arm64: KVM: Fix stage-2 PGD allocation to have per-page refcounting

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 refcount 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 at 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

A possible approach for this is to split the compound page using
split_page() at allocation time, and change the teardown path to
free one page at a time.  It turns out that alloc_pages_exact() and
free_pages_exact() does exactly that.

While we're at it, the PGD allocation code is reworked to reduce
duplication.

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). Also regression-tested
on a Cubietruck (Cortex-A7).

 [ Reworked to use alloc_pages_exact() and free_pages_exact() and to
   return pointers directly instead of by reference as arguments
    - Christoffer ]

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

1 files changed, 49 insertions, 18 deletions

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 3e6859bc3e11..a48a73c6b866 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -632,6 +632,20 @@ int create_hyp_io_mappings(void *from, void *to, phys_addr_t phys_addr)
                                      __phys_to_pfn(phys_addr), PAGE_HYP_DEVICE);
 }
 
+/* Free the HW pgd, one page at a time */
+static void kvm_free_hwpgd(void *hwpgd)
+{
+        free_pages_exact(hwpgd, kvm_get_hwpgd_size());
+}
+
+/* Allocate the HW PGD, making sure that each page gets its own refcount */
+static void *kvm_alloc_hwpgd(void)
+{
+        unsigned int size = kvm_get_hwpgd_size();
+
+        return alloc_pages_exact(size, GFP_KERNEL | __GFP_ZERO);
+}
+
 /**
  * kvm_alloc_stage2_pgd - allocate level-1 table for stage-2 translation.
  * @kvm:        The KVM struct pointer for the VM.
@@ -645,15 +659,31 @@ int create_hyp_io_mappings(void *from, void *to, phys_addr_t phys_addr)
  */
 int kvm_alloc_stage2_pgd(struct kvm *kvm)
 {
-        int ret;
         pgd_t *pgd;
+        void *hwpgd;
 
         if (kvm->arch.pgd != NULL) {
                 kvm_err("kvm_arch already initialized?\n");
                 return -EINVAL;
         }
 
+        hwpgd = kvm_alloc_hwpgd();
+        if (!hwpgd)
+                return -ENOMEM;
+
+        /* When the kernel uses more levels of page tables than the
+         * guest, we allocate a fake PGD and pre-populate it to point
+         * to the next-level page table, which will be the real
+         * initial page table pointed to by the VTTBR.
+         *
+         * When KVM_PREALLOC_LEVEL==2, we allocate a single page for
+         * the PMD and the kernel will use folded pud.
+         * When KVM_PREALLOC_LEVEL==1, we allocate 2 consecutive PUD
+         * pages.
+         */
         if (KVM_PREALLOC_LEVEL > 0) {
+                int i;
+
                 /*
                  * Allocate fake pgd for the page table manipulation macros to
                  * work.  This is not used by the hardware and we have no
@@ -661,30 +691,32 @@ int kvm_alloc_stage2_pgd(struct kvm *kvm)
                  */
                 pgd = (pgd_t *)kmalloc(PTRS_PER_S2_PGD * sizeof(pgd_t),
                                        GFP_KERNEL | __GFP_ZERO);
+
+                if (!pgd) {
+                        kvm_free_hwpgd(hwpgd);
+                        return -ENOMEM;
+                }
+
+                /* Plug the HW PGD into the fake one. */
+                for (i = 0; i < PTRS_PER_S2_PGD; i++) {
+                        if (KVM_PREALLOC_LEVEL == 1)
+                                pgd_populate(NULL, pgd + i,
+                                             (pud_t *)hwpgd + i * PTRS_PER_PUD);
+                        else if (KVM_PREALLOC_LEVEL == 2)
+                                pud_populate(NULL, pud_offset(pgd, 0) + i,
+                                             (pmd_t *)hwpgd + i * PTRS_PER_PMD);
+                }
         } else {
                 /*
                  * 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 *)hwpgd;
         }
 
-        if (!pgd)
-                return -ENOMEM;
-
-        ret = kvm_prealloc_hwpgd(kvm, pgd);
-        if (ret)
-                goto out_err;
-
         kvm_clean_pgd(pgd);
         kvm->arch.pgd = pgd;
         return 0;
-out_err:
-        if (KVM_PREALLOC_LEVEL > 0)
-                kfree(pgd);
-        else
-                free_pages((unsigned long)pgd, S2_PGD_ORDER);
-        return ret;
 }
 
 /**
@@ -785,11 +817,10 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
                 return;
 
         unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
-        kvm_free_hwpgd(kvm);
+        kvm_free_hwpgd(kvm_get_hwpgd(kvm));
         if (KVM_PREALLOC_LEVEL > 0)
                 kfree(kvm->arch.pgd);
-        else
-                free_pages((unsigned long)kvm->arch.pgd, S2_PGD_ORDER);
+
         kvm->arch.pgd = NULL;
 }