Patch "block: nbd: add sanity check for first_minor" has been added to the 5.14-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    block: nbd: add sanity check for first_minor

to the 5.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     block-nbd-add-sanity-check-for-first_minor.patch
and it can be found in the queue-5.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 256eb2c91f93de3eafacf05e06ffb0bee8608ad7
Author: Pavel Skripkin <paskripkin@xxxxxxxxx>
Date:   Thu Aug 12 12:15:01 2021 +0300

    block: nbd: add sanity check for first_minor
    
    [ Upstream commit b1a811633f7321cf1ae2bb76a66805b7720e44c9 ]
    
    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@xxxxxxxxxxxxxxxxxxxxxxxxx
    Reviewed-by: Christoph Hellwig <hch@xxxxxx>
    Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
    Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 7ed888e99f09..93708b1938e8 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1770,7 +1770,17 @@ 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;