In the Linux kernel, the following vulnerability has been resolved: drm/vc4: Prevent shader BO mappings from becoming writable vc4_gem_obj

In the Linux kernel, the following vulnerability has been resolved:

drm/vc4: Prevent shader BO mappings from becoming writable

vc4_gem_object_mmap() rejects a writable mapping of a validated shader
BO, but leaves VM_MAYWRITE set. Userspace can map the BO read-only and
then turn it writable with mprotect().

Validated shader BOs must stay read-only: the validator checks the
instructions once and the GPU trusts them afterwards. A writable
mapping lets userspace rewrite the code after validation, bypassing the
validator.

Clear VM_MAYWRITE on the read-only path so the mapping cannot be
upgraded, as i915 already does for its read-only objects.