Re: [musl] [PATCH] ldso/dynlink: Protect LD_ env vars from getting clobbered by apps

[Rich Felker <dalias@libc.org>]

On Wed, Aug 17, 2022 at 12:10:48PM -0400, James Y Knight wrote:
> Sidenote: Linux does support a less awful way to change the kernel's view
> of argv these days, using prctl(PR_SET_MM, PR_SET_MM_ARG_START (or _END),
> addr, 0, 0). Sadly, it only allows root (CAP_SYS_RESOURCE) to use it. I'm
> not sure why, perhaps that restriction could be relaxed for future
> kernels...
> 
> See
> https://github.com/systemd/systemd/blob/87305b0fbfc0e40a948cf0a683bcf9d47b8a41a3/src/basic/process-util.c#L256
> for an example of use (including ugly workaround for the API being silly
> and setting START/END with separate syscalls, but requiring START <= END at
> all times)

Yes, unfortunately (at least last I checked) it's also only available
if CRIU support was enabled in the kernel since that's the only thing
its creators envisioned it being used for... *sigh*

> 
> On Wed, Aug 17, 2022 at 6:05 AM Elvis Pranskevichus <elvis@edgedb.com>
> wrote:
> 
> > There is no guarantee that the environment block will remain intact.
> > For example, PostgreSQL clobbers argv/environ area to implement its
> > "setproctitle" emulation on non-BSD [1], and there is a popular Python
> > library inspired by it [2].  As a result, setting `LD_LIBRARY_PATH`
> > or `LD_PRELOAD` has no effect on Postgres subprocesses when linking
> > against musl.
> >
> > Protect against this by making a copies instead of storing the
> > original pointers directly.
> >
> > (please CC me, I'm not subscribed to the list)
> >
> > ---
> >  ldso/dynlink.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/ldso/dynlink.c b/ldso/dynlink.c
> > index cc677952..703342b8 100644
> > --- a/ldso/dynlink.c
> > +++ b/ldso/dynlink.c
> > @@ -1756,8 +1756,8 @@ void __dls3(size_t *sp, size_t *auxv)
> >
> >         /* Only trust user/env if kernel says we're not suid/sgid */
> >         if (!libc.secure) {
> > -               env_path = getenv("LD_LIBRARY_PATH");
> > -               env_preload = getenv("LD_PRELOAD");
> > +               env_path = strdup(getenv("LD_LIBRARY_PATH"));
> > +               env_preload = strdup(getenv("LD_PRELOAD"));
> >         }
> >
> >         /* Activate error handler function */
> >
> >
> >
> >
> >