[PATCH 3.11-stable] virtio_ring: plug kmemleak false positive.

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This patch looks like it should be in the 3.11-stable tree, should we apply
it?

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

From: "Rusty Russell <rusty@xxxxxxxxxxxxxxx>"

commit bb478d8b167cf875565ac7d927ffbdc0b6d280e8 upstream

unreferenced object 0xffff88003d467e20 (size 32):
  comm "softirq", pid 0, jiffies 4295197765 (age 6.364s)
  hex dump (first 32 bytes):
    28 19 bf 3d 00 00 00 00 0c 00 00 00 01 00 01 00  (..=............
    02 dc 51 3c 00 00 00 00 56 00 00 00 00 00 00 00  ..Q<....V.......
  backtrace:
    [<ffffffff8152db19>] kmemleak_alloc+0x59/0xc0
    [<ffffffff81102e93>] __kmalloc+0xf3/0x180
    [<ffffffff812db5d6>] vring_add_indirect+0x36/0x280
    [<ffffffff812dc59f>] virtqueue_add_outbuf+0xbf/0x4e0
    [<ffffffff813a8b30>] start_xmit+0x1a0/0x3b0
    [<ffffffff81445861>] dev_hard_start_xmit+0x2d1/0x4d0
    [<ffffffff81460052>] sch_direct_xmit+0xf2/0x1c0
    [<ffffffff81445c28>] dev_queue_xmit+0x1c8/0x460
    [<ffffffff814e3187>] ip6_finish_output2+0x1d7/0x470
    [<ffffffff814e34b0>] ip6_finish_output+0x90/0xb0
    [<ffffffff814e3507>] ip6_output+0x37/0xb0
    [<ffffffff815021eb>] igmp6_send+0x2db/0x470
    [<ffffffff81502645>] igmp6_timer_handler+0x95/0xa0
    [<ffffffff8104b57c>] call_timer_fn+0x2c/0x90
    [<ffffffff8104b7ba>] run_timer_softirq+0x1da/0x1f0
    [<ffffffff81045721>] __do_softirq+0xd1/0x1b0

Address gets embedded in a descriptor via virt_to_phys().  See detach_buf,
which frees it:

	if (vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT)
		kfree(phys_to_virt(vq->vring.desc[i].addr));

Reported-by: Christoph Paasch <christoph.paasch@xxxxxxxxxxxx>
Fix-suggested-by: Christoph Paasch <christoph.paasch@xxxxxxxxxxxx>
Typing-done-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Jonghwan Choi <jhbird.choi@xxxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 6b4a4db..6547d46 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -173,6 +173,8 @@ static inline int vring_add_indirect(struct
vring_virtqueue *vq,
 	head = vq->free_head;
 	vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
 	vq->vring.desc[head].addr = virt_to_phys(desc);
+	/* kmemleak gives a false positive, as it's hidden by virt_to_phys
*/
+	kmemleak_ignore(desc);
 	vq->vring.desc[head].len = i * sizeof(struct vring_desc);
 
 	/* Update free pointer */
-- 
1.7.9.5

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