linux-kernel - [PATCH 4.9 09/64] KVM: PPC: Book3S HV: Protect updates to spapr_tce

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Date:   Tue,  3 Oct 2017 14:23:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Paul Mackerras <paulus@...abs.org>
Subject: [PATCH 4.9 09/64] KVM: PPC: Book3S HV: Protect updates to spapr_tce_tables list

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul Mackerras <paulus@...abs.org>

commit edd03602d97236e8fea13cd76886c576186aa307 upstream.

Al Viro pointed out that while one thread of a process is executing
in kvm_vm_ioctl_create_spapr_tce(), another thread could guess the
file descriptor returned by anon_inode_getfd() and close() it before
the first thread has added it to the kvm->arch.spapr_tce_tables list.
That highlights a more general problem: there is no mutual exclusion
between writers to the spapr_tce_tables list, leading to the
possibility of the list becoming corrupted, which could cause a
host kernel crash.

To fix the mutual exclusion problem, we add a mutex_lock/unlock
pair around the list_del_rce in kvm_spapr_tce_release().

If another thread does guess the file descriptor returned by the
anon_inode_getfd() call in kvm_vm_ioctl_create_spapr_tce() and closes
it, its call to kvm_spapr_tce_release() will not do any harm because
it will have to wait until the first thread has released kvm->lock.

The other things that the second thread could do with the guessed
file descriptor are to mmap it or to pass it as a parameter to a
KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl on a KVM device fd.  An mmap
call won't cause any harm because kvm_spapr_tce_mmap() and
kvm_spapr_tce_fault() don't access the spapr_tce_tables list or
the kvmppc_spapr_tce_table.list field, and the fields that they do use
have been properly initialized by the time of the anon_inode_getfd()
call.

The KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl calls
kvm_spapr_tce_attach_iommu_group(), which scans the spapr_tce_tables
list looking for the kvmppc_spapr_tce_table struct corresponding to
the fd given as the parameter.  Either it will find the new entry
or it won't; if it doesn't, it just returns an error, and if it
does, it will function normally.  So, in each case there is no
harmful effect.

[paulus@...abs.org - moved parts of the upstream patch into the backport
 of 47c5310a8dbe, adjusted this commit message accordingly.]

Fixes: 366baf28ee3f ("KVM: PPC: Use RCU for arch.spapr_tce_tables")
Reviewed-by: David Gibson <david@...son.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@...abs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/kvm/book3s_64_vio.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c
@@ -129,8 +129,11 @@ static int kvm_spapr_tce_mmap(struct fil
 static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
 {
 	struct kvmppc_spapr_tce_table *stt = filp->private_data;
+	struct kvm *kvm = stt->kvm;

+	mutex_lock(&kvm->lock);
 	list_del_rcu(&stt->list);
+	mutex_unlock(&kvm->lock);

 	kvm_put_kvm(stt->kvm);