On Linux, make the Nix store really read-only by using the immutable bit

I was bitten one time too many by Python modifying the Nix store by
creating *.pyc files when run as root.  On Linux, we can prevent this
by setting the immutable bit on files and directories (as in ‘chattr
+i’).  This isn't supported by all filesystems, so it's not an error
if setting the bit fails.  The immutable bit is cleared by the garbage
collector before deleting a path.  The only tricky aspect is in
optimiseStore(), since it's forbidden to create hard links to an
immutable file.  Thus optimiseStore() temporarily clears the immutable
bit before creating the link.

configure.ac

@@ -113,8 +113,8 @@ AC_LANG_POP(C++)
 # Check for chroot support (requires chroot() and bind mounts).
 AC_CHECK_FUNCS([chroot])
 AC_CHECK_FUNCS([unshare])
-AC_CHECK_HEADERS([sched.h], [], [], [])
-AC_CHECK_HEADERS([sys/param.h], [], [], [])
+AC_CHECK_HEADERS([sched.h])
+AC_CHECK_HEADERS([sys/param.h])
 AC_CHECK_HEADERS([sys/mount.h], [], [],
 [#ifdef HAVE_SYS_PARAM_H
 # include <sys/param.h>
@@ -124,7 +124,7 @@ AC_CHECK_HEADERS([sys/mount.h], [], [],
 
 # Check for <locale>.
 AC_LANG_PUSH(C++)
-AC_CHECK_HEADERS([locale], [], [], [])
+AC_CHECK_HEADERS([locale])
 AC_LANG_POP(C++)
 
 
@@ -138,9 +138,13 @@ AC_SUBST([bsddiff_compat_include])
 AC_CHECK_HEADERS([sys/personality.h])
 
 
+# Check for <linux/fs.h> (for immutable file support).
+AC_CHECK_HEADERS([linux/fs.h])
+
+
 # Check for tr1/unordered_set.
 AC_LANG_PUSH(C++)
-AC_CHECK_HEADERS([tr1/unordered_set], [], [], [])
+AC_CHECK_HEADERS([tr1/unordered_set])
 AC_LANG_POP(C++)
 
 

src/libstore/local-store.cc

@@ -5,6 +5,7 @@
 #include "pathlocks.hh"
 #include "worker-protocol.hh"
 #include "derivations.hh"
+#include "immutable.hh"
     
 #include <iostream>
 #include <algorithm>
@@ -405,6 +406,10 @@ void canonicalisePathMetaData(const Path & path, bool recurse)
     if (lstat(path.c_str(), &st))
 	throw SysError(format("getting attributes of path `%1%'") % path);
 
+    /* Really make sure that the path is of a supported type.  This
+       has already been checked in dumpPath(). */
+    assert(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode));
+
     /* Change ownership to the current uid.  If it's a symlink, use
        lchown if available, otherwise don't bother.  Wrong ownership
        of a symlink doesn't matter, since the owning user can't change
@@ -451,6 +456,8 @@ void canonicalisePathMetaData(const Path & path, bool recurse)
 	foreach (Strings::iterator, i, names)
 	    canonicalisePathMetaData(path + "/" + *i, true);
     }
+
+    makeImmutable(path);
 }
 
 

src/libstore/optimise-store.cc

@@ -1,5 +1,6 @@
 #include "util.hh"
 #include "local-store.hh"
+#include "immutable.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -19,6 +20,7 @@ static void makeWritable(const Path & path)
     struct stat st;
     if (lstat(path.c_str(), &st))
 	throw SysError(format("getting attributes of path `%1%'") % path);
+    if (S_ISDIR(st.st_mode) || S_ISREG(st.st_mode)) makeMutable(path);
     if (chmod(path.c_str(), st.st_mode | S_IWUSR) == -1)
         throw SysError(format("changing writability of `%1%'") % path);
 }
@@ -31,6 +33,8 @@ struct MakeReadOnly
     ~MakeReadOnly()
     {
         try {
+            /* This will make the path read-only (and restore the
+               immutable bit on platforms that support it). */
             if (path != "") canonicalisePathMetaData(path, false);
         } catch (...) {
             ignoreException();
@@ -39,6 +43,14 @@ struct MakeReadOnly
 };
 
 
+struct MakeImmutable
+{
+    Path path;
+    MakeImmutable(const Path & path) : path(path) { }
+    ~MakeImmutable() { makeImmutable(path); }
+};
+
+
 static void hashAndLink(bool dryRun, HashToPath & hashToPath,
     OptimiseStats & stats, const Path & path)
 {
@@ -96,14 +108,24 @@ static void hashAndLink(bool dryRun, HashToPath & hashToPath,
 
