CVE-2021-47491: mm: khugepaged: skip huge page collapse for special files

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

mm: khugepaged: skip huge page collapse for special files

The read-only THP for filesystems will collapse THP for files opened readonly and mapped with VMEXEC. The intended usecase is to avoid TLB misses for large text segments. But it doesn't restrict the file types so a THP could be collapsed for a non-regular file, for example, block device, if it is opened readonly and mapped with EXEC permission. This may cause bugs, like [1] and [2].

This is definitely not the intended usecase, so just collapse THP for regular files in order to close the attack surface.

[shy828301@gmail.com: fix vmfile check [3]]

— NVD

Linux Kernel is vulnerable to a denial of service, caused by improper input validation. By sending a specially crafted request, a local authenticated attacker could exploit this vulnerability to cause collapse in read-only THP for filesystems.

— IBM