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Subject[RFC PATCH] virtio_blk: Checking "private_data" to avoid kernel panic when hotplugging
  Hi,

On guest with upstream's kernel(3.3.0-rc7), I
mounted virtblk as:
a) # mkfs /dev/vda
b) # mount /dev/vda /mnt
c) # cd /mnt

Then I did hotplug for virtblk via virsh on host as:
a) # sudo virsh detach-disk guest vda
b) # sudo virsh attach-disk guest /media/data/test.img vda

I encountered guest's kernel panic (*probability *
*event*)whose backtrace liked this:

PID: 2496 TASK: ffff88001f5de080 CPU: 0 COMMAND: "blkid"
#0 [ffff88001afdbb00] machine_kexec at ffffffff81031fcb
#1 [ffff88001afdbb60] crash_kexec at ffffffff810b8f72
#2 [ffff88001afdbc30] oops_end at ffffffff814f04b0
#3 [ffff88001afdbc60] die at ffffffff8100f26b
#4 [ffff88001afdbc90] do_general_protection at ffffffff814f0042
#5 [ffff88001afdbcc0] general_protection at ffffffff814ef815
[exception RIP: virtio_check_driver_offered_feature+27]
RIP: ffffffffa00540cb RSP: ffff88001afdbd78 RFLAGS: 00010206
RAX: ffffffff810ffde0 RBX: ffff88001f14a800 RCX: 000000004cf0758b
RDX: 4ce86d8b4ce0658b RSI: 0000000000000007 RDI: ffffffff81a970a0
RBP: ffff88001afdbd78 R8: ffffffffa009d060 R9: 0000000000000100
R10: 0000000000000006 R11: 0000000000000246 R12: 000000000000101d
R13: 0000000000005331 R14: ffffffff81a970a0 R15: 0000000000000000
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#6 [ffff88001afdbd80] virtblk_ioctl at ffffffffa009c459 [virtio_blk]
#7 [ffff88001afdbdc0] __blkdev_driver_ioctl at ffffffff81257317
#8 [ffff88001afdbe00] blkdev_ioctl at ffffffff8125779d
#9 [ffff88001afdbe50] block_ioctl at ffffffff811aec5c
#10 [ffff88001afdbe60] vfs_ioctl at ffffffff81189342
#11 [ffff88001afdbea0] do_vfs_ioctl at ffffffff811894e4
#12 [ffff88001afdbf30] sys_ioctl at ffffffff81189a61
#13 [ffff88001afdbf80] system_call_fastpath at ffffffff8100b0f2
RIP: 0000003f566dd847 RSP: 00007fffa23e6130 RFLAGS: 00010202
RAX: 0000000000000010 RBX: ffffffff8100b0f2 RCX: 0000000000000008
RDX: 0000000000000000 RSI: 0000000000005331 RDI: 0000000000000003
RBP: 0000000040000000 R8: 0000003f5699b580 R9: 0000000000000100
R10: 0000000000000006 R11: 0000000000000246 R12: 0000000000000000
R13: 0000003f59020ba0 R14: 0000000000000003 R15: 000000000241f030
ORIG_RAX: 0000000000000010 CS: 0033 SS: 002b
This panic was triggered by the command of "blkid" in
udev's rule "60-persistent-storage.rules".

So, when the virtblk was reattached, command "blkid"
probed filesystem metadata of disks. At that moment,
virtblk_ioctl() was called, but the data of "virtio_blk"
"bdev->bd_disk->private_data" pointing to may not be
updated for the sync reason.

This patch do this check to avoid panic.

Any comment will be appreciated.

Signed-off-by: Ren Mingxin <renmx@cn.fujitsu.com>
---
virtio_blk.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c4a60ba..4ac81f8 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -245,6 +245,12 @@ static int virtblk_ioctl(struct block_device *bdev,
fmode_t mode,
struct virtio_blk *vblk = disk->private_data;

/*
+ * Check whether the private data pointer has been updated.
+ */
+ if (vblk != vblk->vdev->priv)
+ return -EFAULT;
+
+ /*
* Only allow the generic SCSI ioctls if the host can support it.
*/
if (!virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_SCSI))

Thanks,
Ren


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