[PATCH v18 0/4] Add trusted_for(2) (was O_MAYEXEC)

[PATCH v18 0/4] Add trusted_for(2) (was O_MAYEXEC)

[Mickaël Salaün]

Hi,

This new patch series moves some code (from sysctl.c to fs.c) to fit
with the recent sysctl refactoring included in next-20220104.  As a
result, a new patch moves back the proc_dointvec_minmax_sysadmin()
helper to sysctl.c to make it also usable by the new fs.trusted_for
sysctl.  I also increased the syscall IDs to align with the new
set_mempolicy_home_node syscall.  These are cosmetic changes and I kept
the Acked-by and Signed-off-by from the original patches.  I'd like to
get one for the new patch moving proc_dointvec_minmax_sysadmin() though.

This patch series has been open for review for a long time and got a lot
of feedbacks (and bikeshedding) which were all considered.

Andrew, can you please consider to merge this into your tree?  Without
reply and since I heard no objection, I'll go ahead and merge it in
-next after the merge window closes.

Overview
========

The final goal of this patch series is to enable the kernel to be a
global policy manager by entrusting processes with access control at
their level.  To reach this goal, two complementary parts are required:
* user space needs to be able to know if it can trust some file
  descriptor content for a specific usage;
* and the kernel needs to make available some part of the policy
  configured by the system administrator.

Primary goal of trusted_for(2)
==============================

This new syscall enables user space to ask the kernel: is this file
descriptor's content trusted to be used for this purpose?  The set of
usage currently only contains execution, but other may follow (e.g.
configuration, sensitive data).  If the kernel identifies the file
descriptor as trustworthy for this usage, user space should then take
this information into account.  The "execution" usage means that the
content of the file descriptor is trusted according to the system policy
to be executed by user space, which means that it interprets the content
or (try to) maps it as executable memory.

A simple system-wide security policy can be set by the system
administrator through a sysctl configuration consistent with the mount
points or the file access rights.  The documentation explains the
prerequisites.

It is important to note that this can only enable to extend access
control managed by the kernel.  Hence it enables current access control
mechanism to be extended and become a superset of what they can
currently control.  Indeed, the security policy could also be delegated
to an LSM, either a MAC system or an integrity system.  For instance,
this is required to close a major IMA measurement/appraisal interpreter
integrity gap by bringing the ability to check the use of scripts [1].
Other uses are expected, such as for magic-links [2], SGX integration
[3], bpffs [4].

Complementary W^X protections can be brought by SELinux, IPE [5] and
trampfd [6].

System call description
=======================

trusted_for(int fd, enum trusted_for_usage usage, u32 flags);

@fd is the file descriptor to check.

@usage identifies the user space usage intended for @fd: only
TRUSTED_FOR_EXECUTION for now, but trusted_for_usage could be extended
to identify other usages (e.g. configuration, sensitive data).

@flags must be 0 for now but it could be used in the future to do
complementary checks (e.g. signature or integrity requirements, origin
of the file).

This system call returns 0 on success, or -EACCES if the kernel policy
denies the specified usage (which should be enforced by the caller).

The first patch contains the full syscall and sysctl documentation.

Prerequisite of its use
=======================

User space needs to adapt to take advantage of this new feature.  For
example, the PEP 578 [7] (Runtime Audit Hooks) enables Python 3.8 to be
extended with policy enforcement points related to code interpretation,
which can be used to align with the PowerShell audit features.
Additional Python security improvements (e.g. a limited interpreter
without -c, stdin piping of code) are on their way [8].

Examples
========

The initial idea comes from CLIP OS 4 and the original implementation
has been used for more than 13 years:
https://github.com/clipos-archive/clipos4_doc
Chrome OS has a similar approach:
https://chromium.googlesource.com/chromiumos/docs/+/master/security/noexec_shell_scripts.md

Userland patches can be found here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
Actually, there is more than the O_MAYEXEC changes (which matches this search)
e.g., to prevent Python interactive execution. There are patches for
Bash, Wine, Java (Icedtea), Busybox's ash, Perl and Python. There are
also some related patches which do not directly rely on O_MAYEXEC but
which restrict the use of browser plugins and extensions, which may be
seen as scripts too:
https://github.com/clipos-archive/clipos4_portage-overlay/tree/master/www-client

An introduction to O_MAYEXEC was given at the Linux Security Summit
Europe 2018 - Linux Kernel Security Contributions by ANSSI:
https://www.youtube.com/watch?v=chNjCRtPKQY&t=17m15s
The "write xor execute" principle was explained at Kernel Recipes 2018 -
CLIP OS: a defense-in-depth OS:
https://www.youtube.com/watch?v=PjRE0uBtkHU&t=11m14s
See also a first LWN article about O_MAYEXEC and a new one about
trusted_for(2) and its background:
* https://lwn.net/Articles/820000/
* https://lwn.net/Articles/832959/

This can be tested with CONFIG_SYSCTL.  I would really appreciate
constructive comments on this patch series.

[1] https://lore.kernel.org/lkml/20211014130125.6991-1-zohar@linux.ibm.com/
[2] https://lore.kernel.org/lkml/20190904201933.10736-6-cyphar@cyphar.com/
[3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=z2T_cNrpvMig@mail.gmail.com/
[4] https://lore.kernel.org/lkml/CALCETrVeZ0eufFXwfhtaG_j+AdvbzEWE0M3wjXMWVEO7pj+xkw@mail.gmail.com/
[5] https://lore.kernel.org/lkml/20200406221439.1469862-12-deven.desai@linux.microsoft.com/
[6] https://lore.kernel.org/lkml/20200922215326.4603-1-madvenka@linux.microsoft.com/
[7] https://www.python.org/dev/peps/pep-0578/
[8] https://lore.kernel.org/lkml/0c70debd-e79e-d514-06c6-4cd1e021fa8b@python.org/

