efi: fix panic in kdump kernel

Check if get_next_variable() is actually valid pointer before
calling it. In kdump kernel this method is set to NULL that causes
panic during the kexec-ed kernel boot.

Tested with QEMU and OVMF firmware.

Fixes: bad267f ("efi: verify that variable services are supported")
Signed-off-by: Oleksandr Tymoshenko <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>

Author: gonzoua

drivers/firmware/efi/efi.c

@@ -203,6 +203,8 @@ static bool generic_ops_supported(void)
 
 	name_size = sizeof(name);
 
+	if (!efi.get_next_variable)
+		return false;
 	status = efi.get_next_variable(&name_size, &name, &guid);
 	if (status == EFI_UNSUPPORTED)
 		return false;