vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
VMA.  This is unreliable as ->mmap may not set ->vm_ops.

False-positive vma_is_anonymous() may lead to crashes:

	next ffff8801ce5e7040 prev ffff8801d20eca50 mm ffff88019c1e13c0
	prot 27 anon_vma ffff88019680cdd8 vm_ops 0000000000000000
	pgoff 0 file ffff8801b2ec2d00 private_data 0000000000000000
	flags: 0xff(read|write|exec|shared|mayread|maywrite|mayexec|mayshare)
	------------[ cut here ]------------
	kernel BUG at mm/memory.c:1422!
	invalid opcode: 0000 [#1] SMP KASAN
	CPU: 0 PID: 18486 Comm: syz-executor3 Not tainted 4.18.0-rc3+ #136
	Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google
	01/01/2011
	RIP: 0010:zap_pmd_range mm/memory.c:1421 [inline]
	RIP: 0010:zap_pud_range mm/memory.c:1466 [inline]
	RIP: 0010:zap_p4d_range mm/memory.c:1487 [inline]
	RIP: 0010:unmap_page_range+0x1c18/0x2220 mm/memory.c:1508
	Call Trace:
	 unmap_single_vma+0x1a0/0x310 mm/memory.c:1553
	 zap_page_range_single+0x3cc/0x580 mm/memory.c:1644
	 unmap_mapping_range_vma mm/memory.c:2792 [inline]
	 unmap_mapping_range_tree mm/memory.c:2813 [inline]
	 unmap_mapping_pages+0x3a7/0x5b0 mm/memory.c:2845
	 unmap_mapping_range+0x48/0x60 mm/memory.c:2880
	 truncate_pagecache+0x54/0x90 mm/truncate.c:800
	 truncate_setsize+0x70/0xb0 mm/truncate.c:826
	 simple_setattr+0xe9/0x110 fs/libfs.c:409
	 notify_change+0xf13/0x10f0 fs/attr.c:335
	 do_truncate+0x1ac/0x2b0 fs/open.c:63
	 do_sys_ftruncate+0x492/0x560 fs/open.c:205
	 __do_sys_ftruncate fs/open.c:215 [inline]
	 __se_sys_ftruncate fs/open.c:213 [inline]
	 __x64_sys_ftruncate+0x59/0x80 fs/open.c:213
	 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reproducer:

	#include <stdio.h>
	#include <stddef.h>
	#include <stdint.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <sys/ioctl.h>
	#include <sys/mman.h>
	#include <unistd.h>
	#include <fcntl.h>

	#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
	#define KCOV_ENABLE			_IO('c', 100)
	#define KCOV_DISABLE			_IO('c', 101)
	#define COVER_SIZE			(1024<<10)

	#define KCOV_TRACE_PC  0
	#define KCOV_TRACE_CMP 1

	int main(int argc, char **argv)
	{
		int fd;
		unsigned long *cover;

		system("mount -t debugfs none /sys/kernel/debug");
		fd = open("/sys/kernel/debug/kcov", O_RDWR);
		ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE);
		cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long),
				PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
		munmap(cover, COVER_SIZE * sizeof(unsigned long));
		cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long),
				PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
		memset(cover, 0, COVER_SIZE * sizeof(unsigned long));
		ftruncate(fd, 3UL << 20);
		return 0;
	}

This can be fixed by assigning anonymous VMAs own vm_ops and not relying
on it being NULL.

If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
dummy_vm_ops.  This way we will have non-NULL ->vm_ops for all VMAs.

Link: http://lkml.kernel.org/r/20180724121139.62570-4-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: syzbot+3f84280d52be9b7083cc@syzkaller.appspotmail.com
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>