Previous versions:
v17: https://lore.kernel.org/r/20211115185304.198460-1-mic@digikod.net/
v16: https://lore.kernel.org/r/20211110190626.257017-1-mic@digikod.net/
v15: https://lore.kernel.org/r/20211012192410.2356090-1-mic@digikod.net/
v14: https://lore.kernel.org/r/20211008104840.1733385-1-mic@digikod.net/
v13: https://lore.kernel.org/r/20211007182321.872075-1-mic@digikod.net/
v12: https://lore.kernel.org/r/20201203173118.379271-1-mic@digikod.net/
v11: https://lore.kernel.org/r/20201019164932.1430614-1-mic@digikod.net/
v10: https://lore.kernel.org/r/20200924153228.387737-1-mic@digikod.net/
v9: https://lore.kernel.org/r/20200910164612.114215-1-mic@digikod.net/
v8: https://lore.kernel.org/r/20200908075956.1069018-1-mic@digikod.net/
v7: https://lore.kernel.org/r/20200723171227.446711-1-mic@digikod.net/
v6: https://lore.kernel.org/r/20200714181638.45751-1-mic@digikod.net/
v5: https://lore.kernel.org/r/20200505153156.925111-1-mic@digikod.net/
v4: https://lore.kernel.org/r/20200430132320.699508-1-mic@digikod.net/
v3: https://lore.kernel.org/r/20200428175129.634352-1-mic@digikod.net/
v2: https://lore.kernel.org/r/20190906152455.22757-1-mic@digikod.net/
v1: https://lore.kernel.org/r/20181212081712.32347-1-mic@digikod.net/

Regards,

Mickaël Salaün (4):
  printk: Move back proc_dointvec_minmax_sysadmin() to sysctl.c
  fs: Add trusted_for(2) syscall implementation and related sysctl
  arch: Wire up trusted_for(2)
  selftest/interpreter: Add tests for trusted_for(2) policies

 Documentation/admin-guide/sysctl/fs.rst       |  50 +++
 arch/alpha/kernel/syscalls/syscall.tbl        |   1 +
 arch/arm/tools/syscall.tbl                    |   1 +
 arch/arm64/include/asm/unistd.h               |   2 +-
 arch/arm64/include/asm/unistd32.h             |   2 +
 arch/ia64/kernel/syscalls/syscall.tbl         |   1 +
 arch/m68k/kernel/syscalls/syscall.tbl         |   1 +
 arch/microblaze/kernel/syscalls/syscall.tbl   |   1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl     |   1 +
 arch/parisc/kernel/syscalls/syscall.tbl       |   1 +
 arch/powerpc/kernel/syscalls/syscall.tbl      |   1 +
 arch/s390/kernel/syscalls/syscall.tbl         |   1 +
 arch/sh/kernel/syscalls/syscall.tbl           |   1 +
 arch/sparc/kernel/syscalls/syscall.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
 arch/xtensa/kernel/syscalls/syscall.tbl       |   1 +
 fs/open.c                                     | 133 +++++++
 fs/proc/proc_sysctl.c                         |   2 +-
 include/linux/syscalls.h                      |   1 +
 include/linux/sysctl.h                        |   3 +
 include/uapi/asm-generic/unistd.h             |   5 +-
 include/uapi/linux/trusted-for.h              |  18 +
 kernel/printk/sysctl.c                        |   9 -
 kernel/sysctl.c                               |   9 +
 tools/testing/selftests/Makefile              |   1 +
 .../testing/selftests/interpreter/.gitignore  |   2 +
 tools/testing/selftests/interpreter/Makefile  |  21 +
 tools/testing/selftests/interpreter/config    |   1 +
 .../selftests/interpreter/trust_policy_test.c | 362 ++++++++++++++++++
 32 files changed, 625 insertions(+), 12 deletions(-)
 create mode 100644 include/uapi/linux/trusted-for.h
 create mode 100644 tools/testing/selftests/interpreter/.gitignore
 create mode 100644 tools/testing/selftests/interpreter/Makefile
 create mode 100644 tools/testing/selftests/interpreter/config
 create mode 100644 tools/testing/selftests/interpreter/trust_policy_test.c


base-commit: 6b8d4927540e416878113f0f7e273ddc939291f3
-- 
2.34.1

[PATCH v18 1/4] printk: Move back proc_dointvec_minmax_sysadmin() to sysctl.c

[Mickaël Salaün]

From: Mickaël Salaün <mic@linux.microsoft.com>

The proc_dointvec_minmax_sysadmin() helper is useful for the
fs.trusted_for_policy sysctl brought by the next commit.

This partially revert commit 642fd23fb826 ("printk: move printk sysctl
to printk/sysctl.c") from Luis Chamberlain's 20211129-sysctl-cleanups
branch [1], to share the proc_dointvec_minmax_sysadmin() helper.  FYI,
this previous commit also got the buffer pointer an __user attribute.

Also remove the forgotten ten_thousand static variable (moved to
kernel/printk/sysctl.c).

Link: https://lkml.kernel.org/r/20211124231435.1445213-6-mcgrof@kernel.org [1]
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20220104155024.48023-2-mic@digikod.net
---
 include/linux/sysctl.h | 2 ++
 kernel/printk/sysctl.c | 9 ---------
 kernel/sysctl.c        | 9 +++++++++
 3 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 180adf7da785..cf1ba98aab50 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -69,6 +69,8 @@ int proc_dobool(struct ctl_table *table, int write, void *buffer,
 int proc_dointvec(struct ctl_table *, int, void *, size_t *, loff_t *);
 int proc_douintvec(struct ctl_table *, int, void *, size_t *, loff_t *);
 int proc_dointvec_minmax(struct ctl_table *, int, void *, size_t *, loff_t *);
+int proc_dointvec_minmax_sysadmin(struct ctl_table *, int, void *, size_t *,
+		loff_t *);
 int proc_douintvec_minmax(struct ctl_table *table, int write, void *buffer,
 		size_t *lenp, loff_t *ppos);
 int proc_dou8vec_minmax(struct ctl_table *table, int write, void *buffer,
diff --git a/kernel/printk/sysctl.c b/kernel/printk/sysctl.c
index 653ae04aab7f..c7129428ee9b 100644
--- a/kernel/printk/sysctl.c
+++ b/kernel/printk/sysctl.c
@@ -11,15 +11,6 @@
 
 static const int ten_thousand = 10000;
 
-static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
-				void __user *buffer, size_t *lenp, loff_t *ppos)
-{
-	if (write && !capable(CAP_SYS_ADMIN))
-		return -EPERM;
-
-	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
-}
-
 static struct ctl_table printk_sysctls[] = {
 	{
 		.procname	= "printk",
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 5ae443b2882e..2e2027e323fd 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -888,6 +888,15 @@ static int proc_taint(struct ctl_table *table, int write,
 	return err;
 }
 
+int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (write && !capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+}
+
 /**
  * struct do_proc_dointvec_minmax_conv_param - proc_dointvec_minmax() range checking structure
  * @min: pointer to minimum allowable value
-- 
2.34.1

[PATCH v18 2/4] fs: Add trusted_for(2) syscall implementation and related sysctl

[Mickaël Salaün]

From: Mickaël Salaün <mic@linux.microsoft.com>

The trusted_for() syscall enables user space tasks to check that files
are trusted to be executed or interpreted by user space.  This may allow
script interpreters to check execution permission before reading
commands from a file, or dynamic linkers to allow shared object loading.
This may be seen as a way for a trusted task (e.g. interpreter) to check
the trustworthiness of files (e.g. scripts) before extending its control
flow graph with new ones originating from these files.

