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

This is a followup to 8f4ffb3fae.

Commit 8f4ffb3fae 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

nix/libstore/build.cc

@@ -1387,13 +1387,14 @@ void DerivationGoal::buildDone()
                make sure that there's no stale file descriptor pointing to it
                (CVE-2024-27297).  */
 	    foreach (DerivationOutputs::iterator, i, drv.outputs) {
-		if (pathExists(i->second.path)) {
+		Path output = chrootRootDir + i->second.path;
-		    Path pivot = i->second.path + ".tmp";
+		if (pathExists(output)) {
-		    copyFileRecursively(i->second.path, pivot, true);
+		    Path pivot = output + ".tmp";
-		    int err = rename(pivot.c_str(), i->second.path.c_str());
+		    copyFileRecursively(output, pivot, true);
+		    int err = rename(pivot.c_str(), output.c_str());
 		    if (err != 0)
 			throw SysError(format("renaming `%1%' to `%2%'")
-				       % pivot % i->second.path);
+				       % pivot % output);
 		}
 	    }
 	}

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)

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
+/* Copy SOURCE to DESTINATION, recursively, preserving ownership.  Throw if
-   that is not a regular file, symlink, or directory.  When DELETESOURCE is
+   SOURCE contains a file that is not a regular file, symlink, or directory.
-   true, delete source files once they have been copied.  */
+   When DELETESOURCE is true, delete source files once they have been
+   copied.  */
 void copyFileRecursively(const Path &source, const Path &destination,
     bool deleteSource = false);