Executive brief
In the Linux kernel, the following vulnerability has been resolved:
ALSA: us122l: Prevent write upgrades for read mappings
The hwdep mmap callback rejects read-buffer mappings that are initially writable, but leaves VM_MAYWRITE set on mappings created with PROT_READ. A process that can open the hwdep node O_RDWR can later use mprotect() to make the mapping writable.
The read allocation begins with struct usb_stream. Its read_size member is used by the fault handler to decide which pages belong to the read buffer. The read VMA intentionally remains expandable because pcm_usb_stream uses mremap() after reading that size. Changing read_size first can therefore map and access pages beyond the allocation. The same member is also consumed by usb_stream_free(), where changing it can make free_pages_exact() release pages outside the allocation.
Clear VM_MAYWRITE for read-buffer mappings after rejecting an initially writable VMA. This keeps the separate output-buffer mapping writable while preventing later permission upgrades.