Btrfs: fix an integer overflow check

This isn't super serious because you need CAP_ADMIN to run this code.

I added this integer overflow check last year but apparently I am
rubbish at writing integer overflow checks...  There are two issues.
First, access_ok() works on unsigned long type and not u64 so on 32 bit
systems the access_ok() could be checking a truncated size.  The other
issue is that we should be using a stricter limit so we don't overflow
the kzalloc() setting ctx->clone_roots later in the function after the
access_ok():

	alloc_size = sizeof(struct clone_root) * (arg->clone_sources_count + 1);
	sctx->clone_roots = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN);

Fixes: f5ecec3c ("btrfs: send: silence an integer overflow warning")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ added comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -6305,8 +6305,13 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 		goto out;
 	}
 
+	/*
+	 * Check that we don't overflow at later allocations, we request
+	 * clone_sources_count + 1 items, and compare to unsigned long inside
+	 * access_ok.
+	 */
 	if (arg->clone_sources_count >
-	    ULLONG_MAX / sizeof(*arg->clone_sources)) {
+	    ULONG_MAX / sizeof(struct clone_root) - 1) {
 		ret = -EINVAL;
 		goto out;
 	}