr: 14, prev: 14, has_error: 1, error_code: 0x2, reinj: 1
reinj:1 means that previous injection failed due to another #PF that
happened during the event injection itself This may happen if GDT or fist
instruction of a fault handler is not mapped by shadow pages, but here
it says that the new page fault is at the same address as the previous
one as if GDT is or #PF handler is mapped there. Strange. Especially
since #DF is injected successfully, so GDT should be fine. May be wrong
cpl makes svm crazy?


> qemu-system-x86-5007 [007] ...1 346.126225: kvm_page_fault: address 7fff0b0f8908 error_code 2
> qemu-system-x86-5007 [007] ...1 346.126225: kvm_mmu_pagetable_walk: addr 7fff0b0f8908 pferr 0
> qemu-system-x86-5007 [007] ...1 346.126226: kvm_mmu_paging_element: pte 7b2b6067 level 4
> qemu-system-x86-5007 [007] ...1 346.126227: kvm_mmu_paging_element: pte 0 level 3
> qemu-system-x86-5007 [007] ...1 346.126227: kvm_mmu_walker_error: pferr 0
> qemu-system-x86-5007 [007] ...1 346.126228: kvm_mmu_pagetable_walk: addr 7fff0b0f8908 pferr 2 W
> qemu-system-x86-5007 [007] ...1 346.126229: kvm_mmu_paging_element: pte 7b2b6067 level 4
> qemu-system-x86-5007 [007] ...1 346.126230: kvm_mmu_paging_element: pte 0 level 3
> qemu-system-x86-5007 [007] ...1 346.126230: kvm_mmu_walker_error: pferr 2 W
> qemu-system-x86-5007 [007] ...1 346.126231: kvm_multiple_exception: nr: 14, prev: 14, has_error: 1, error_code: 0x2, reinj: 0
Here we getting a #PF while delivering another #PF which is, rightfully, transformed to #DF.

> qemu-system-x86-5007 [007] ...1 346.126231: kvm_inj_exception: #DF (0x0)
> qemu-system-x86-5007 [007] d..2 346.126232: kvm_entry: vcpu 0
> qemu-system-x86-5007 [007] d..2 346.126371: kvm_exit: reason io rip 0xffffffff8131e623 info 3d40220 ffffffff8131e625
> qemu-system-x86-5007 [007] ...1 346.126372: kvm_pio: pio_write at 0x3d4 size 2 count 1 val 0x130e
> qemu-system-x86-5007 [007] ...1 346.126374: kvm_userspace_exit: reason KVM_EXIT_IO (2)
> qemu-system-x86-5007 [007] d..2 346.126383: kvm_entry: vcpu 0
>

--
Gleb.
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