fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create anonymous inodes with proper security context. This replaces the current pattern of calling alloc_anon_inode() followed by inode_init_security_anon() for creating security context manually. This change also fixes a security regression in secretmem where the S_PRIVATE flag was not cleared after alloc_anon_inode(), causing LSM/SELinux checks to be bypassed for secretmem file descriptors. As guest_memfd currently resides in the KVM module, we need to export this symbol for use outside the core kernel. In the future, guest_memfd might be moved to core-mm, at which point the symbols no longer would have to be exported. When/if that happens is still unclear.

References

• https://git.kernel.org/stable/c/66d29d757c968d2bee9124816da5d718eb352959
• https://git.kernel.org/stable/c/e3eed01347721cd7a8819568161c91d538fbf229
• https://git.kernel.org/stable/c/f94c422157f3e43dd31990567b3e5d54b3e5b32b
• https://git.kernel.org/stable/c/6ca45ea48530332a4ba09595767bd26d3232743b
• https://git.kernel.org/stable/c/cbe4134ea4bc493239786220bd69cb8a13493190