The security policy is consistently managed by the kernel through the
new sysctl: fs.trusted_for_policy .  This enables 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.

For this to be possible, script interpreters must use trusted_for(2)
with the TRUSTED_FOR_EXECUTION usage.  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.

Even without enforced security policy, user space interpreters can use
this syscall to try as much as possible to enforce the system policy at
their level, knowing that it will not break anything on running systems
which do not care about this feature.  However, on systems which want
this feature enforced, there will be knowledgeable people (i.e. system
administrator who configured fs.trusted_for_policy deliberately) to
manage it.

Because trusted_for(2) is a mean to enforce a system-wide security
policy (but not application-centric policies), it does not make sense
for user space to check the sysctl value.  Indeed, this new syscall only
enables to extend the system ability to enforce a policy thanks to (some
trusted) user space 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/

trusted_for(2) with TRUSTED_FOR_EXECUTION should not be confused with
the O_EXEC flag (for open) which is intended for execute-only, which
obviously doesn't work for scripts.  However, a similar behavior could
be implemented in user space with O_PATH:
https://lore.kernel.org/lkml/1e2f6913-42f2-3578-28ed-567f6a4bdda1@digikod.net/

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).

This is a new implementation of a patch initially written by
Vincent Strubel for CLIP OS 4:
https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
This patch has been used for more than 13 years with customized script
interpreters.  Some examples (with the original O_MAYEXEC) can be found
here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Co-developed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Signed-off-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220104155024.48023-3-mic@digikod.net
---

Changes since v17:
* Move the trusted_for_policy sysctl to a dedicated table in fs/open.c
  to make kernel/sysctl.c easier to maintain.

Changes since v16:
* Replace the enum trusted_for_usage with an int in the syscall
  signature to avoid C unspecified behavior (suggested by Alejandro
  Colomar).  This is mostly a cosmetic fix according to the current
  kernel compilers and used options, but let's remove
  implementation-defined hazard.

Changes since v14:
* Add full syscall documentation (requested by Andrew Morton).

Changes since v13:
* Rename sysctl from "trust_policy" to "trusted_for_policy" (suggested
  by Kees Cook).
* Add Acked-by Kees Cook.

Changes since v12:
* Update inode_permission() call to allign with commit 47291baa8ddf
  ("namei: make permission helpers idmapped mount aware").
* Switch from d_backing_inode(f.file->f_path.dentry) to
  file_inode(f.file).

Changes since v10:
* Add enum definition to syscalls.h .

Changes since v9:
* Rename the syscall to trusted_for(2) and the sysctl to fs.trust_policy
* Add a dedicated enum trusted_for_usage with include/uapi/linux/trusted-for.h
* Remove the extra MAY_INTROSPECTION_EXEC bit.  LSMs can still implement
  this feature themselves.

Changes since v8:
* Add a dedicated syscall introspect_access() (requested by Al Viro).
* Rename MAY_INTERPRETED_EXEC to MAY_INTROSPECTION_EXEC .
* Rename the sysctl fs.interpreted_access to fs.introspection_policy .
* Update documentation.

Changes since v7:
* Replaces openat2/O_MAYEXEC with faccessat2/X_OK/AT_INTERPRETED .
  Switching to an FD-based syscall was suggested by Al Viro and Jann
  Horn.
* Handle special file descriptors.
* Add a compatibility mode for execute/read check.
* Move the sysctl policy from fs/namei.c to fs/open.c for the new
  faccessat2/AT_INTERPRETED.
* Rename the sysctl from fs.open_mayexec_enforce to
  fs.interpreted_access .
* Update documentation accordingly.

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).
* Do not set __FMODE_EXEC for now because of inconsistent behavior:
  https://lore.kernel.org/lkml/202007160822.CCDB5478@keescook/
* Returns EISDIR when opening a directory with O_MAYEXEC.
* Removed Deven Bowers and Kees Cook Reviewed-by tags because of the
  current update.

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:
* Switch back to O_MAYEXEC, but only handle it with openat2(2) which
  checks unknown flags (suggested by Aleksa Sarai). Cf.
  https://lore.kernel.org/lkml/20200430015429.wuob7m5ofdewubui@yavin.dot.cyphar.com/

Changes since v2:
* Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2).  This change
  enables to not break existing application using bogus O_* flags that
  may be ignored by current kernels by using a new dedicated flag, only
  usable through openat2(2) (suggested by Jeff Layton).  Using this flag
  will results in an error if the running kernel does not support it.
  User space needs to manage this case, as with other RESOLVE_* flags.
  The best effort approach to security (for most common distros) will
  simply consists of ignoring such an error and retry without
  RESOLVE_MAYEXEC.  However, a fully controlled system may which to
  error out if such an inconsistency is detected.
* Cosmetic changes.

Changes since v1:
* Set __FMODE_EXEC when using O_MAYEXEC to make this information
  available through the new fanotify/FAN_OPEN_EXEC event (suggested by
  Jan Kara and Matthew Bobrowski):
  https://lore.kernel.org/lkml/20181213094658.GA996@lithium.mbobrowski.org/
* 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 |  50 +++++++++
 fs/open.c                               | 133 ++++++++++++++++++++++++
 fs/proc/proc_sysctl.c                   |   2 +-
 include/linux/syscalls.h                |   1 +
 include/linux/sysctl.h                  |   1 +
 include/uapi/linux/trusted-for.h        |  18 ++++
 6 files changed, 204 insertions(+), 1 deletion(-)
 create mode 100644 include/uapi/linux/trusted-for.h

