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

Vulnerability Description

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;

References

• https://git.kernel.org/stable/c/a508a102edf8735adc9bb73d37dd13c38d1a1b10
• https://git.kernel.org/stable/c/3ae63a8c1685e16958560ec08d30defdc5b9cca0
• https://git.kernel.org/stable/c/2219e5f97244b79c276751a1167615b9714db1b0
• https://git.kernel.org/stable/c/1b4a8b89bf6787090b56424d269bf84ba00c3263
• https://git.kernel.org/stable/c/03c71c42809ef4b17f5d874cdb2d3bf40e847b86
• https://git.kernel.org/stable/c/39bc27bd688066a63e56f7f64ad34fae03fbe3b8