block: Fix the maximum minor value is blk_alloc_ext_minor()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: block: Fix the maximum minor value is blk_alloc_ext_minor() ida_alloc_range(..., min, max, ...) returns values from min to max, inclusive. So, NR_EXT_DEVT is a valid idx returned by blk_alloc_ext_minor(). This is an issue because in device_add_disk(), this value is used in: ddev->devt = MKDEV(disk->major, disk->first_minor); and NR_EXT_DEVT is '(1 << MINORBITS)'. So, should 'disk->first_minor' be NR_EXT_DEVT, it would overflow.

References

• https://git.kernel.org/stable/c/5b9ac3727e4abb11c9cfbe9c0781fc05dfdd7cfb
• https://git.kernel.org/stable/c/0f8cf8f5ccbad25ed6828875b222dbab29d5c272
• https://git.kernel.org/stable/c/fbe2cc4525480ddd20c866bb5c0578071e01451a
• https://git.kernel.org/stable/c/d1868328dec5ae2cf210111025fcbc71f78dd5ca