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a501c9ddc55..e364d6c45790 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -48,6 +48,7 @@ Currently, these files are in /proc/sys/fs:
 - suid_dumpable
 - super-max
 - super-nr
+- trusted_for_policy
 
 
 aio-nr & aio-max-nr
@@ -382,3 +383,52 @@ Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
 on a 64bit one.
 The current default value for  max_user_watches  is the 1/25 (4%) of the
 available low memory, divided for the "watch" cost in bytes.
+
+
+trusted_for_policy
+------------------
+
+An interpreter can call :manpage:`trusted_for(2)` with a
+``TRUSTED_FOR_EXECUTION`` usage to check that opened regular files 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 :manpage:`trusted_for(2)`, 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.trusted_for_policy`` 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 file is marked as
+    executable for the current process (e.g., POSIX permissions, ACLs).
+
+Note that as long as a policy is enforced, checking any non-regular file with
+:manpage:`trusted_for(2)` returns -EACCES (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/interpreter/trust_policy_test.c and interpreter patches
+(for the original O_MAYEXEC) are available at
+https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC .
+See also an overview article: https://lwn.net/Articles/820000/ .
diff --git a/fs/open.c b/fs/open.c
index 9ff2f621b760..cf4cddc4e3f5 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -33,6 +33,8 @@
 #include <linux/dnotify.h>
 #include <linux/compat.h>
 #include <linux/mnt_idmapping.h>
+#include <linux/sysctl.h>
+#include <uapi/linux/trusted-for.h>
 
 #include "internal.h"
 
@@ -481,6 +483,137 @@ SYSCALL_DEFINE2(access, const char __user *, filename, int, mode)
 	return do_faccessat(AT_FDCWD, filename, mode, 0);
 }
 
+#define TRUST_POLICY_EXEC_MOUNT			BIT(0)
+#define TRUST_POLICY_EXEC_FILE			BIT(1)
+
+static int sysctl_trusted_for_policy __read_mostly;
+
+#ifdef CONFIG_SYSCTL
+static struct ctl_table open_sysctls[] = {
+	{
+		.procname       = "trusted_for_policy",
+		.data           = &sysctl_trusted_for_policy,
+		.maxlen         = sizeof(int),
+		.mode           = 0600,
+		.proc_handler	= proc_dointvec_minmax_sysadmin,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_THREE,
+	},
+	{ }
+};
+
+static int __init init_fs_open_sysctls(void)
+{
+	register_sysctl_init("fs", open_sysctls);
+	return 0;
+}
+fs_initcall(init_fs_open_sysctls);
+#endif /* CONFIG_SYSCTL */
+
+/**
+ * sys_trusted_for - Check that a FD is trusted for a specific usage
+ *
+ * @fd: File descriptor to check.
+ * @usage: Identify the user space usage (defined by enum trusted_for_usage)
+ *         intended for the file descriptor (only TRUSTED_FOR_EXECUTION for
+ *         now).
+ *
+ * @flags: Must be 0.
+ *
+ * This system call enables user space to ask the kernel: is this file
+ * descriptor's content trusted to be used for this purpose?  The set of @usage
+ * currently only contains TRUSTED_FOR_EXECUTION, but other may follow (e.g.
+ * configuration, sensitive data).  If the kernel identifies the file
+ * descriptor as trustworthy for this usage, this call returns 0 and the caller
+ * should then take this information into account.
+ *
+ * The execution usage means that the content of the file descriptor is trusted
+ * according to the system policy to be executed by user space, which means
+ * that it interprets the content or (try to) maps it as executable memory.
+ *
+ * A simple system-wide security policy can be set by the system administrator
+ * through a sysctl configuration consistent with the mount points or the file
+ * access rights: Documentation/admin-guide/sysctl/fs.rst
+ *
+ * @flags could be used in the future to do complementary checks (e.g.
+ * signature or integrity requirements, origin of the file).
+ *
+ * Possible returned errors are:
+ *
+ * - EINVAL: unknown @usage or unknown @flags;
+ * - EBADF: @fd is not a file descriptor for the calling thread;
+ * - EACCES: the requested usage is denied (and user space should enforce it).
+ */
+SYSCALL_DEFINE3(trusted_for, const int, fd, const int, usage, const u32, flags)
+{
+	int mask, err = -EACCES;
+	struct fd f;
+	struct inode *inode;
+
+	if (flags)
+		return -EINVAL;
+
+	/* Only handles execution for now. */
+	if (usage != TRUSTED_FOR_EXECUTION)
+		return -EINVAL;
+	mask = MAY_EXEC;
+
+	f = fdget(fd);
+	if (!f.file)
+		return -EBADF;
+	inode = file_inode(f.file);
+
+	/*
+	 * For compatibility reasons, without a defined security policy, we
+	 * must map the execute permission to the read permission.  Indeed,
+	 * from user space point of view, being able to execute data (e.g.
+	 * scripts) implies to be able to read this data.
+	 */
+	if ((mask & MAY_EXEC)) {
+		/*
+		 * If there is a system-wide execute policy enforced, then
+		 * forbids access to non-regular files and special superblocks.
+		 */
+		if ((sysctl_trusted_for_policy & (TRUST_POLICY_EXEC_MOUNT |
+						TRUST_POLICY_EXEC_FILE))) {
+			if (!S_ISREG(inode->i_mode))
+				goto out_fd;
+			/*
+			 * Denies access to pseudo filesystems that will never
+			 * be mountable (e.g. sockfs, pipefs) but can still be
+			 * reachable through /proc/self/fd, or memfd-like file
+			 * descriptors, or nsfs-like files.
+			 *
+			 * According to the selftests, SB_NOEXEC seems to be
+			 * only used by proc and nsfs filesystems.
+			 */
+			if ((f.file->f_path.dentry->d_sb->s_flags &
+						(SB_NOUSER | SB_KERNMOUNT | SB_NOEXEC)))
+				goto out_fd;
+		}
+
+		if ((sysctl_trusted_for_policy & TRUST_POLICY_EXEC_MOUNT) &&
+				path_noexec(&f.file->f_path))
+			goto out_fd;
+		/*
+		 * For compatibility reasons, if the system-wide policy doesn't
+		 * enforce file permission checks, then replaces the execute
+		 * permission request with a read permission request.
+		 */
+		if (!(sysctl_trusted_for_policy & TRUST_POLICY_EXEC_FILE))
+			mask &= ~MAY_EXEC;
+		/* To be executed *by* user space, files must be readable. */
+		mask |= MAY_READ;
+	}
+
+	err = inode_permission(file_mnt_user_ns(f.file), inode,
+			mask | MAY_ACCESS);
+
+out_fd:
+	fdput(f);
+	return err;
+}
+
 SYSCALL_DEFINE1(chdir, const char __user *, filename)
 {
 	struct path path;
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index a553ed36c0b8..949281dbc6a8 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -26,7 +26,7 @@ static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
 /* shared constants to be used in various sysctls */
-const int sysctl_vals[] = { -1, 0, 1, 2, 4, 100, 200, 1000, 3000, INT_MAX, 65535 };
+const int sysctl_vals[] = { -1, 0, 1, 2, 4, 100, 200, 1000, 3000, INT_MAX, 65535, 3 };
 EXPORT_SYMBOL(sysctl_vals);
 
 const unsigned long sysctl_long_vals[] = { 0, 1, LONG_MAX };
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 819c0cb00b6d..f812521be8e7 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -462,6 +462,7 @@ asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len);
 asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
 asmlinkage long sys_faccessat2(int dfd, const char __user *filename, int mode,
 			       int flags);
+asmlinkage long sys_trusted_for(int fd, int usage, u32 flags);
 asmlinkage long sys_chdir(const char __user *filename);
 asmlinkage long sys_fchdir(unsigned int fd);
 asmlinkage long sys_chroot(const char __user *filename);
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index cf1ba98aab50..7e55f3475952 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -51,6 +51,7 @@ struct ctl_dir;
 
