[PATCH 4.9 09/64] KVM: PPC: Book3S HV: Protect updates to spapr_tce_tables list

From: Greg Kroah-Hartman
Date: Tue Oct 03 2017 - 09:02:32 EST

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


From: Paul Mackerras <paulus@xxxxxxxxxx>

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()

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@xxxxxxxxxx - 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@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Mackerras <paulus@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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);
+ mutex_unlock(&kvm->lock);