[PATCH] block: nbd: add sanity check for first_minor

Syzbot hit WARNING in internal_create_group(). The problem was in
too big disk->first_minor.

disk->first_minor is initialized by value, which comes from userspace
and there wasn't any sanity checks about value correctness. It can cause
duplicate creation of sysfs files/links, because disk->first_minor will
be passed to MKDEV() which causes truncation to byte. Since maximum
minor value is 0xff, let's check if first_minor is correct minor number.

NOTE: the root case of the reported warning was in wrong error handling
in register_disk(), but we can avoid passing knowingly wrong values to
sysfs API, because sysfs error messages can confuse users. For example:
user passed 1048576 as index, but sysfs complains about duplicate
creation of /dev/block/43:0. It's not obvious how 1048576 becomes 0.
Log and reproducer for above example can be found on syzkaller bug
report page.

Link: https://syzkaller.appspot.com/bug?id=03c2ae9146416edf811958d5fd7acfab75b143d1
Fixes: b0d9111a2d53 ("nbd: use an idr to keep track of nbd devices")
Reported-by: syzbot+9937dc42271cd87d4b98@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <redacted>
---
 drivers/block/nbd.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index c38317979f74..600e9bab5d43 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -1725,7 +1725,17 @@ static int nbd_dev_add(int index)
 	refcount_set(&nbd->refs, 1);
 	INIT_LIST_HEAD(&nbd->list);
 	disk->major = NBD_MAJOR;
+
+	/* Too big first_minor can cause duplicate creation of
+	 * sysfs files/links, since first_minor will be truncated to
+	 * byte in __device_add_disk().
+	 */
 	disk->first_minor = index << part_shift;
+	if (disk->first_minor > 0xff) {
+		err = -EINVAL;
+		goto out_free_idr;
+	}
+
 	disk->minors = 1 << part_shift;
 	disk->fops = &nbd_fops;
 	disk->private_data = nbd;

-- 
2.32.0

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

On 8/12/21 12:15 PM, Pavel Skripkin wrote:

Fun thing: I got a reply to this email from
nsd-public@police.gov.hk, which is Hong Kong Police office email. Does 
anyone know what is going on? :) It's a bit scary...



With regards,
Pavel Skripkin

On Thu, Aug 12, 2021 at 12:42:38PM +0300, Pavel Skripkin wrote:

Fun thing: I got a reply to this email from
nsd-public@police.gov.hk, which is Hong Kong Police office email. Does
anyone know what is going on? :) It's a bit scary...

You are not alone.  Apparently, someone subscribed that address to the
nbd@other.debian.org list and it is auto-responding to every message
it receives; hopefully, a list administrator (I am not one) will be
willing to forcefully unsubscribe that address.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On Thu, Aug 12, 2021 at 10:35:25AM -0500, Eric Blake wrote:

On Thu, Aug 12, 2021 at 12:42:38PM +0300, Pavel Skripkin wrote:

quoted

Fun thing: I got a reply to this email from
nsd-public@police.gov.hk, which is Hong Kong Police office email. Does
anyone know what is going on? :) It's a bit scary...

You are not alone.  Apparently, someone subscribed that address to the
nbd@other.debian.org list and it is auto-responding to every message
it receives; hopefully, a list administrator (I am not one) will be
willing to forcefully unsubscribe that address.

FWIW, this has now happened, so you should no longer see such autoreplies.

-- 
     w@uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}

On Mon, Aug 16, 2021 at 05:26:23PM +0200, Wouter Verhelst wrote:

On Thu, Aug 12, 2021 at 10:35:25AM -0500, Eric Blake wrote:

quoted

On Thu, Aug 12, 2021 at 12:42:38PM +0300, Pavel Skripkin wrote:

quoted

Fun thing: I got a reply to this email from
nsd-public@police.gov.hk, which is Hong Kong Police office email. Does
anyone know what is going on? :) It's a bit scary...

You are not alone.  Apparently, someone subscribed that address to the
nbd@other.debian.org list and it is auto-responding to every message
it receives; hopefully, a list administrator (I am not one) will be
willing to forcefully unsubscribe that address.

FWIW, this has now happened, so you should no longer see such autoreplies.

... except I just received another one myself. I've taken it up with the
list admins again.

I won't update the whole Cc list of this anymore, but rest assured I'll
stay on this until it's been taken care of.

-- 
     w@uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}

On 8/12/21 3:15 AM, Pavel Skripkin wrote:

Syzbot hit WARNING in internal_create_group(). The problem was in
too big disk->first_minor.

disk->first_minor is initialized by value, which comes from userspace
and there wasn't any sanity checks about value correctness. It can cause
duplicate creation of sysfs files/links, because disk->first_minor will
be passed to MKDEV() which causes truncation to byte. Since maximum
minor value is 0xff, let's check if first_minor is correct minor number.

NOTE: the root case of the reported warning was in wrong error handling
in register_disk(), but we can avoid passing knowingly wrong values to
sysfs API, because sysfs error messages can confuse users. For example:
user passed 1048576 as index, but sysfs complains about duplicate
creation of /dev/block/43:0. It's not obvious how 1048576 becomes 0.
Log and reproducer for above example can be found on syzkaller bug
report page.

Applied, thanks.

-- 
Jens Axboe