 /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
 #define SYSCTL_MAXOLDUID		((void *)&sysctl_vals[10])
+#define SYSCTL_THREE			((void *)&sysctl_vals[11])
 
 extern const int sysctl_vals[];
 
diff --git a/include/uapi/linux/trusted-for.h b/include/uapi/linux/trusted-for.h
new file mode 100644
index 000000000000..cc4f030c5103
--- /dev/null
+++ b/include/uapi/linux/trusted-for.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_TRUSTED_FOR_H
+#define _UAPI_LINUX_TRUSTED_FOR_H
+
+/**
+ * enum trusted_for_usage - Usage for which a file descriptor is trusted
+ *
+ * Argument of trusted_for(2).
+ */
+enum trusted_for_usage {
+	/**
+	 * @TRUSTED_FOR_EXECUTION: Check that the data read from a file
+	 * descriptor is trusted to be executed or interpreted (e.g. scripts).
+	 */
+	TRUSTED_FOR_EXECUTION = 1,
+};
+
+#endif /* _UAPI_LINUX_TRUSTED_FOR_H */
-- 
2.34.1

[PATCH v18 3/4] arch: Wire up trusted_for(2)

[Mickaël Salaün]

From: Mickaël Salaün <mic@linux.microsoft.com>

Wire up trusted_for(2) for all architectures.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220104155024.48023-4-mic@digikod.net
---

Changes since v17:
* Update syscall IDs to align with the new set_mempolicy_home_node.

Changes since v15:
* Update syscall IDs to align with the new futex_waitv.

Changes since v13:
* Add Reviewed-by Kees Cook.

Changes since v12:
* Update syscall IDs to align with the new ones.

Changes since v11:
* Add Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
* Rebase and leave space for watch_mount(2) and epoll_pwait2(2) from
  -next.

Changes since v9:
* Rename introspect_access(2) to trusted_for(2).
* Increase syscall number to leave space for memfd_secret(2) in -next.

Changes since v7:
* New patch for the new syscall.
* Increase syscall numbers by 2 to leave space for new ones (in
  linux-next): watch_mount(2) and process_madvise(2).
---
 arch/alpha/kernel/syscalls/syscall.tbl      | 1 +
 arch/arm/tools/syscall.tbl                  | 1 +
 arch/arm64/include/asm/unistd.h             | 2 +-
 arch/arm64/include/asm/unistd32.h           | 2 ++
 arch/ia64/kernel/syscalls/syscall.tbl       | 1 +
 arch/m68k/kernel/syscalls/syscall.tbl       | 1 +
 arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   | 1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   | 1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   | 1 +
 arch/parisc/kernel/syscalls/syscall.tbl     | 1 +
 arch/powerpc/kernel/syscalls/syscall.tbl    | 1 +
 arch/s390/kernel/syscalls/syscall.tbl       | 1 +
 arch/sh/kernel/syscalls/syscall.tbl         | 1 +
 arch/sparc/kernel/syscalls/syscall.tbl      | 1 +
 arch/x86/entry/syscalls/syscall_32.tbl      | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl      | 1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     | 1 +
 include/uapi/asm-generic/unistd.h           | 5 ++++-
 19 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 3515bc4f16a4..2bf65d76e188 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -490,3 +490,4 @@
 558	common	process_mrelease		sys_process_mrelease
 559	common  futex_waitv                     sys_futex_waitv
 560	common	set_mempolicy_home_node		sys_ni_syscall
+561	common	trusted_for			sys_trusted_for
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index ac964612d8b0..31f747a4f35c 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -464,3 +464,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common	futex_waitv			sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 4e65da3445c7..14c95844f6c8 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		451
+#define __NR_compat_syscalls		452
 #endif
 
 #define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 604a2053d006..9bd1841e1d47 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -907,6 +907,8 @@ __SYSCALL(__NR_process_mrelease, sys_process_mrelease)
 __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
 #define __NR_set_mempolicy_home_node 450
 __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
+#define __NR_trusted_for 451
+__SYSCALL(__NR_trusted_for, sys_trusted_for)
 
 /*
  * Please add new compat syscalls above this comment and update
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index 78b1d03e86e1..dc97eab8bbea 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -371,3 +371,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index b1f3940bc298..fabb49d38c74 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -450,3 +450,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 820145e47350..a7aa26d5b143 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -456,3 +456,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 253ff994ed2e..ad9a968fee63 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -389,3 +389,4 @@
 448	n32	process_mrelease		sys_process_mrelease
 449	n32	futex_waitv			sys_futex_waitv
 450	n32	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	n32	trusted_for			sys_trusted_for
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 3f1886ad9d80..294e0ab651e7 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -365,3 +365,4 @@
 448	n64	process_mrelease		sys_process_mrelease
 449	n64	futex_waitv			sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	n64	trusted_for			sys_trusted_for
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 8f243e35a7b2..4644c7c7de6a 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -438,3 +438,4 @@
 448	o32	process_mrelease		sys_process_mrelease
 449	o32	futex_waitv			sys_futex_waitv
 450	o32	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	o32	trusted_for			sys_trusted_for
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 68b46fe2f17c..208dc6eed4ca 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -448,3 +448,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common	futex_waitv			sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index 2600b4237292..7281bb9c1cba 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -530,3 +530,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450 	nospu	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 799147658dee..badd1fe09c97 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
 448  common	process_mrelease	sys_process_mrelease		sys_process_mrelease
 449  common	futex_waitv		sys_futex_waitv			sys_futex_waitv
 450  common	set_mempolicy_home_node	sys_set_mempolicy_home_node	sys_set_mempolicy_home_node
+451  common	trusted_for		sys_trusted_for			sys_trusted_for
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 2de85c977f54..8f92f449617b 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 4398cc6fb68d..910fdd8d9443 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -496,3 +496,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..47150d9e039a 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -455,3 +455,4 @@
 448	i386	process_mrelease	sys_process_mrelease
 449	i386	futex_waitv		sys_futex_waitv
 450	i386	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	i386	trusted_for		sys_trusted_for
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..29024e64fb30 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
 448	common	process_mrelease	sys_process_mrelease
 449	common	futex_waitv		sys_futex_waitv
 450	common	set_mempolicy_home_node	sys_set_mempolicy_home_node
+451	common	trusted_for		sys_trusted_for
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 52c94ab5c205..efc45e1a53ce 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -421,3 +421,4 @@
 448	common	process_mrelease		sys_process_mrelease
 449	common  futex_waitv                     sys_futex_waitv
 450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
+451	common	trusted_for			sys_trusted_for
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 1c48b0ae3ba3..26d1ffaca07b 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
 #define __NR_set_mempolicy_home_node 450
 __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
 
