[PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC

Mickaël Salaün
2020-07-23T17:12:57Z

Allow for the enforcement of the O_MAYEXEC openat2(2) flag. Thanks to
the noexec option from the underlying VFS mount, or to the file execute
permission, userspace can enforce these execution policies. This may
allow script interpreters to check execution permission before reading
commands from a file, or dynamic linkers to allow shared object loading.

Add a new sysctl fs.open_mayexec_enforce to enable system administrators
to enforce two complementary security policies according to the
installed system: enforce the noexec mount option, and enforce
executable file permission. Indeed, because of compatibility with
installed systems, only system administrators are able to check that
this new enforcement is in line with the system mount points and file
permissions. A following patch adds documentation.

Being able to restrict execution also enables to protect the kernel by
restricting arbitrary syscalls that an attacker could perform with a
crafted binary or certain script languages. It also improves multilevel
isolation by reducing the ability of an attacker to use side channels
with specific code. These restrictions can natively be enforced for ELF
binaries (with the noexec mount option) but require this kernel
extension to properly handle scripts (e.g., Python, Perl). To get a
consistent execution policy, additional memory restrictions should also
be enforced (e.g. thanks to SELinux).

Because the O_MAYEXEC flag is a meant to enforce a system-wide security
policy (but not application-centric policies), it does not make sense
for userland to check the sysctl value. Indeed, this new flag only
enables to extend the system ability to enforce a policy thanks to (some
trusted) userland collaboration. Moreover, additional security policies
could be managed by LSMs. This is a best-effort approach from the
application developer point of view:
https://lore.kernel.org/lkml/1477d3d7-4b36-afad-7077-a38f42322238@digikod.net/

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
---

Changes since v6:
* Allow opening pipes, block devices and character devices with
O_MAYEXEC when there is no enforced policy, but forbid any non-regular
file opened with O_MAYEXEC otherwise (i.e. for any enforced policy).
* Add a paragraph about the non-regular files policy.
* Move path_noexec() calls out of the fast-path (suggested by Kees
Cook).

Changes since v5:
* Remove the static enforcement configuration through Kconfig because it
makes the code more simple like this, and because the current sysctl
configuration can only be set with CAP_SYS_ADMIN, the same way mount
options (i.e. noexec) can be set. If an harden distro wants to
enforce a configuration, it should restrict capabilities or sysctl
configuration. Furthermore, an LSM can easily leverage O_MAYEXEC to
fit its need.
* Move checks from inode_permission() to may_open() and make the error
codes more consistent according to file types (in line with a previous
commit): opening a directory with O_MAYEXEC returns EISDIR and other
non-regular file types may return EACCES.
* In may_open(), when OMAYEXEC_ENFORCE_FILE is set, replace explicit
call to generic_permission() with an artificial MAY_EXEC to avoid
double calls. This makes sense especially when an LSM policy forbids
execution of a file.
* Replace the custom proc_omayexec() with
proc_dointvec_minmax_sysadmin(), and then replace the CAP_MAC_ADMIN
check with a CAP_SYS_ADMIN one (suggested by Kees Cook and Stephen
Smalley).
* Use BIT() (suggested by Kees Cook).
* Rename variables (suggested by Kees Cook).
* Reword the kconfig help.
* Import the documentation patch (suggested by Kees Cook):
https://lore.kernel.org/lkml/20200505153156.925111-6-mic@digikod.net/
* Update documentation and add LWN.net article.

Changes since v4:
* Add kernel configuration options to enforce O_MAYEXEC at build time,
and disable the sysctl in such case (requested by James Morris).
* Reword commit message.

Changes since v3:
* Update comment with O_MAYEXEC.

Changes since v2:
* Cosmetic changes.

Changes since v1:
* Move code from Yama to the FS subsystem (suggested by Kees Cook).
* Make omayexec_inode_permission() static (suggested by Jann Horn).
* Use mode 0600 for the sysctl.
* Only match regular files (not directories nor other types), which
follows the same semantic as commit 73601ea5b7b1 ("fs/open.c: allow
opening only regular files during execve()").
---
Documentation/admin-guide/sysctl/fs.rst | 49 +++++++++++++++++++++++++
fs/namei.c | 24 ++++++++++++
include/linux/fs.h | 1 +
kernel/sysctl.c | 12 +++++-
4 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a45119e3331..ce6e2081d3a9 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -37,6 +37,7 @@ Currently, these files are in /proc/sys/fs:
- inode-nr
- inode-state
- nr_open
+- open_mayexec_enforce
- overflowuid
- overflowgid
- pipe-user-pages-hard
@@ -165,6 +166,54 @@ system needs to prune the inode list instead of allocating
more.

