xen: Prevent buffer overflow in privcmd ioctl

BugLink: https://bugs.launchpad.net/bugs/1828420

commit 42d8644b upstream.

The "call" variable comes from the user in privcmd_ioctl_hypercall().
It's an offset into the hypercall_page[] which has (PAGE_SIZE / 32)
elements.  We need to put an upper bound on it to prevent an out of
bounds access.

Cc: stable@vger.kernel.org
Fixes: 1246ae0b ("xen: add variable hypercall caller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/include/asm/xen/hypercall.h

@@ -215,6 +215,9 @@ privcmd_call(unsigned call,
 	__HYPERCALL_DECLS;
 	__HYPERCALL_5ARG(a1, a2, a3, a4, a5);
 
+	if (call >= PAGE_SIZE / sizeof(hypercall_page[0]))
+		return -EINVAL;
+
 	stac();
 	asm volatile(CALL_NOSPEC
 		     : __HYPERCALL_5PARAM