Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

Florian Weimer
2021-08-12T12:21:07Z

* David Hildenbrand:

> There are some (minor) user-visible changes with this series:
> 1. We no longer deny write access to shared libaries loaded via legacy
> uselib(); this behavior matches modern user space e.g., via dlopen().
> 2. We no longer deny write access to the elf interpreter after exec
> completed, treating it just like shared libraries (which it often is).

We have a persistent issue with people using cp (or similar tools) to
replace system libraries. Since the file is truncated first, all
relocations and global data are replaced by file contents, result in
difficult-to-diagnose crashes. It would be nice if we had a way to
prevent this mistake. It doesn't have to be MAP_DENYWRITE or MAP_COPY.
It could be something completely new, like an option that turns every
future access beyond the truncation point into a signal (rather than
getting bad data or bad code and crashing much later).

I don't know how many invalid copy operations are currently thwarted by
the current program interpreter restriction. I doubt that lifting the
restriction matters.

> 3. We always deny write access to the file linked via /proc/pid/exe:
> sys_prctl(PR_SET_MM_EXE_FILE) will fail if write access to the file
> cannot be denied, and write access to the file will remain denied
> until the link is effectivel gone (exec, termination,
> PR_SET_MM_EXE_FILE) -- just as if exec'ing the file.
>
> I was wondering if we really care about permanently disabling write access
> to the executable, or if it would be good enough to just disable write
> access while loading the new executable during exec; but I don't know
> the history of that -- and it somewhat makes sense to deny write access
> at least to the main executable. With modern user space -- dlopen() -- we
> can effectively modify the content of shared libraries while being used.

Is there a difference between ET_DYN and ET_EXEC executables?

Thanks,
Florian