drm/shmem-helper: Fix BUG_ON() on mmap(PROT_WRITE, MAP_PRIVATE)

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

drm/shmem-helper: Fix BUG_ON() on mmap(PROT_WRITE, MAP_PRIVATE)

Lack of check for copy-on-write (COW) mapping in drm_gem_shmem_mmap

allows users to call mmap with PROT_WRITE and MAP_PRIVATE flag

causing a kernel panic due to BUG_ON in vmf_insert_pfn_prot:

BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));

Return -EINVAL early if COW mapping is detected.

This bug affects all drm drivers using default shmem helpers.

It can be reproduced by this simple example:

void *ptr = mmap(0, size, PROT_WRITE, MAP_PRIVATE, fd, mmap_offset);

ptr[0] = 0;