virtio: force vring descriptors to be allocated from lowmem

virtio: force vring descriptors to be allocated from lowmem

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: [email protected]
Cc: Sasha Levin <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Rusty Russell <[email protected]>