Restore parent mount namespace before executing a child process

This ensures that they can't write to /nix/store. Fixes #2535.

src/libstore/local-store.cc

@@ -366,6 +366,8 @@ void LocalStore::makeStoreWritable()
         throw SysError("getting info about the Nix store mount point");
 
     if (stat.f_flag & ST_RDONLY) {
+        saveMountNamespace();
+
         if (unshare(CLONE_NEWNS) == -1)
             throw SysError("setting up a private mount namespace");
 

src/libstore/ssh.cc

@@ -1,4 +1,5 @@
 #include "ssh.hh"
+#include "affinity.hh"
 
 namespace nix {
 
@@ -34,7 +35,9 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string
 
     auto conn = std::make_unique<Connection>();
     conn->sshPid = startProcess([&]() {
+        restoreAffinity();
         restoreSignals();
+        restoreMountNamespace();
 
         close(in.writeSide.get());
         close(out.readSide.get());

src/libutil/util.cc

@@ -936,6 +936,7 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
                 throw SysError("setting death signal");
 #endif
             restoreAffinity();
+            restoreMountNamespace();
             fun();
         } catch (std::exception & e) {
             try {
@@ -1504,4 +1505,26 @@ std::unique_ptr<InterruptCallback> createInterruptCallback(std::function<void()>
     return std::unique_ptr<InterruptCallback>(res.release());
 }
 
+static AutoCloseFD fdSavedMountNamespace;
+
+void saveMountNamespace()
+{
+#if __linux__
+    std::once_flag done;
+    std::call_once(done, []() {
+        fdSavedMountNamespace = open("/proc/self/ns/mnt", O_RDONLY);
+        if (!fdSavedMountNamespace)
+            throw SysError("saving parent mount namespace");
+    });
+#endif
+}
+
+void restoreMountNamespace()
+{
+#if __linux__
+    if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
+        throw SysError("restoring parent mount namespace");
+#endif
+}
+
 }

src/libutil/util.hh

@@ -514,4 +514,13 @@ typedef std::function<bool(const Path & path)> PathFilter;
 extern PathFilter defaultPathFilter;
 
 
+/* Save the current mount namespace. Ignored if called more than
+   once. */
+void saveMountNamespace();
+
+/* Restore the mount namespace saved by saveMountNamespace(). Ignored
+   if saveMountNamespace() was never called. */
+void restoreMountNamespace();
+
+
 }

src/nix-build/nix-build.cc

@@ -401,8 +401,6 @@ static void _main(int argc, char * * argv)
             } else
                 env[var.first] = var.second;
 
-        restoreAffinity();
-
         /* Run a shell using the derivation's environment.  For
            convenience, source $stdenv/setup to setup additional
            environment variables and shell functions.  Also don't
@@ -446,7 +444,9 @@ static void _main(int argc, char * * argv)
 
         auto argPtrs = stringsToCharPtrs(args);
 
+        restoreAffinity();
         restoreSignals();
+        restoreMountNamespace();
 
         execvp(shell.c_str(), argPtrs.data());
 

src/nix/edit.cc

@@ -3,6 +3,7 @@
 #include "eval.hh"
 #include "attr-path.hh"
 #include "progress-bar.hh"
+#include "affinity.hh"
 
 #include <unistd.h>
 
@@ -72,6 +73,10 @@ struct CmdEdit : InstallableCommand
 
         stopProgressBar();
 
+        restoreAffinity();
+        restoreSignals();
+        restoreMountNamespace();
+
         execvp(args.front().c_str(), stringsToCharPtrs(args).data());
 
         throw SysError("cannot run editor '%s'", editor);

src/nix/repl.cc

@@ -285,6 +285,8 @@ static int runProgram(const string & program, const Strings & args)
     if (pid == -1) throw SysError("forking");
     if (pid == 0) {
         restoreAffinity();
+        restoreSignals();
+        restoreMountNamespace();
         execvp(program.c_str(), stringsToCharPtrs(args2).data());
         _exit(1);
     }

src/nix/run.cc

@@ -153,9 +153,9 @@ struct CmdRun : InstallablesCommand
 
         stopProgressBar();
 
-        restoreSignals();
-
         restoreAffinity();
+        restoreSignals();
+        restoreMountNamespace();
 
         /* If this is a diverted store (i.e. its "logical" location
            (typically /nix/store) differs from its "physical" location