+#define __NR_trusted_for 451
+__SYSCALL(__NR_trusted_for, sys_trusted_for)
+
 #undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452
 
 /*
  * 32 bit systems traditionally used different
-- 
2.34.1

[PATCH v18 4/4] selftest/interpreter: Add tests for trusted_for(2) policies

[Mickaël Salaün]

From: Mickaël Salaün <mic@linux.microsoft.com>

Test that checks performed by trusted_for(2) on file descriptors are
consistent with noexec mount points and file execute permissions,
according to the policy configured with the fs.trust_policy sysctl.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220104155024.48023-5-mic@digikod.net
---

Changes since v14:
* Add Reviewed-by Kees Cook.

Changes since v13:
* Move -I to CFLAGS (suggested by Kees Cook).
* Update sysctl name.

Changes since v12:
* Fix Makefile's license.

Changes since v10:
* Update selftest Makefile.

Changes since v9:
* Rename the syscall and the sysctl.
* Update tests for enum trusted_for_usage

Changes since v8:
* Update with the dedicated syscall introspect_access(2) and the renamed
  fs.introspection_policy sysctl.
* Remove check symlink which can't be use as is anymore.
* Use socketpair(2) to test UNIX socket.

Changes since v7:
* Update tests with faccessat2/AT_INTERPRETED, including new ones to
  check that setting R_OK or W_OK returns EINVAL.
* Add tests for memfd, pipefs and nsfs.
* Rename and move back tests to a standalone directory.

Changes since v6:
* Add full combination tests for all file types, including block
  devices, character devices, fifos, sockets and symlinks.
* Properly save and restore initial sysctl value for all tests.

Changes since v5:
* Refactor with FIXTURE_VARIANT, which make the tests much more easy to
  read and maintain.
* Save and restore initial sysctl value (suggested by Kees Cook).
* Test with a sysctl value of 0.
* Check errno in sysctl_access_write test.
* Update tests for the CAP_SYS_ADMIN switch.
* Update tests to check -EISDIR (replacing -EACCES).
* Replace FIXTURE_DATA() with FIXTURE() (spotted by Kees Cook).
* Use global const strings.

Changes since v3:
* Replace RESOLVE_MAYEXEC with O_MAYEXEC.
* Add tests to check that O_MAYEXEC is ignored by open(2) and openat(2).

Changes since v2:
* Move tests from exec/ to openat2/ .
* Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2).
* Cleanup tests.

Changes since v1:
* Move tests from yama/ to exec/ .
* Fix _GNU_SOURCE in kselftest_harness.h .
* Add a new test sysctl_access_write to check if CAP_MAC_ADMIN is taken
  into account.
* Test directory execution which is always forbidden since commit
  73601ea5b7b1 ("fs/open.c: allow opening only regular files during
  execve()"), and also check that even the root user can not bypass file
  execution checks.
* Make sure delete_workspace() always as enough right to succeed.
* Cosmetic cleanup.
---
 tools/testing/selftests/Makefile              |   1 +
 .../testing/selftests/interpreter/.gitignore  |   2 +
 tools/testing/selftests/interpreter/Makefile  |  21 +
 tools/testing/selftests/interpreter/config    |   1 +
 .../selftests/interpreter/trust_policy_test.c | 362 ++++++++++++++++++
 5 files changed, 387 insertions(+)
 create mode 100644 tools/testing/selftests/interpreter/.gitignore
 create mode 100644 tools/testing/selftests/interpreter/Makefile
 create mode 100644 tools/testing/selftests/interpreter/config
 create mode 100644 tools/testing/selftests/interpreter/trust_policy_test.c

