Re: [RFC][PATCH] net/bpfilter: Remove this broken and apparently unmantained

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On 2020/06/09 1:23, Alexei Starovoitov wrote:
> On Sun, Jun 07, 2020 at 11:31:05AM +0900, Tetsuo Handa wrote:
>> On 2020/06/07 10:49, Alexei Starovoitov wrote:
>>> So you're right that for most folks user space is the
>>> answer. But there are cases where kernel has to have these things before
>>> systemd starts.
>>
>> Why such cases can't use init= kernel command line argument?
>> The program specified via init= kernel command line argument can do anything
>> before systemd (a.k.a. global init process) starts.
>>
>> By the way, from the LSM perspective, doing a lot of things before global init
>> process starts is not desirable, for access decision can be made only after policy
>> is loaded (which is generally when /sbin/init on a device specified via root=
>> kernel command line argument becomes ready). Since
>> fork_usermode_blob((void *) "#!/bin/true\n", 12, info) is possible, I worry that
>> the ability to start userspace code is abused for bypassing dentry/inode-based
>> permission checks.
> 
> bpf_lsm is that thing that needs to load and start acting early.
> It's somewhat chicken and egg. fork_usermode_blob() will start a process
> that will load and apply security policy to all further forks and execs.

fork_usermode_blob() would start a process in userspace, but early in the boot
stage means that things in the kernel might not be ready to serve for userspace
processes (e.g. we can't open a shared library before a filesystem containing
that file becomes ready, we can't mount a filesystem before mount point becomes
ready, we can't access mount point before a device that contains that directory
becomes ready).

TOMOYO LSM module uses call_usermodehelper() from tomoyo_load_policy() in order to
load and apply security policy. What is so nice with fork_usermode_blob() compared
to existing call_usermodehelper(), at the cost of confusing LSM modules by allowing
file-less execve() request from fork_usermode_blob() ?



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