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Fixes #18124: atomically replace /var/setuid-wrappers/ (#18186)

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Before this commit updating /var/setuid-wrappers/ folder introduced
a small window where NixOS activation scripts could be terminated
and resulted into empty /var/setuid-wrappers/ folder.

That's very unfortunate because one might lose sudo binary.

Instead we use two atomic operations mv and ln (as described in
https://axialcorps.com/2013/07/03/atomically-replacing-files-and-directories/)
to achieve atomicity.

Since /var/setuid-wrappers is not a directory anymore, tmpfs mountpoints
were removed in installation scripts and in boot process.

Tested:

- upgrade /var/setuid-wrappers/ from folder to a symlink
- make sure /run/setuid-wrappers-dirs/ legacy symlink is really deleted
This commit is contained in:
Domen Kožar 2016-09-01 20:57:51 +02:00 • committed by GitHub
parent 78cd9f8ebc
commit a6670c1a0b
4 changed files with 35 additions and 16 deletions
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34
nixos/modules/security/setuid-wrappers.nix
View file

@ -12,7 +12,7 @@ let
installPhase = ''
mkdir -p $out/bin
cp ${./setuid-wrapper.c} setuid-wrapper.c
gcc -Wall -O2 -DWRAPPER_DIR=\"${wrapperDir}\" \
gcc -Wall -O2 -DWRAPPER_DIR=\"/run/setuid-wrapper-dirs\" \
setuid-wrapper.c -o $out/bin/setuid-wrapper
'';
};
@ -102,11 +102,11 @@ in
source=/nix/var/nix/profiles/default/bin/${program}
fi
cp ${setuidWrapper}/bin/setuid-wrapper ${wrapperDir}/${program}
echo -n "$source" > ${wrapperDir}/${program}.real
chmod 0000 ${wrapperDir}/${program} # to prevent races
chown ${owner}.${group} ${wrapperDir}/${program}
chmod "u${if setuid then "+" else "-"}s,g${if setgid then "+" else "-"}s,${permissions}" ${wrapperDir}/${program}
cp ${setuidWrapper}/bin/setuid-wrapper $wrapperDir/${program}
echo -n "$source" > $wrapperDir/${program}.real
chmod 0000 $wrapperDir/${program} # to prevent races
chown ${owner}.${group} $wrapperDir/${program}
chmod "u${if setuid then "+" else "-"}s,g${if setgid then "+" else "-"}s,${permissions}" $wrapperDir/${program}
'';
in stringAfter [ "users" ]
@ -115,9 +115,29 @@ in
# programs to be wrapped.
SETUID_PATH=${config.system.path}/bin:${config.system.path}/sbin
rm -f ${wrapperDir}/* # */
mkdir -p /run/setuid-wrapper-dirs
wrapperDir=$(mktemp --directory --tmpdir=/run/setuid-wrapper-dirs setuid-wrappers.XXXXXXXXXX)
${concatMapStrings makeSetuidWrapper setuidPrograms}
if [ -L ${wrapperDir} ]; then
# Atomically replace the symlink
# See https://axialcorps.com/2013/07/03/atomically-replacing-files-and-directories/
old=$(readlink ${wrapperDir})
ln --symbolic --force --no-dereference $wrapperDir ${wrapperDir}-tmp
mv --no-target-directory ${wrapperDir}-tmp ${wrapperDir}
rm --force --recursive $old
elif [ -d ${wrapperDir} ]; then
# Compatibility with old state, just remove the folder and symlink
rm -f ${wrapperDir}/*
# if it happens to be a tmpfs
umount ${wrapperDir} || true
rm -d ${wrapperDir}
ln -d --symbolic $wrapperDir ${wrapperDir}
else
# For initial setup
ln --symbolic $wrapperDir ${wrapperDir}
fi
'';
};