[PATCH 20/20] KVM: PPC: Book3S HV: Avoid crash from THP collapse during radix page fault

Leonardo Bras leonardo at linux.ibm.com
Mon Mar 11 16:50:51 UTC 2019

From: Paul Mackerras <paulus at ozlabs.org>

Commit 71d29f43b633 ("KVM: PPC: Book3S HV: Don't use compound_order to
determine host mapping size", 2018-09-11) added a call to
__find_linux_pte() and a dereference of the returned PTE pointer to the
radix page fault path in the common case where the page is normal
system memory.  Previously, __find_linux_pte() was only called for
mappings to physical addresses which don't have a page struct (e.g.
memory-mapped I/O) or where the page struct is marked as reserved

This exposes us to the possibility that the returned PTE pointer
could be NULL, for example in the case of a concurrent THP collapse
operation.  Dereferencing the returned NULL pointer causes a host

To fix this, we check for NULL, and if it is NULL, we retry the
operation by returning to the guest, with the expectation that it
will generate the same page fault again (unless of course it has
been fixed up by another CPU in the meantime).

Fixes: 71d29f43b633 ("KVM: PPC: Book3S HV: Don't use compound_order to determine host mapping size")
Signed-off-by: Paul Mackerras <paulus at ozlabs.org>
Signed-off-by: Leonardo Bras <leonardo at linux.ibm.com>
 arch/powerpc/kvm/book3s_64_mmu_radix.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index ae023d2256ef..26d927bf2fdb 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -659,6 +659,16 @@ int kvmppc_book3s_radix_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	ptep = __find_linux_pte(vcpu->arch.pgdir, hva, NULL, &shift);
+	/*
+	 * If the PTE disappeared temporarily due to a THP
+	 * collapse, just return and let the guest try again.
+	 */
+	if (!ptep) {
+		local_irq_enable();
+		if (page)
+			put_page(page);
+		return RESUME_GUEST;
+	}
 	pte = *ptep;

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