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.
This commit is contained in:
Eelco Dolstra 2012-02-15 01:31:56 +01:00
parent 5e57047d87
commit bd013b6f98
7 changed files with 130 additions and 8 deletions

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@ -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++)

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@ -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);
}

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@ -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

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@ -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 += \

67
src/libutil/immutable.cc Normal file
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@ -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);
}
}

19
src/libutil/immutable.hh Normal file
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@ -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 */

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@ -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;