bug#41499: /proc/filesystems impurity in build environment

[Ludovic Courtès]

Hi,

Chris Marusich <cmmarusich@gmail.com> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> The daemon mounts /proc in the build environment (see
>> libstore/build.cc):
>>
>>     /* Bind a new instance of procfs on /proc to reflect our
>>        private PID namespace. */
>>     createDirs(chrootRootDir + "/proc");
>>     if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
>>         throw SysError("mounting /proc");
>>
>> /proc is needed for many things on GNU/Linux.  For example, libc’s
>> loader relies on /proc/self/exe to implement $ORIGIN, ‘getlogin_r’
>> relies on /proc/self/loginuid, ‘ttyname’ uses /proc/self/fd, ‘sysconf’
>> uses /proc/sys/kernel, etc.  So we have to have /proc in there.
>>
>> The problem is that /proc appears to be all-or-nothing.
>>
>> What we could do maybe is bind-mount our own statically-defined
>> ‘filesystems’ file on top of the procfs mount above.
>>
>> There would still be many leaks in /proc anyway, so perhaps a better
>> approach is to patch ‘sed’ to not refer to it.
>
> That makes sense.  I have submitted an upstream patch to fix sed:
>
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36150
>
> It could be fun to investigate how far we can go with respect to
> limiting access in the sandbox to a minimal, uniform set of kernel
> features.  However, unless issues like this become more common, it may
> not be that big of an issue.

There’s /proc, but there are also syscalls that leak info, such as
uname(2).

Fortunately these issues are quite rare in practice (it’s mentioned in
<https://guix.gnu.org/blog/2015/reproducible-builds-a-means-to-an-end/>,
which references an earlier NixOS paper that discusses it.)

> Shall we close this bug report, then?

I think so.

Thanks,
Ludo’.