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Revision as of 07:09, 4 June 2021


Broken RPATH will fail rpmbuild

Summary

Enable broken RPATH detection buildroot policy script by default. This will make the RPM build fail once a broken RPATH was detected within a binary or a shared library file. An opt-out mechanism will be provided as well.

Owner

Current status

Detailed Description

The dynamic linker and loader (ld.so) is responsible for resolving runtime dependencies of executables and shared library files through a search hierarchy. However some packages (usually through their upstream buildsystems) contain a hard-coded path within their binaries or .so files, by using the -R or -rpath flag during compilation, which is called an RPATH. By utilizing RPATH, ELF files can point to directories to be included in the search path, on runtime, to resolve their dependencies.

While RPATH can be used for non-standard directories, such as a place containing private libraries of the project, when it points to a value already provided by the search path of ld.so, it changes the hierarchy of the search by placing the system defaults first.

(a) DT_RPATH -> (b) LD_LIBRARY_PATH -> (c) DT_RUNPATH -> (d) cache (/etc/ld.so.cache) -> (e) system defaults

This could present a variety of issues, such as LD_LIBRARY_PATH overrides not working, incomplete dependency resolution, loading of wrong libraries etc. In general, changing the default search hierarchy could lead to unforeseen bugs and issues - in a similar manner as adding /usr/lib64 to LD_LIBRARY_PATH.

Another problem of a hardcoded RPATH is security. When an ELF object contains an RPATH pointed to a directory not managed by the system, where some malicious actor has write permissions to, it's relatively easy to execute arbitrary code.

Performance can be also affected, since probing explicitly e.g. /usr/lib64 through RPATH adds extra open/openat system calls to the process startup.

In Fedora the use of such RPATH is forbidden, but it was never enforced. This change intends to ratify that by executing /usr/lib/rpm/check-rpaths during rpmbuild, after %install, and fail the build if an RPATH entry was detected. This change will not affect RPATH's pointing to private libraries.

Definition of a broken RPATH

This change will use the rpm script for checking the broken RPATH's.

The categories are:

  • standard RPATHs (e.g. /usr/lib or /usr/lib64); such RPATHs are a minor issue but are introducing redundant searchpaths without providing a benefit. They can also cause errors in multilib environments.
  • invalid RPATHs; these are RPATHs which are neither absolute nor relative filenames and can therefore be a SECURITY risk
  • insecure RPATHs; these are relative RPATHs which are a SECURITY risk
  • the special $ORIGIN RPATHs are appearing after other RPATHs; this is just a minor issue but usually unwanted
  • the RPATH is empty; there is no reason for such RPATHs and they cause unneeded work while loading libraries
  • an RPATH references .. of an absolute path; this will break the functionality when the path before .. is a symlink

Opting out

A standard opt-out mechanism is provided, same as the other buildroot policy scripts, if the script provides incorrect results for your package. Simply add %global __brp_check_rpaths %{nil} on top of your SPEC. According to the guidelines, any package that disables a BRP script this way, MUST also note the reason in an accompanying comment.

Feedback

The change has been proposed a long time ago through FPC and the general consensus is that it needs to be done along with an overhaul of the Fedora documentation in regards to RPATH.

An email thread was also started on Fedora devel regarding this change.

There have been multiple requests in the past to enable that check, as well as various attempts to remove RPATH's from packages in the distro. 0 123

As for other distributions, Debian discourages the use of RPATH, openSUSE forbids it by running the check from rpmlint after every package build and Arch and Gentoo point out to possible insecure usage at their respective documentation pages.

Also there has been a relevant discussion in the upstream python community.

Benefit to Fedora

The main benefit of this change is avoiding bugs that might stem from hardcoded RPATH values which include but not limited to: loading of wrong or malicious code, wrong dependency resolution of object files etc. There will also be a performance increase by derived from avoiding unnecessary system calls required for handling RPATH's.

In addition the RPATH related packaging guidelines will be enforced by the build system.

