mm/shmem.c: cast the type of unmap_start to u64

In 64bit system. sb->s_maxbytes of shmem filesystem is MAX_LFS_FILESIZE,
which equal LLONG_MAX.

If offset > LLONG_MAX - PAGE_SIZE, offset + len < LLONG_MAX in
shmem_fallocate, which will pass the checking in vfs_fallocate.

	/* Check for wrap through zero too */
	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
		return -EFBIG;

loff_t unmap_start = round_up(offset, PAGE_SIZE) in shmem_fallocate
causes a overflow.

Syzkaller reports a overflow problem in mm/shmem:

  UBSAN: Undefined behaviour in mm/shmem.c:2014:10
  signed integer overflow: '9223372036854775807 + 1' cannot be represented in type 'long long int'
  CPU: 0 PID:17076 Comm: syz-executor0 Not tainted 4.1.46+ #1
  Hardware name: linux, dummy-virt (DT)
  Call trace:
     dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100
     show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238
     __dump_stack lib/dump_stack.c:15 [inline]
     ubsan_epilogue+0x18/0x70 lib/ubsan.c:164
     handle_overflow+0x158/0x1b0 lib/ubsan.c:195
     shmem_fallocate+0x6d0/0x820 mm/shmem.c:2104
     vfs_fallocate+0x238/0x428 fs/open.c:312
     SYSC_fallocate fs/open.c:335 [inline]
     SyS_fallocate+0x54/0xc8 fs/open.c:239

The highest bit of unmap_start will be appended with sign bit 1
(overflow) when calculate shmem_falloc.start:

    shmem_falloc.start = unmap_start >> PAGE_SHIFT.

Fix it by casting the type of unmap_start to u64, when right shifted.

This bug is found in LTS Linux 4.1.  It also seems to exist in mainline.

Link: http://lkml.kernel.org/r/1573867464-5107-1-git-send-email-chenjun102@huawei.comSigned-off-by: Chen Jun <chenjun102@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -2747,7 +2747,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		}
 
 		shmem_falloc.waitq = &shmem_falloc_waitq;
-		shmem_falloc.start = unmap_start >> PAGE_SHIFT;
+		shmem_falloc.start = (u64)unmap_start >> PAGE_SHIFT;
 		shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
 		spin_lock(&inode->i_lock);
 		inode->i_private = &shmem_falloc;