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.
Fixes: 030a07e441 ("ALSA: Add USB US122L driver")
Cc: stable@vger.kernel.org
Signed-off-by: Kazuki Hanai <hnkz.64@gmail.com>
Link: https://patch.msgid.link/20260908110053.2950767-1-hnkz.64@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Kazuki Hanai
71c610aeb1