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index d08fe4cfe811..d6a54a1d9d3a 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -21,6 +21,7 @@ TARGETS += ftrace
 TARGETS += futex
 TARGETS += gpio
 TARGETS += intel_pstate
+TARGETS += interpreter
 TARGETS += ipc
 TARGETS += ir
 TARGETS += kcmp
diff --git a/tools/testing/selftests/interpreter/.gitignore b/tools/testing/selftests/interpreter/.gitignore
new file mode 100644
index 000000000000..82a4846cbc4b
--- /dev/null
+++ b/tools/testing/selftests/interpreter/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/*_test
diff --git a/tools/testing/selftests/interpreter/Makefile b/tools/testing/selftests/interpreter/Makefile
new file mode 100644
index 000000000000..7402fdb6533f
--- /dev/null
+++ b/tools/testing/selftests/interpreter/Makefile
@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += -Wall -O2 -I$(khdr_dir)
+LDLIBS += -lcap
+
+src_test := $(wildcard *_test.c)
+TEST_GEN_PROGS := $(src_test:.c=)
+
+KSFT_KHDR_INSTALL := 1
+include ../lib.mk
+
+khdr_dir = $(top_srcdir)/usr/include
+
+$(khdr_dir)/asm-generic/unistd.h: khdr
+	@:
+
+$(khdr_dir)/linux/trusted-for.h: khdr
+	@:
+
+$(OUTPUT)/%_test: %_test.c $(khdr_dir)/asm-generic/unistd.h $(khdr_dir)/linux/trusted-for.h ../kselftest_harness.h
+	$(LINK.c) $< $(LDLIBS) -o $@
diff --git a/tools/testing/selftests/interpreter/config b/tools/testing/selftests/interpreter/config
new file mode 100644
index 000000000000..dd53c266bf52
--- /dev/null
+++ b/tools/testing/selftests/interpreter/config
@@ -0,0 +1 @@
+CONFIG_SYSCTL=y
diff --git a/tools/testing/selftests/interpreter/trust_policy_test.c b/tools/testing/selftests/interpreter/trust_policy_test.c
new file mode 100644
index 000000000000..b59f07f537ad
--- /dev/null
+++ b/tools/testing/selftests/interpreter/trust_policy_test.c
@@ -0,0 +1,362 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test trusted_for(2) with fs.trusted_for_policy sysctl
+ *
+ * Copyright © 2018-2020 ANSSI
+ *
+ * Author: Mickaël Salaün <mic@digikod.net>
+ */
+
+#define _GNU_SOURCE
+#include <asm-generic/unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/trusted-for.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/capability.h>
+#include <sys/mman.h>
+#include <sys/mount.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/sysmacros.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "../kselftest_harness.h"
+
+#ifndef trusted_for
+static int trusted_for(const int fd, const enum trusted_for_usage usage,
+		const __u32 flags)
+{
+	errno = 0;
+	return syscall(__NR_trusted_for, fd, usage, flags);
+}
+#endif
+
+static const char sysctl_path[] = "/proc/sys/fs/trusted_for_policy";
+
+static const char workdir_path[] = "./test-mount";
+static const char reg_file_path[] = "./test-mount/regular_file";
+static const char dir_path[] = "./test-mount/directory";
+static const char block_dev_path[] = "./test-mount/block_device";
+static const char char_dev_path[] = "./test-mount/character_device";
+static const char fifo_path[] = "./test-mount/fifo";
+
+static void ignore_dac(struct __test_metadata *_metadata, int override)
+{
+	cap_t caps;
+	const cap_value_t cap_val[2] = {
+		CAP_DAC_OVERRIDE,
+		CAP_DAC_READ_SEARCH,
+	};
+
+	caps = cap_get_proc();
+	ASSERT_NE(NULL, caps);
+	ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_val,
+				override ? CAP_SET : CAP_CLEAR));
+	ASSERT_EQ(0, cap_set_proc(caps));
+	EXPECT_EQ(0, cap_free(caps));
+}
+
+static void ignore_sys_admin(struct __test_metadata *_metadata, int override)
+{
+	cap_t caps;
+	const cap_value_t cap_val[1] = {
+		CAP_SYS_ADMIN,
+	};
+
+	caps = cap_get_proc();
+	ASSERT_NE(NULL, caps);
+	ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_val,
+				override ? CAP_SET : CAP_CLEAR));
+	ASSERT_EQ(0, cap_set_proc(caps));
+	EXPECT_EQ(0, cap_free(caps));
+}
+
+static void test_omx(struct __test_metadata *_metadata,
+		const char *const path, const int err_access)
+{
+	int flags = O_RDONLY | O_CLOEXEC;
+	int fd, access_ret, access_errno;
+
+	/* Do not block on pipes. */
+	if (path == fifo_path)
+		flags |= O_NONBLOCK;
+
+	fd = open(path, flags);
+	ASSERT_LE(0, fd) {
+		TH_LOG("Failed to open %s: %s", path, strerror(errno));
+	}
+	access_ret = trusted_for(fd, TRUSTED_FOR_EXECUTION, 0);
+	access_errno = errno;
+	if (err_access) {
+		ASSERT_EQ(err_access, access_errno) {
+			TH_LOG("Wrong error for trusted_for(2) with %s: %s",
+					path, strerror(access_errno));
+		}
+		ASSERT_EQ(-1, access_ret);
+	} else {
+		ASSERT_EQ(0, access_ret) {
+			TH_LOG("Access denied for %s: %s", path, strerror(access_errno));
+		}
+	}
+
+	/* Tests unsupported trusted usage. */
+	access_ret = trusted_for(fd, 0, 0);
+	ASSERT_EQ(-1, access_ret);
+	ASSERT_EQ(EINVAL, errno);
+
+	access_ret = trusted_for(fd, 2, 0);
+	ASSERT_EQ(-1, access_ret);
+	ASSERT_EQ(EINVAL, errno);
+
+	EXPECT_EQ(0, close(fd));
+}
+
+static void test_policy_fd(struct __test_metadata *_metadata, const int fd,
+		const bool has_policy)
+{
+	const int ret = trusted_for(fd, TRUSTED_FOR_EXECUTION, 0);
+
+	if (has_policy) {
+		ASSERT_EQ(-1, ret);
+		ASSERT_EQ(EACCES, errno) {
+			TH_LOG("Wrong error for trusted_for(2) with FD: %s", strerror(errno));
+		}
+	} else {
+		ASSERT_EQ(0, ret) {
+			TH_LOG("Access denied for FD: %s", strerror(errno));
+		}
+	}
+}
+
+FIXTURE(access) {
+	char initial_sysctl_value;
+	int memfd, pipefd;
+	int pipe_fds[2], socket_fds[2];
+};
+
+static void test_file_types(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self,
+		const int err_code, const bool has_policy)
+{
+	/* Tests are performed on a tmpfs mount point. */
+	test_omx(_metadata, reg_file_path, err_code);
+	test_omx(_metadata, dir_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, block_dev_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, char_dev_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, fifo_path, has_policy ? EACCES : 0);
+
