[PATCH 3.16 130/132] KVM: coalesced_mmio: add bounds checking

From: Ben Hutchings
Date: Fri Sep 20 2019 - 10:34:24 EST


3.16.74-rc1 review patch. If anyone has any objections, please let me know.

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

From: Matt Delco <delco@xxxxxxxxxxxx>

commit b60fe990c6b07ef6d4df67bc0530c7c90a62623a upstream.

The first/last indexes are typically shared with a user app.
The app can change the 'last' index that the kernel uses
to store the next result. This change sanity checks the index
before using it for writing to a potentially arbitrary address.

This fixes CVE-2019-14821.

Fixes: 5f94c1741bdc ("KVM: Add coalesced MMIO support (common part)")
Signed-off-by: Matt Delco <delco@xxxxxxxxxxxx>
Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
Reported-by: syzbot+983c866c3dd6efa3662a@xxxxxxxxxxxxxxxxxxxxxxxxx
[Use READ_ONCE. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16:
- Use ACCESS_ONCE() instead of READ_ONCE()
- kvm_coalesced_mmio_zone::pio field is not supported]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
virt/kvm/coalesced_mmio.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -39,7 +39,7 @@ static int coalesced_mmio_in_range(struc
return 1;
}

-static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev)
+static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
{
struct kvm_coalesced_mmio_ring *ring;
unsigned avail;
@@ -51,7 +51,7 @@ static int coalesced_mmio_has_room(struc
* there is always one unused entry in the buffer
*/
ring = dev->kvm->coalesced_mmio_ring;
- avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
+ avail = (ring->first - last - 1) % KVM_COALESCED_MMIO_MAX;
if (avail == 0) {
/* full */
return 0;
@@ -65,24 +65,27 @@ static int coalesced_mmio_write(struct k
{
struct kvm_coalesced_mmio_dev *dev = to_mmio(this);
struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
+ __u32 insert;

if (!coalesced_mmio_in_range(dev, addr, len))
return -EOPNOTSUPP;

spin_lock(&dev->kvm->ring_lock);

- if (!coalesced_mmio_has_room(dev)) {
+ insert = ACCESS_ONCE(ring->last);
+ if (!coalesced_mmio_has_room(dev, insert) ||
+ insert >= KVM_COALESCED_MMIO_MAX) {
spin_unlock(&dev->kvm->ring_lock);
return -EOPNOTSUPP;
}

/* copy data in first free entry of the ring */

- ring->coalesced_mmio[ring->last].phys_addr = addr;
- ring->coalesced_mmio[ring->last].len = len;
- memcpy(ring->coalesced_mmio[ring->last].data, val, len);
+ ring->coalesced_mmio[insert].phys_addr = addr;
+ ring->coalesced_mmio[insert].len = len;
+ memcpy(ring->coalesced_mmio[insert].data, val, len);
smp_wmb();
- ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+ ring->last = (insert + 1) % KVM_COALESCED_MMIO_MAX;
spin_unlock(&dev->kvm->ring_lock);
return 0;
}