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

[Chris Marusich <cmmarusich@gmail.com>]

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Hi Ludo,

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.

Shall we close this bug report, then?

-- 
Chris

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