vfs: Define usercopy region in names_cache slab caches

vfs: Define usercopy region in names_cache slab caches

VFS pathnames are stored in the names_cache slab cache, either inline
or across an entire allocation entry (when approaching PATH_MAX). These
are copied to/from userspace, so they must be entirely whitelisted.

cache object allocation:
    include/linux/fs.h:
        #define __getname()    kmem_cache_alloc(names_cachep, GFP_KERNEL)

example usage trace:
    strncpy_from_user+0x4d/0x170
    getname_flags+0x6f/0x1f0
    user_path_at_empty+0x23/0x40
    do_mount+0x69/0xda0
    SyS_mount+0x83/0xd0

    fs/namei.c:
        getname_flags(...):
            ...
            result = __getname();
            ...
            kname = (char *)result->iname;
            result->name = kname;
            len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
            ...
            if (unlikely(len == EMBEDDED_NAME_MAX)) {
                const size_t size = offsetof(struct filename, iname[1]);
                kname = (char *)result;

                result = kzalloc(size, GFP_KERNEL);
                ...
                result->name = kname;
                len = strncpy_from_user(kname, filename, PATH_MAX);

In support of usercopy hardening, this patch defines the entire cache
object in the names_cache slab cache as whitelisted, since it may entirely
hold name strings to be copied to/from userspace.

This patch is verbatim from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@nullcore.net>
[kees: adjust commit log, add usage trace]
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>