FreeBSD: VID-a9c6e9be-61fb-11eb-b87a-901b0ef719ab (CVE-2020-25579): FreeBSD -- Uninitialized kernel stack leaks in several file systems

Problem Description: Several file systems were not properly initializing the d_off field of the dirent structures returned by VOP_READDIR. In particular, tmpfs(5), smbfs(5), autofs(5) and mqueuefs(5) were failing to do so. As a result, eight uninitialized kernel stack bytes may be leaked to userspace by these file systems. This problem is not present in FreeBSD 11. Additionally, msdosfs(5) was failing to zero-fill a pair of padding fields in the dirent structure, resulting in a leak of three uninitialized bytes. Impact: Kernel stack disclosures may leak sensitive information which could be used to compromise the security of the system.