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Date: Sun, 18 Apr 2021 14:51:52 +0200
From: Solar Designer <>
Subject: Re: xscreensaver package caps gets raw socket

On Sat, Apr 17, 2021 at 09:51:38PM -0300, √Črico Nogueira wrote:
> Em 17/04/2021 11:31, Tavis Ormandy escreveu:
> >Summary of discussion so far:
> >
> >- In theory, mesa support running in a privileged context, their
> >   documentation says they disable dangerous features in setuid/setgid
> >   binaries:
> >
> >
> >
> >   In fact, this is broken because they only check if (geteuid() !=
> >   getuid()) { ... }. That check doesn't even handle setgid, let alone file
> >   caps. If mesa agree this is a bug, simply changing their checks to if
> >   (getauxval(AT_SECURE)) { ... } might make this bug go away, and handle
> >   file caps and setgid for free. I filed a bug for that, but there
> >   hasn't been a response:
> >
> The linked issue appears to be private... Not sure it makes sense, since 
> the problem has been explained in this public email. FWIW, libglvnd has 
> the same issue, though it at leasts (E)GID as well. Sending it here 
> because I couldn't find a security contact.
>     if (getuid() == geteuid() && getgid() == getegid()) {
>         env = getenv("__EGL_VENDOR_LIBRARY_FILENAMES");
>     }
> I will look into opening an issue with them and finding a fix.


"search for LIBGL_DRIVERS_PATH finds that Mesa appears to have the same
issue, and it also finds that we should also search for GBM_DRIVERS_PATH
(apparently, for older Mesa) and maybe EGL_DRIVERS_PATH and EGL_DRIVER,
and LIBVA_DRIVERS_PATH and LIBVA_DRIVER_NAME.  There are probably more."

> Using `secure_getenv` in some of these cases would probably work as well 
> as checking `getauxval(AT_SECURE)`, especially because it seems (from my 
> quick search over at <>) that both are Linux specific 
> anyway.
> It would be nice to define a `is_privileged_context()` function that 
> works on most platforms to be shared across projects or used as a 
> library.

Historically, that's __libc_enable_secure on glibc (although if
secure_getenv() does what's needed in a given context, then you don't
need to use __libc_enable_secure directly) and issetugid(2) on OpenBSD.


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