Re: [PATCH] procfs: change the owner of non-dumpable and writeable files

[Aleksa Sarai <asarai@suse.de>]

>> In order to protect against ptrace(2) and similar attacks on container
>> runtimes when they join namespaces, many runtimes set mm->dumpable to
>> SUID_DUMP_DISABLE. However, doing this means that attempting to set up
>> an unprivileged user namespace will fail because an unprivileged process
>> can no longer access /proc/self/{setgroups,{uid,gid}_map} for the
>> container process (which is the same uid as the runtime process).
>>
>> Fix this by changing pid_getattr to *also* change the owner of regular
>> files that have a mode of 0644 (when the process is not dumpable). This
>> ensures that the important /proc/[pid]/... files mentioned above are
>> properly accessible by a container runtime in a rootless container
>> context.
>>
>> The most blantant issue is that a non-dumpable process in a rootless
>> container context is unable to open /proc/self/setgroups, because it
>> doesn't own the file.
>
> This changes a lot more than just setgroups, doesn't it? This bypasses
> the task_dumpable check for all kinds of things.

Yeah. I can special case /proc/self/setgroups as well as uid_map, 
gid_map and the other *specific* things that runC needs. But ultimately 
I think we should come up with agreement on what things should always 
appear to be owned by the process's user.

> Though, I expect the
> has_pid_permissions() check to be the harder one to pass. Why does
> has_pid_permissions() succeed in the case you've given?

Because the group id of the container process is the same as the parent 
process, so is_group_p() will succeed. Also hide_pid_min is set such 
that it will work in either case.

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/