Scope

  • Proposal owners: Merge the pull request to redhat-rpm-config to enable running the check-rpaths script after %install.
  • Other developers: After merging the changes to redhat-rpm-config the affected package maintainers that will see their packages' builds fail, will need to review their usage of RPATH and either remove it or workaround the issue. The packages currently failing to build due to RPATH issues, so far, are listed bellow:
  • Release engineering: This change doesn't require coordination with rel-eng, as any issues will be caught during the regular mass rebuild of packages.
  • Policies and guidelines: TODO: The guidelines will be overhauled to take into account accepted usage or RPATH, clarification of the policy and ways to opt-out.

FPC ticket: https://pagure.io/packaging-committee/issue/886

  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

There will be no visible impact to non-packagers. Packagers will need to fix their packages if an broken RPATH entry was detected, as a broken RPATH will make the rpmbuild fail either in koji or locally.

How To Test

  • Mock build testing:

Initiate a Fedora rawhide mock chroot and install the modified redhat-rpm-config:

 mock -r fedora-rawhide-x86_64 --install https://download.copr.fedorainfracloud.org/results/cstratak/rpath/fedora-rawhide-x86_64/02160930-redhat-rpm-config/redhat-rpm-config-190-1.fc35.noarch.rpm

Build your package with the --no-clean option:

mock -r fedora-rawhide-x86_64 --no-clean <srpm>  
  • Local testing: Building rpm's locally should already reveal the issue as the .rpmmacros file defines the RPATH check. Sample of a vanilla .rpmmacros file:
%_topdir %(echo $HOME)/rpmbuild
 
%__arch_install_post \
   [ "%{buildarch}" = "noarch" ] || QA_CHECK_RPATHS=1 ; \
   case "${QA_CHECK_RPATHS:-}" in [1yY]*) /usr/lib/rpm/check-rpaths ;; esac \
   /usr/lib/rpm/check-buildroot
  • rpmlint test: The binary-or-shlib-defines-rpath test from the BinariesCheck module will reveal RPATH usage.
 rpmlint -c BinariesCheck <binary rpm>
 e.g.: rpmlint -c BinariesCheck -v audiofile-0.3.6-27.fc34.x86_64.rpm 
       audiofile.x86_64: I: checking
       audiofile.x86_64: E: binary-or-shlib-defines-rpath /usr/bin/sfconvert ['/usr/lib64']
       audiofile.x86_64: E: binary-or-shlib-defines-rpath /usr/bin/sfinfo ['/usr/lib64']
       1 packages and 0 specfiles checked; 2 errors, 0 warnings.
  • rpminspect test: Using the rpminspect-fedora tool can reveal RPATH as well as RUNPATH usage.
rpminspect-fedora --tests runpath <binary rpm>
e.g. rpminspect-fedora --tests runpath audiofile-0.3.6-27.fc34.x86_64.rpm
     
     runpath:
     --------
    1) /usr/bin/sfconvert has an invalid-looking DT_RPATH on x86_64: /usr/lib64 
    Result: VERIFY
    Waiver Authorization: Anyone

    Suggested Remedy: Either DT_RPATH or DT_RUNPATH properties were found on ELF shared objects in this package.  The use of
    DT_RPATH and DT_RUNPATH is discouraged except in certain situations.  Check to see that you a disabling rpath during the
    %build stage of the spec file.  If you are unable to do this easily, you can try using a program such as patchelf to remove
    these properties from the ELF files.

    2) /usr/bin/sfinfo has an invalid-looking DT_RPATH on x86_64: /usr/lib64 
    Result: VERIFY
    Waiver Authorization: Anyone

    Suggested Remedy: Either DT_RPATH or DT_RUNPATH properties were found on ELF shared objects in this package. The use of
    DT_RPATH and DT_RUNPATH is discouraged except in certain situations.  Check to see that you a disabling rpath during
    the %build stage of the spec file.  If you are unable to do this easily, you can try using a program such as patchelf to
    remove these properties from the ELF files.

User Experience

N/A (While this is a system wide change the impact is not visible to regular users, only to packagers as described above).

Dependencies

The change depends on modifying redhat-rpm-config.

Contingency Plan

  • Contingency mechanism: The change to redhat-rpm-config will be reverted
  • Contingency deadline: Beta freeze
  • Blocks release? No

Documentation

TODO:

The documentation of the packaging guidelines will be updated to reflect the changes that this change brings. Updated guidelines will be https://docs.fedoraproject.org/en-US/packaging-guidelines/#_beware_of_rpath

Release Notes