daemon: Address shortcoming in previous security fix for CVE-2024-27297.

This is a followup to 8f4ffb3fae133bb21d7991e97c2f19a7108b1143.

Commit 8f4ffb3fae133bb21d7991e97c2f19a7108b1143 fell short in two
ways: (1) it didn’t have any effet for fixed-output derivations
performed in a chroot, which is the case for all of them except those
using “builtin:download” and “builtin:git-download”, and (2) it did not
preserve ownership when copying, leading to “suspicious ownership or
permission […] rejecting this build output” errors.

* nix/libstore/build.cc (DerivationGoal::buildDone): Account for
‘chrootRootDir’ when copying ‘drv.outputs’.
* nix/libutil/util.cc (copyFileRecursively): Add ‘fchown’ and ‘fchownat’
calls to preserve file ownership; this is necessary for chrooted
fixed-output derivation builds.
* nix/libutil/util.hh: Update comment.

Change-Id: Ib59f040e98fed59d1af81d724b874b592cbef156

2 files changed, 8 insertions, 3 deletions

diff --git a/nix/libutil/util.cc b/nix/libutil/util.cc
index 493f06f357a..578d6572934 100644
--- a/nix/libutil/util.cc
+++ b/nix/libutil/util.cc
@@ -422,6 +422,7 @@ static void copyFileRecursively(int sourceroot, const Path &source,
         if (destinationFd == -1) throw SysError(format("opening `%1%'") % source);
 
         copyFile(sourceFd, destinationFd);
+        fchown(destinationFd, st.st_uid, st.st_gid);
     } else if (S_ISLNK(st.st_mode)) {
         char target[st.st_size + 1];
         ssize_t result = readlinkat(sourceroot, source.c_str(), target, st.st_size);
@@ -430,6 +431,8 @@ static void copyFileRecursively(int sourceroot, const Path &source,
         int err = symlinkat(target, destinationroot, destination.c_str());
         if (err != 0)
             throw SysError(format("creating symlink `%1%'") % destination);
+        fchownat(destinationroot, destination.c_str(),
+                 st.st_uid, st.st_gid, AT_SYMLINK_NOFOLLOW);
     } else if (S_ISDIR(st.st_mode)) {
         int err = mkdirat(destinationroot, destination.c_str(), 0755);
         if (err != 0)
@@ -455,6 +458,7 @@ static void copyFileRecursively(int sourceroot, const Path &source,
         for (auto & i : readDirectory(sourceFd))
             copyFileRecursively((int)sourceFd, i.name, (int)destinationFd, i.name,
                                 deleteSource);
+        fchown(destinationFd, st.st_uid, st.st_gid);
     } else throw Error(format("refusing to copy irregular file `%1%'") % source);
 
     if (deleteSource)
diff --git a/nix/libutil/util.hh b/nix/libutil/util.hh
index 058f5f84465..377aac06847 100644
--- a/nix/libutil/util.hh
+++ b/nix/libutil/util.hh
@@ -102,9 +102,10 @@ void deletePath(const Path & path);
 void deletePath(const Path & path, unsigned long long & bytesFreed,
     size_t linkThreshold = 1);
 
-/* Copy SOURCE to DESTINATION, recursively.  Throw if SOURCE contains a file
-   that is not a regular file, symlink, or directory.  When DELETESOURCE is
-   true, delete source files once they have been copied.  */
+/* Copy SOURCE to DESTINATION, recursively, preserving ownership.  Throw if
+   SOURCE contains a file that is not a regular file, symlink, or directory.
+   When DELETESOURCE is true, delete source files once they have been
+   copied.  */
 void copyFileRecursively(const Path &source, const Path &destination,
     bool deleteSource = false);