+open_mayexec_enforce
+--------------------
+
+While being ignored by :manpage:`open(2)` and :manpage:`openat(2)`, the
+``O_MAYEXEC`` flag can be passed to :manpage:`openat2(2)` to only open regular
+files that are expected to be executable. If the file is not identified as
+executable, then the syscall returns -EACCES. This may allow a script
+interpreter to check executable permission before reading commands from a file,
+or a dynamic linker to only load executable shared objects. One interesting
+use case is to enforce a "write xor execute" policy through interpreters.
+
+The ability to restrict code execution must be thought as a system-wide policy,
+which first starts by restricting mount points with the ``noexec`` option.
+This option is also automatically applied to special filesystems such as /proc .
+This prevents files on such mount points to be directly executed by the kernel
+or mapped as executable memory (e.g. libraries). With script interpreters
+using the ``O_MAYEXEC`` flag, the executable permission can then be checked
+before reading commands from files. This makes it possible to enforce the
+``noexec`` at the interpreter level, and thus propagates this security policy
+to scripts. To be fully effective, these interpreters also need to handle the
+other ways to execute code: command line parameters (e.g., option ``-e`` for
+Perl), module loading (e.g., option ``-m`` for Python), stdin, file sourcing,
+environment variables, configuration files, etc. According to the threat
+model, it may be acceptable to allow some script interpreters (e.g. Bash) to
+interpret commands from stdin, may it be a TTY or a pipe, because it may not be
+enough to (directly) perform syscalls.
+
+There are two complementary security policies: enforce the ``noexec`` mount
+option, and enforce executable file permission. These policies are handled by
+the ``fs.open_mayexec_enforce`` sysctl (writable only with ``CAP_SYS_ADMIN``)
+as a bitmask:
+
+1 - Mount restriction: checks that the mount options for the underlying VFS
+ mount do not prevent execution.
+
+2 - File permission restriction: checks that the to-be-opened file is marked as
+ executable for the current process (e.g., POSIX permissions).
+
+Note that as long as a policy is enforced, opening any non-regular file with
+``O_MAYEXEC`` is denied (e.g. TTYs, pipe), even when such a file is marked as
+executable or is on an executable mount point.
+
+Code samples can be found in tools/testing/selftests/openat2/omayexec_test.c
+and interpreter patches (for the original O_MAYEXEC version) may be found at
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
+See also an overview article: https://lwn.net/Articles/820000/ .
+
+
overflowgid & overflowuid
-------------------------

diff --git a/fs/namei.c b/fs/namei.c
index 3f074ec77390..8ec13c7fd403 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -39,6 +39,7 @@
#include <linux/bitops.h>
#include <linux/init_task.h>
#include <linux/uaccess.h>
+#include <linux/sysctl.h>

#include "internal.h"
#include "mount.h"
@@ -425,6 +426,11 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
return 0;
}

+#define OPEN_MAYEXEC_ENFORCE_MOUNT BIT(0)
+#define OPEN_MAYEXEC_ENFORCE_FILE BIT(1)
+
+int sysctl_open_mayexec_enforce __read_mostly;
+
/**
* inode_permission - Check for access rights to a given inode
* @inode: Inode to check permission on
@@ -2861,11 +2867,29 @@ static int may_open(const struct path *path, int acc_mode, int flag)
case S_IFSOCK:
if (acc_mode & MAY_EXEC)
return -EACCES;
+ /*
+ * Opening devices (e.g. TTYs) or pipes with O_MAYEXEC may be
+ * legitimate when there is no enforced policy.
+ */
+ if ((acc_mode & MAY_OPENEXEC) && sysctl_open_mayexec_enforce)
+ return -EACCES;
flag &= ~O_TRUNC;
break;
case S_IFREG:
if ((acc_mode & MAY_EXEC) && path_noexec(path))
return -EACCES;
+ if (acc_mode & MAY_OPENEXEC) {
+ if ((sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_MOUNT)
+ && path_noexec(path))
+ return -EACCES;
+ if (sysctl_open_mayexec_enforce & OPEN_MAYEXEC_ENFORCE_FILE)
+ /*
+ * Because acc_mode may change here, the next and only
+ * use of acc_mode should then be by the following call
+ * to inode_permission().
+ */
+ acc_mode |= MAY_EXEC;
+ }
break;
}

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 56f835c9a87a..071f37707ccc 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -83,6 +83,7 @@ extern int sysctl_protected_symlinks;
extern int sysctl_protected_hardlinks;
extern int sysctl_protected_fifos;
extern int sysctl_protected_regular;
+extern int sysctl_open_mayexec_enforce;

typedef __kernel_rwf_t rwf_t;

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index db1ce7af2563..5008a2566e79 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -113,6 +113,7 @@ static int sixty = 60;

static int __maybe_unused neg_one = -1;
static int __maybe_unused two = 2;
+static int __maybe_unused three = 3;
static int __maybe_unused four = 4;
static unsigned long zero_ul;
static unsigned long one_ul = 1;
@@ -888,7 +889,6 @@ static int proc_taint(struct ctl_table *table, int write,
return err;
}

-#ifdef CONFIG_PRINTK
static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
void *buffer, size_t *lenp, loff_t *ppos)
{
@@ -897,7 +897,6 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,

return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
}
-#endif

/**
* struct do_proc_dointvec_minmax_conv_param - proc_dointvec_minmax() range checking structure
@@ -3264,6 +3263,15 @@ static struct ctl_table fs_table[] = {
.extra1 = SYSCTL_ZERO,
.extra2 = &two,
},
+ {
+ .procname = "open_mayexec_enforce",
+ .data = &sysctl_open_mayexec_enforce,
+ .maxlen = sizeof(int),
+ .mode = 0600,
+ .proc_handler = proc_dointvec_minmax_sysadmin,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = &three,
+ },
#if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
{
.procname = "binfmt_misc",
--
2.27.0