linux-kernel - [PATCH] KVM: Prevent double-free on HPT resize commit path

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Date:   Wed, 15 Feb 2017 14:40:04 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     paulus@...ba.org
Cc:     dan.carpenter@...cle.com, linuxppc-dev@...ts.ozlabs.org,
        kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        David Gibson <david@...son.dropbear.id.au>
Subject: [PATCH] KVM: Prevent double-free on HPT resize commit path

resize_hpt_release(), called once the HPT resize of a KVM guest is
completed (successfully or unsuccessfully) free()s the state structure for
the resize.  It is currently not safe to call with a NULL pointer.

However, one of the error paths in kvm_vm_ioctl_resize_hpt_commit() can
invoke it with a NULL pointer.  This will occur if userspace improperly
invokes KVM_PPC_RESIZE_HPT_COMMIT without previously calling
KVM_PPC_RESIZE_HPT_PREPARE, or if it calls COMMIT twice without an
intervening PREPARE.

To fix this potential crash bug - and maybe others like it, make it safe
(and a no-op) to call resize_hpt_release() with a NULL resize pointer.

Found by Dan Carpenter with a static checker.

Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
Signed-off-by: David Gibson <david@...son.dropbear.id.au>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 013552f..72ccac2 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -1407,6 +1407,9 @@ static void resize_hpt_release(struct kvm *kvm, struct kvm_resize_hpt *resize)
 {
 	BUG_ON(kvm->arch.resize_hpt != resize);

+	if (!resize)
+		return;
+
 	if (resize->hpt.virt)
 		kvmppc_free_hpt(&resize->hpt);

-- 
2.9.3