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

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Message-Id: <1350651813-8694-3-git-send-email-will.deacon@arm.com>
Date:	Fri, 19 Oct 2012 14:03:33 +0100
From:	Will Deacon <will.deacon@....com>
To:	linux-kernel@...r.kernel.org
Cc:	rusty@...tcorp.com.au, levinsasha928@...il.com,
	Marc.Zyngier@....com, virtualization@...ts.linux-foundation.org,
	akpm@...ux-foundation.org, ericvh@...il.com,
	Will Deacon <will.deacon@....com>
Subject: [PATCH v2 3/3] 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: Rusty Russell <rusty@...tcorp.com.au>
Cc: Sasha Levin <levinsasha928@...il.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---
 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;
-- 
1.7.4.1

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