             /* Make the containing directory writable, but only if
                it's not the store itself (we don't want or need to
-               mess with  its permissions). */
+               mess with its permissions). */
             bool mustToggle = !isStorePath(path);
             if (mustToggle) makeWritable(dirOf(path));
             
             /* When we're done, make the directory read-only again and
                reset its timestamp back to 0. */
             MakeReadOnly makeReadOnly(mustToggle ? dirOf(path) : "");
-        
+
+            /* If ‘prevPath’ is immutable, we can't create hard links
+               to it, so make it mutable first (and make it immutable
+               again when we're done).  We also have to make ‘path’
+               mutable, otherwise rename() will fail to delete it. */
+            makeMutable(prevPath.first);
+            MakeImmutable mk1(prevPath.first);
+            
+            makeMutable(path);
+            MakeImmutable mk2(path);
+
             if (link(prevPath.first.c_str(), tempLink.c_str()) == -1) {
                 if (errno == EMLINK) {
                     /* Too many links to the same file (>= 32000 on

src/libutil/Makefile.am

@@ -1,12 +1,12 @@
 pkglib_LTLIBRARIES = libutil.la
 
 libutil_la_SOURCES = util.cc hash.cc serialise.cc \
-  archive.cc xml-writer.cc
+  archive.cc xml-writer.cc immutable.cc
 
 libutil_la_LIBADD = ../boost/format/libformat.la ${aterm_lib} ${sqlite_lib}
 
 pkginclude_HEADERS = util.hh hash.hh serialise.hh \
-  archive.hh xml-writer.hh types.hh
+  archive.hh xml-writer.hh types.hh immutable.hh
 
 if !HAVE_OPENSSL
 libutil_la_SOURCES += \

src/libutil/immutable.cc

@@ -0,0 +1,67 @@
+#include "config.h"
+
+#include "immutable.hh"
+#include "util.hh"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#if HAVE_LINUX_FS_H
+#include <linux/fs.h>
+#include <sys/ioctl.h>
+#include <errno.h>
+#endif
+
+namespace nix {
+
+
+void changeMutable(const Path & path, bool mut)
+{
+#if defined(FS_IOC_SETFLAGS) && defined(FS_IOC_GETFLAGS) && defined(FS_IMMUTABLE_FL)
+
+    /* Don't even try if we're not root.  One day we should support
+       the CAP_LINUX_IMMUTABLE capability. */
+    if (getuid() != 0) return;
+
+    /* The O_NOFOLLOW is important to prevent us from changing the
+       mutable bit on the target of a symlink (which would be a
+       security hole). */
+    AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_NOFOLLOW);
+    if (fd == -1) {
+        if (errno == ELOOP) return; // it's a symlink
+        throw SysError(format("opening file `%1%'") % path);
+    }
+
+    unsigned int flags = 0, old;
+
+    /* Silently ignore errors getting/setting the immutable flag so
+       that we work correctly on filesystems that don't support it. */
+    if (ioctl(fd, FS_IOC_GETFLAGS, &flags)) return;
+
+    old = flags;
+    
+    if (mut) flags &= ~FS_IMMUTABLE_FL;
+    else flags |= FS_IMMUTABLE_FL;
+
+    if (old == flags) return;
+
+    if (ioctl(fd, FS_IOC_SETFLAGS, &flags)) return;
+    
+#endif
+}
+
+
+void makeImmutable(const Path & path)
+{
+    changeMutable(path, false);
+}
+
+
+void makeMutable(const Path & path)
+{
+    changeMutable(path, true);
+}
+
+
+}

src/libutil/immutable.hh

@@ -0,0 +1,19 @@
+#ifndef __IMMUTABLE_H
+#define __IMMUTABLE_H
+
+#include <types.hh>
+
+namespace nix {
+
+/* Make the given path immutable, i.e., prevent it from being modified
+   in any way, even by root.  This is a no-op on platforms that do not
+   support this, or if the calling user is not privileged.  On Linux,
+   this is implemented by doing the equivalent of ‘chattr +i path’. */
+void makeImmutable(const Path & path);
+
+/* Make the given path mutable. */
+void makeMutable(const Path & path);
+
+}
+
+#endif /* !__IMMUTABLE_H */

src/libutil/util.cc

@@ -12,6 +12,7 @@
 #include <limits.h>
 
 #include "util.hh"
+#include "immutable.hh"
 
 
 extern char * * environ;
@@ -304,6 +305,8 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed,
 
     struct stat st = lstat(path);
 
+    if (S_ISDIR(st.st_mode) || S_ISREG(st.st_mode)) makeMutable(path);
+
     if (!S_ISDIR(st.st_mode) && st.st_nlink == 1) {
         bytesFreed += st.st_size;
         blocksFreed += st.st_blocks;