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Re: [Xen-devel] [PATCH] xen-blkfront: fix kernel panic with negotiate_mq error path

On 10/30/2018 3:39 AM, Roger Pau Monné wrote:

On Mon, Oct 29, 2018 at 11:31:56AM -0700, Manjunath Patil wrote:
info->nr_rings isn't adjusted in case of ENOMEM error from
negotiate_mq(). This leads to kernel panic in error path.

Typical call stack involving panic -
  #8 page_fault at ffffffff8175936f
     [exception RIP: blkif_free_ring+33]
     RIP: ffffffffa0149491  RSP: ffff8804f7673c08  RFLAGS: 00010292
  #9 blkif_free at ffffffffa0149aaa [xen_blkfront]
  #10 talk_to_blkback at ffffffffa014c8cd [xen_blkfront]
  #11 blkback_changed at ffffffffa014ea8b [xen_blkfront]
  #12 xenbus_otherend_changed at ffffffff81424670
  #13 backend_changed at ffffffff81426dc3
  #14 xenwatch_thread at ffffffff81422f29
  #15 kthread at ffffffff810abe6a
  #16 ret_from_fork at ffffffff81754078

Though either of my changes avoid the panic, I included both the
changes. This issue got introduced with "7ed8ce1 xen-blkfront: move
negotiate_mq to cover all cases of new VBDs"

Signed-off-by: Manjunath Patil <manjunath.b.patil@xxxxxxxxxx>
  drivers/block/xen-blkfront.c |    7 +++++--
  1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 429d201..dc8fe25 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1350,8 +1350,10 @@ static void blkif_free(struct blkfront_info *info, int 
        if (info->rq)
- for (i = 0; i < info->nr_rings; i++)
-               blkif_free_ring(&info->rinfo[i]);
+       if (info->rinfo) {
+               for (i = 0; i < info->nr_rings; i++)
+                       blkif_free_ring(&info->rinfo[i]);
+       }
I don't see the point in the if case here, you already fixed it by
setting the nr_rings value which I think it's the correct way of
fixing it.

Thanks, Roger.
Thank you Roger for your comments.
I will exclude this change and send for review again.


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