Re: [PATCH] kmod: verify module name before invoking modprobe

[Song Chen]

Hi Petr,

Please see my inline comments, many thanks.

BR

Song

在 2024/11/12 20:56, Petr Pavlu 写道:
> On 11/10/24 12:42, Song Chen wrote:
> > Sometimes when kernel calls request_module to load a module
> > into kernel space, it doesn't pass the module name appropriately,
> > and request_module doesn't verify it as well.
> >
> > As a result, modprobe is invoked anyway and spend a lot of time
> > searching a nonsense name.
> >
> > For example reported from a customer, he runs a user space process
> > to call ioctl(fd, SIOCGIFINDEX, &ifr), the callstack in kernel is
> > like that:
> > dev_ioctl(net/core/dev_iovtl.c)
> >    dev_load
> >       request_module("netdev-%s", name);
> >       or request_module("%s", name);
> >
> > However if name of NIC is empty, neither dev_load nor request_module
> > checks it at the first place, modprobe will search module "netdev-"
> > in its default path, env path and path configured in etc for nothing,
> > increase a lot system overhead.
> >
> > To address this problem, this patch copies va_list and introduces
> > a helper is_module_name_valid to verify the parameters validity
> > one by one, either null or empty. if it fails, no modprobe invoked.
>
> I'm not sure if I fully follow why this should be addressed at the
> request_module() level. If the user repeatedly invokes SIOCGIFINDEX with
> an empty name and this increases their system load, wouldn't it be
> better to update the userspace to prevent this non-sense request in the
> first place? 

If the user process knew, it wouldn't make the mistake. moreover, what 
happened in dev_load was quite confusing, please see the code below:

    no_module = !dev;
    if (no_module && capable(CAP_NET_ADMIN))
        no_module = request_module("netdev-%s", name);
    if (no_module && capable(CAP_SYS_MODULE))
        request_module("%s", name);

Running the same process, sys admin or root user spends more time than 
normal user, it took a while for us to find the cause, that's why i 
tried to fix it in kernel.

Similarly, if something should be done in the kernel,
> wouldn't it be more straightforward for dev_ioctl()/dev_load() to check
> this case?

I thought about it at the beginning, not only dev_ioctl/dev_load but 
also other request_module callers should check this case as well, that 
would be too much effort, then I switched to check it at the beginning 
of request_module which every caller goes through.

> I think the same should in principle apply to other places that might
> invoke request_module() with "%s" and a bogus value. The callers can
> appropriately decide if their request makes sense and should be
> fixed/improved.

Callees are obliged to do fault tolerance for callers, or at least let 
them know what is going on inside, what kinds of mistake they are 
making, there are a lot of such cases in kernel, such as call_modprobe 
in kernel/module/kmod.c, it checks if orig_module_name is NULL.

Song