Re: [PATCH 1/2] kvm: arm/arm64: Fix race in resetting stage2 PGD

[Marc Zyngier]

On 03/05/17 15:17, Suzuki K Poulose wrote:
> In kvm_free_stage2_pgd() we check the stage2 PGD before holding
> the lock and proceed to take the lock if it is valid. And we unmap
> the page tables, followed by releasing the lock. We reset the PGD
> only after dropping this lock, which could cause a race condition
> where another thread waiting on or even holding the lock, could
> potentially see that the PGD is still valid and proceed to perform
> a stage2 operation and later encounter a NULL PGD.
> 
> [223090.242280] Unable to handle kernel NULL pointer dereference at
> virtual address 00000040
> [223090.262330] PC is at unmap_stage2_range+0x8c/0x428
> [223090.262332] LR is at kvm_unmap_hva_handler+0x2c/0x3c
> [223090.262531] Call trace:
> [223090.262533] [<ffff0000080adb78>] unmap_stage2_range+0x8c/0x428
> [223090.262535] [<ffff0000080adf40>] kvm_unmap_hva_handler+0x2c/0x3c
> [223090.262537] [<ffff0000080ace2c>] handle_hva_to_gpa+0xb0/0x104
> [223090.262539] [<ffff0000080af988>] kvm_unmap_hva+0x5c/0xbc
> [223090.262543] [<ffff0000080a2478>]
> kvm_mmu_notifier_invalidate_page+0x50/0x8c
> [223090.262547] [<ffff0000082274f8>]
> __mmu_notifier_invalidate_page+0x5c/0x84
> [223090.262551] [<ffff00000820b700>] try_to_unmap_one+0x1d0/0x4a0
> [223090.262553] [<ffff00000820c5c8>] rmap_walk+0x1cc/0x2e0
> [223090.262555] [<ffff00000820c90c>] try_to_unmap+0x74/0xa4
> [223090.262557] [<ffff000008230ce4>] migrate_pages+0x31c/0x5ac
> [223090.262561] [<ffff0000081f869c>] compact_zone+0x3fc/0x7ac
> [223090.262563] [<ffff0000081f8ae0>] compact_zone_order+0x94/0xb0
> [223090.262564] [<ffff0000081f91c0>] try_to_compact_pages+0x108/0x290
> [223090.262569] [<ffff0000081d5108>] __alloc_pages_direct_compact+0x70/0x1ac
> [223090.262571] [<ffff0000081d64a0>] __alloc_pages_nodemask+0x434/0x9f4
> [223090.262572] [<ffff0000082256f0>] alloc_pages_vma+0x230/0x254
> [223090.262574] [<ffff000008235e5c>] do_huge_pmd_anonymous_page+0x114/0x538
> [223090.262576] [<ffff000008201bec>] handle_mm_fault+0xd40/0x17a4
> [223090.262577] [<ffff0000081fb324>] __get_user_pages+0x12c/0x36c
> [223090.262578] [<ffff0000081fb804>] get_user_pages_unlocked+0xa4/0x1b8
> [223090.262579] [<ffff0000080a3ce8>] __gfn_to_pfn_memslot+0x280/0x31c
> [223090.262580] [<ffff0000080a3dd0>] gfn_to_pfn_prot+0x4c/0x5c
> [223090.262582] [<ffff0000080af3f8>] kvm_handle_guest_abort+0x240/0x774
> [223090.262584] [<ffff0000080b2bac>] handle_exit+0x11c/0x1ac
> [223090.262586] [<ffff0000080ab99c>] kvm_arch_vcpu_ioctl_run+0x31c/0x648
> [223090.262587] [<ffff0000080a1d78>] kvm_vcpu_ioctl+0x378/0x768
> [223090.262590] [<ffff00000825df5c>] do_vfs_ioctl+0x324/0x5a4
> [223090.262591] [<ffff00000825e26c>] SyS_ioctl+0x90/0xa4
> [223090.262595] [<ffff000008085d84>] el0_svc_naked+0x38/0x3c
> 
> This patch moves the stage2 PGD manipulation under the lock.
> 
> Reported-by: Alexander Graf <agraf@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Reviewed-by: Christoffer Dall <cdall@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx>

	M.
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