[PATCH v2 3/3] virtio: force vring descriptors to be allocated from lowmem

From: Will Deacon
Date: Fri Oct 19 2012 - 09:04:14 EST

Virtio devices may attempt to add descriptors to a virtqueue from atomic
context using GFP_ATOMIC allocation. This is problematic because such
allocations can fall outside of the lowmem mapping, causing virt_to_phys
to report bogus physical addresses which are subsequently passed to
userspace via the buffers for the virtual device.

This patch masks out __GFP_HIGH and __GFP_HIGHMEM from the requested
flags when allocating descriptors for a virtqueue. If an atomic
allocation is requested and later fails, we will return -ENOSPC which
will be handled by the driver.

Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Sasha Levin <levinsasha928@xxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
drivers/virtio/virtio_ring.c | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e639584..286c30c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -135,6 +135,13 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
unsigned head;
int i;

+ /*
+ * We require lowmem mappings for the descriptors because
+ * otherwise virt_to_phys will give us bogus addresses in the
+ * virtqueue.
+ */
+ gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
if (!desc)
return -ENOMEM;

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