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RHEL7 glibc sample

Most of the binaries in the system are coming from distribution packages so building patches for them is different from the above. Here is how to do it.

This example builds glibc patch for an old fashioned CVE-2015-0235 GHOST vulnerability for RHEL7. The build is done using `scripts/pkgbuild`_ and package files are stored in packages/rhel7/glibc/glibc-2.17-55.el7.

Preparing environment

First, we need the exact versions of tools and libs. Let's build a Docker image and a container for it:

$ docker build -t kernelcare/centos7:gcc-4.8.2-16.el7 \
        docker/kernelcare/centos7/gcc-4.8.2-16.el7
...
$ docker run -v $PWD:/libcare --cap-add SYS_PTRACE -it \
        kernelcare/centos7:gcc-4.8.2-16.el7 /bin/bash
[root@... /]#

Now, from inside the container let's install vulnerable version of glibc:

[root@... /]# yum downgrade -y --enablerepo=C7.0.1406-base \
        glibc-2.17-55.el7 glibc-devel-2.17-55.el7 \
        glibc-headers-2.17-55.el7 glibc-common-2.17-55.el7
...

Build the libcare tools:

[root@... /]# make -C /libcare/src clean all && make -C /libcare/tests/execve
...

Now build and run the sample GHOST app that runs 16 threads to constantly check whether the glibc is vulnerable to GHOST and prints a dot every time it detects a buffer overflow in the gethostbyname_r function. The downgraded glibc is vulnerable:

[root@... /]# cd /libcare/samples/ghost
[root@... ghost]# make
...
[root@... ghost]# ./GHOST
............^C

Press Ctrl+C to get your console back and let's start building the patch for glibc.

Building and applying the patch

The build is done in two stages.

First, the original package build is repeated with all the `intermediate assembly files`_ stored and saved for later. This greatly helps to speed up builds against the same base code. Run the following from inside our docker container to pre-build glibc package:

[root@... /]# cd /libcare/
[root@... /libcare]# ./scripts/pkgbuild -p packages/rhel7/glibc/glibc-2.17-55.el7
...

This should download the package, do a regular RPM build with kpatch_cc wrapper substituted for GCC and store the pre-built data into the archive under /kcdata directory:

[root@... /libcare]# ls /kcdata
build.orig-glibc-2.17-55.el7.x86_64.rpm.tgz  glibc-2.17-55.el7.src.rpm

Now let's build the patch, the output will be verbose since it contains tests run by the kp_patch_test defined in packages/rhel7/glibc/glibc-2.17-55.el7/info:

[root@... /libcare]# ./scripts/pkgbuild packages/rhel7/glibc/glibc-2.17-55.el7
...
[root@... /libcare]# ls /kcdata/kpatch*
/kcdata/kpatch-glibc-2.17-55.el7.x86_64.tgz

Unwrap patches and run the GHOST sample:

[root@... /libcare]# cd /kcdata
[root@... /kcdata]# tar xf kpatch*
[root@... /kcdata]# /libcare/samples/ghost/GHOST 2>/dev/null &
[root@... /kcdata]# patient_pid=$!

And, finally, patch it. All the threads of the sample must stop when the GHOST vulnerability is patched:

[root@... /kcdata]# /libcare/src/libcare-ctl -v patch -p $patient_pid \
                root/kpatch-glibc-2.17-55.el7.x86_64
...
1 patch hunk(s) have been successfully applied to PID '...'
(Press Enter again)
[1]+  Done                    /libcare/samples/ghost/GHOST 2> /dev/null

You can patch any running application this way:

[root@... /kcdata]# sleep 100 &
[root@... /kcdata]# patient_pid=$!
[root@... /kcdata]# /libcare/src/libcare-ctl -v patch -p $patient_pid \
                root/kpatch-glibc-2.17-55.el7.x86_64
...
1 patch hunk(s) have been successfully applied to PID '...'

Congratulations on finishing this rather confusing sample!