linux-kernel - Re: __schedule #DF splat

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Message-ID: <20140629131443.GA5199@pd.tnic>
Date:	Sun, 29 Jun 2014 15:14:43 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Jan Kiszka <jan.kiszka@....de>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>, x86-ml <x86@...nel.org>,
	kvm@...r.kernel.org, Jörg Rödel <joro@...tes.org>
Subject: Re: __schedule #DF splat

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.

> 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
 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
 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)
 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
 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
 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

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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