Patch "nbd: fix max value for 'first_minor'" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    nbd: fix max value for 'first_minor'

to the 5.15-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:
     nbd-fix-max-value-for-first_minor.patch
and it can be found in the queue-5.15 subdirectory.

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



commit 5a70811f30c42ce7ee152663caffd3f835640a1c
Author: Yu Kuai <yukuai3@xxxxxxxxxx>
Date:   Tue Nov 2 09:52:34 2021 +0800

    nbd: fix max value for 'first_minor'
    
    [ Upstream commit e4c4871a73944353ea23e319de27ef73ce546623 ]
    
    commit b1a811633f73 ("block: nbd: add sanity check for first_minor")
    checks that 'first_minor' should not be greater than 0xff, which is
    wrong. Whitout the commit, the details that when user pass 0x100000,
    it ends up create sysfs dir "/sys/block/43:0" are as follows:
    
    nbd_dev_add
     disk->first_minor = index << part_shift
      -> default part_shift is 5, first_minor is 0x2000000
      device_add_disk
       ddev->devt = MKDEV(disk->major, disk->first_minor)
        -> (0x2b << 20) | (0x2000000) = 0x2b00000
       device_add
        device_create_sys_dev_entry
             format_dev_t
              sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));
               -> got 43:0
              sysfs_create_link -> /sys/block/43:0
    
    By the way, with the wrong fix, when part_shift is the default value,
    only 8 ndb devices can be created since 8 << 5 is greater than 0xff.
    
    Since the max bits for 'first_minor' should be the same as what
    MKDEV() does, which is 20. Change the upper bound of 'first_minor'
    from 0xff to 0xfffff.
    
    Fixes: b1a811633f73 ("block: nbd: add sanity check for first_minor")
    Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
    Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20211102015237.2309763-2-yebin10@xxxxxxxxxx
    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 4f1b591c3a555..0d820c4dec176 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1749,11 +1749,11 @@ static struct nbd_device *nbd_dev_add(int index, unsigned int refs)
 	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().
+	 * sysfs files/links, since MKDEV() expect that the max bits of
+	 * first_minor is 20.
 	 */
 	disk->first_minor = index << part_shift;
-	if (disk->first_minor > 0xff) {
+	if (disk->first_minor > MINORMASK) {
 		err = -EINVAL;
 		goto out_free_idr;
 	}