+	/* Checks that exec is denied for any socket FD. */
+	test_policy_fd(_metadata, self->socket_fds[0], has_policy);
+
+	/* Checks that exec is denied for any memfd. */
+	test_policy_fd(_metadata, self->memfd, has_policy);
+
+	/* Checks that exec is denied for any pipefs FD. */
+	test_policy_fd(_metadata, self->pipefd, has_policy);
+}
+
+static void test_files(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self,
+		const int err_code, const bool has_policy)
+{
+	/* Tests as root. */
+	ignore_dac(_metadata, 1);
+	test_file_types(_metadata, self, err_code, has_policy);
+
+	/* Tests without bypass. */
+	ignore_dac(_metadata, 0);
+	test_file_types(_metadata, self, err_code, has_policy);
+}
+
+static void sysctl_write_char(struct __test_metadata *_metadata, const char value)
+{
+	int fd;
+
+	fd = open(sysctl_path, O_WRONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ASSERT_EQ(1, write(fd, &value, 1));
+	EXPECT_EQ(0, close(fd));
+}
+
+static char sysctl_read_char(struct __test_metadata *_metadata)
+{
+	int fd;
+	char sysctl_value;
+
+	fd = open(sysctl_path, O_RDONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ASSERT_EQ(1, read(fd, &sysctl_value, 1));
+	EXPECT_EQ(0, close(fd));
+	return sysctl_value;
+}
+
+FIXTURE_VARIANT(access) {
+	const bool mount_exec;
+	const bool file_exec;
+	const int sysctl_err_code[3];
+};
+
+FIXTURE_VARIANT_ADD(access, mount_exec_file_exec) {
+	.mount_exec = true,
+	.file_exec = true,
+	.sysctl_err_code = {0, 0, 0},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_exec_file_noexec)
+{
+	.mount_exec = true,
+	.file_exec = false,
+	.sysctl_err_code = {0, EACCES, EACCES},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_exec)
+{
+	.mount_exec = false,
+	.file_exec = true,
+	.sysctl_err_code = {EACCES, 0, EACCES},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_noexec)
+{
+	.mount_exec = false,
+	.file_exec = false,
+	.sysctl_err_code = {EACCES, EACCES, EACCES},
+};
+
+FIXTURE_SETUP(access)
+{
+	int procfd_path_size;
+	static const char path_template[] = "/proc/self/fd/%d";
+	char procfd_path[sizeof(path_template) + 10];
+
+	/*
+	 * Cleans previous workspace if any error previously happened (don't
+	 * check errors).
+	 */
+	umount(workdir_path);
+	rmdir(workdir_path);
+
+	/* Creates a clean mount point. */
+	ASSERT_EQ(0, mkdir(workdir_path, 00700));
+	ASSERT_EQ(0, mount("test", workdir_path, "tmpfs", MS_MGC_VAL |
+				(variant->mount_exec ? 0 : MS_NOEXEC),
+				"mode=0700,size=4k"));
+
+	/* Creates a regular file. */
+	ASSERT_EQ(0, mknod(reg_file_path, S_IFREG | (variant->file_exec ? 0500 : 0400), 0));
+	/* Creates a directory. */
+	ASSERT_EQ(0, mkdir(dir_path, variant->file_exec ? 0500 : 0400));
+	/* Creates a character device: /dev/null. */
+	ASSERT_EQ(0, mknod(char_dev_path, S_IFCHR | 0400, makedev(1, 3)));
+	/* Creates a block device: /dev/loop0 */
+	ASSERT_EQ(0, mknod(block_dev_path, S_IFBLK | 0400, makedev(7, 0)));
+	/* Creates a fifo. */
+	ASSERT_EQ(0, mknod(fifo_path, S_IFIFO | 0400, 0));
+
+	/* Creates a regular file without user mount point. */
+	self->memfd = memfd_create("test-interpreted", MFD_CLOEXEC);
+	ASSERT_LE(0, self->memfd);
+	/* Sets mode, which must be ignored by the exec check. */
+	ASSERT_EQ(0, fchmod(self->memfd, variant->file_exec ? 0500 : 0400));
+
+	/* Creates a pipefs file descriptor. */
+	ASSERT_EQ(0, pipe(self->pipe_fds));
+	procfd_path_size = snprintf(procfd_path, sizeof(procfd_path),
+			path_template, self->pipe_fds[0]);
+	ASSERT_LT(procfd_path_size, sizeof(procfd_path));
+	self->pipefd = open(procfd_path, O_RDONLY | O_CLOEXEC);
+	ASSERT_LE(0, self->pipefd);
+	ASSERT_EQ(0, fchmod(self->pipefd, variant->file_exec ? 0500 : 0400));
+
+	/* Creates a socket file descriptor. */
+	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0, self->socket_fds));
+
+	/* Saves initial sysctl value. */
+	self->initial_sysctl_value = sysctl_read_char(_metadata);
+
+	/* Prepares for sysctl writes. */
+	ignore_sys_admin(_metadata, 1);
+}
+
+FIXTURE_TEARDOWN(access)
+{
+	EXPECT_EQ(0, close(self->memfd));
+	EXPECT_EQ(0, close(self->pipefd));
+	EXPECT_EQ(0, close(self->pipe_fds[0]));
+	EXPECT_EQ(0, close(self->pipe_fds[1]));
+	EXPECT_EQ(0, close(self->socket_fds[0]));
+	EXPECT_EQ(0, close(self->socket_fds[1]));
+
+	/* Restores initial sysctl value. */
+	sysctl_write_char(_metadata, self->initial_sysctl_value);
+
+	/* There is no need to unlink the test files. */
+	ASSERT_EQ(0, umount(workdir_path));
+	ASSERT_EQ(0, rmdir(workdir_path));
+}
+
+TEST_F(access, sysctl_0)
+{
+	/* Do not enforce anything. */
+	sysctl_write_char(_metadata, '0');
+	test_files(_metadata, self, 0, false);
+}
+
+TEST_F(access, sysctl_1)
+{
+	/* Enforces mount exec check. */
+	sysctl_write_char(_metadata, '1');
+	test_files(_metadata, self, variant->sysctl_err_code[0], true);
+}
+
+TEST_F(access, sysctl_2)
+{
+	/* Enforces file exec check. */
+	sysctl_write_char(_metadata, '2');
+	test_files(_metadata, self, variant->sysctl_err_code[1], true);
+}
+
+TEST_F(access, sysctl_3)
+{
+	/* Enforces mount and file exec check. */
+	sysctl_write_char(_metadata, '3');
+	test_files(_metadata, self, variant->sysctl_err_code[2], true);
+}
+
+FIXTURE(cleanup) {
+	char initial_sysctl_value;
+};
+
+FIXTURE_SETUP(cleanup)
+{
+	/* Saves initial sysctl value. */
+	self->initial_sysctl_value = sysctl_read_char(_metadata);
+}
+
+FIXTURE_TEARDOWN(cleanup)
+{
+	/* Restores initial sysctl value. */
+	ignore_sys_admin(_metadata, 1);
+	sysctl_write_char(_metadata, self->initial_sysctl_value);
+}
+
+TEST_F(cleanup, sysctl_access_write)
+{
+	int fd;
+	ssize_t ret;
+
+	ignore_sys_admin(_metadata, 1);
+	sysctl_write_char(_metadata, '0');
+
+	ignore_sys_admin(_metadata, 0);
+	fd = open(sysctl_path, O_WRONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ret = write(fd, "0", 1);
+	ASSERT_EQ(-1, ret);
+	ASSERT_EQ(EPERM, errno);
+	EXPECT_EQ(0, close(fd));
+}
+
+TEST_HARNESS_MAIN
-- 
2.34.1