[PATCH 1/3] arm64: KVM: Fix stage-2 PGD allocation to have per-page refcounting

[marc.zyngier@arm.com (Marc Zyngier)]

We're using __get_free_pages with to allocate the guest's stage-2
PGD. The standard behaviour of this function is to return a set of
pages where only the head page has a valid refcount.

This behaviour gets us into trouble when we're trying to increment
the refount on a non-head page:

page:ffff7c00cfb693c0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x4000000000000000()
page dumped because: VM_BUG_ON_PAGE((*({ __attribute__((unused)) typeof((&page->_count)->counter) __var = ( typeof((&page->_count)->counter)) 0; (volatile typeof((&page->_count)->counter) *)&((&page->_count)->counter); })) <= 0)
BUG: failure@include/linux/mm.h:548/get_page()!
Kernel panic - not syncing: BUG!
CPU: 1 PID: 1695 Comm: kvm-vcpu-0 Not tainted 4.0.0-rc1+ #3825
Hardware name: APM X-Gene Mustang board (DT)
Call trace:
[<ffff80000008a09c>] dump_backtrace+0x0/0x13c
[<ffff80000008a1e8>] show_stack+0x10/0x1c
[<ffff800000691da8>] dump_stack+0x74/0x94
[<ffff800000690d78>] panic+0x100/0x240
[<ffff8000000a0bc4>] stage2_get_pmd+0x17c/0x2bc
[<ffff8000000a1dc4>] kvm_handle_guest_abort+0x4b4/0x6b0
[<ffff8000000a420c>] handle_exit+0x58/0x180
[<ffff80000009e7a4>] kvm_arch_vcpu_ioctl_run+0x114/0x45c
[<ffff800000099df4>] kvm_vcpu_ioctl+0x2e0/0x754
[<ffff8000001c0a18>] do_vfs_ioctl+0x424/0x5c8
[<ffff8000001c0bfc>] SyS_ioctl+0x40/0x78
CPU0: stopping

Passing the (unintuitively named) __GFP_COMP flag to __get_free_pages
forces the allocator to maintain a per-page refcount, which is exactly
what we need.

This has been tested on an X-Gene platform with a 4kB/48bit-VA host
kernel, and kvmtool hacked to place memory in the second page of
the hardware PGD (PUD for the host kernel).

Reported-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
 arch/arm/include/asm/kvm_mmu.h   | 7 +++++++
 arch/arm/kvm/mmu.c               | 2 +-
 arch/arm64/include/asm/kvm_mmu.h | 9 ++++++++-
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
index 37ca2a4..1cac89b 100644
--- a/arch/arm/include/asm/kvm_mmu.h
+++ b/arch/arm/include/asm/kvm_mmu.h
@@ -162,6 +162,13 @@ static inline bool kvm_page_empty(void *ptr)
 
 #define KVM_PREALLOC_LEVEL	0
 
+/*
+ * We need to ensure that stage-2 PGDs are allocated with a per-page
+ * refcount, as we fiddle with the refcounts of non-head pages.
+ * __GFP_COMP forces the allocator to do what we want.
+ */
+#define KVM_GFP_S2_PGD	(GFP_KERNEL | __GFP_ZERO | __GFP_COMP)
+
 static inline int kvm_prealloc_hwpgd(struct kvm *kvm, pgd_t *pgd)
 {
 	return 0;
diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 3e6859b..a6a8252 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -666,7 +666,7 @@ int kvm_alloc_stage2_pgd(struct kvm *kvm)
 		 * Allocate actual first-level Stage-2 page table used by the
 		 * hardware for Stage-2 page table walks.
 		 */
-		pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, S2_PGD_ORDER);
+		pgd = (pgd_t *)__get_free_pages(KVM_GFP_S2_PGD, S2_PGD_ORDER);
 	}
 
 	if (!pgd)
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 6458b53..06c733a 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -171,6 +171,13 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
 #define KVM_PREALLOC_LEVEL	(0)
 #endif
 
+/*
+ * We need to ensure that stage-2 PGDs are allocated with a per-page
+ * refcount, as we fiddle with the refcounts of non-head pages.
+ * __GFP_COMP forces the allocator to do what we want.
+ */
+#define KVM_GFP_S2_PGD	(GFP_KERNEL | __GFP_ZERO | __GFP_COMP)
+
 /**
  * kvm_prealloc_hwpgd - allocate inital table for VTTBR
  * @kvm:	The KVM struct pointer for the VM.
@@ -192,7 +199,7 @@ static inline int kvm_prealloc_hwpgd(struct kvm *kvm, pgd_t *pgd)
 	if (KVM_PREALLOC_LEVEL == 0)
 		return 0;
 
-	hwpgd = __get_free_pages(GFP_KERNEL | __GFP_ZERO, PTRS_PER_S2_PGD_SHIFT);
+	hwpgd = __get_free_pages(KVM_GFP_S2_PGD, PTRS_PER_S2_PGD_SHIFT);
 	if (!hwpgd)
 		return -ENOMEM;
 
-- 
2.1.4