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

From: Manjunath Patil
Date: Tue Oct 30 2018 - 12:29:13 EST

On 10/30/2018 3:39 AM, Roger Pau Monnà 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 suspend)
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.