linux-kernel - Re: [PATCH v5 1/1] NAX LSM: Add initial support

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Date:   Mon, 30 Aug 2021 16:29:51 -0700
From:   J Freyensee <why2jjj.linux@...il.com>
To:     Igor Zhbanov <izh1979@...il.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        THOBY Simon <Simon.THOBY@...eris.fr>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/1] NAX LSM: Add initial support



On 8/21/21 2:47 AM, Igor Zhbanov wrote:
> Add initial support for NAX (No Anonymous Execution), which is a Linux
> Security Module that extends DAC by making impossible to make anonymous
> and modified pages executable for privileged processes.
>
> Intercepts anonymous executable pages created with mmap() and mprotect()
> system calls.
>
> Log violations (in non-quiet mode) and block the action or kill the
> offending process, depending on the enabled settings.
>
> See Documentation/admin-guide/LSM/NAX.rst.
>
> Signed-off-by: Igor Zhbanov <izh1979@...il.com>
> ---
>   Documentation/admin-guide/LSM/NAX.rst         |  72 +++
>   Documentation/admin-guide/LSM/index.rst       |   1 +
>   .../admin-guide/kernel-parameters.rst         |   1 +
>   .../admin-guide/kernel-parameters.txt         |  32 ++
>   security/Kconfig                              |  11 +-
>   security/Makefile                             |   2 +
>   security/nax/Kconfig                          | 113 +++++
>   security/nax/Makefile                         |   4 +
>   security/nax/nax-lsm.c                        | 472 ++++++++++++++++++
>   9 files changed, 703 insertions(+), 5 deletions(-)
>   create mode 100644 Documentation/admin-guide/LSM/NAX.rst
>   create mode 100644 security/nax/Kconfig
>   create mode 100644 security/nax/Makefile
>   create mode 100644 security/nax/nax-lsm.c
>
> diff --git a/Documentation/admin-guide/LSM/NAX.rst b/Documentation/admin-guide/LSM/NAX.rst
> new file mode 100644
> index 000000000000..da54b3be4cda
> --- /dev/null
> +++ b/Documentation/admin-guide/LSM/NAX.rst
> @@ -0,0 +1,72 @@
> +=======
> +NAX LSM
> +=======
> +
> +:Author: Igor Zhbanov
> +
> +NAX (No Anonymous Execution) is a Linux Security Module that extends DAC
> +by making impossible to make anonymous and modified pages executable for
> +processes. The module intercepts anonymous executable pages created with
> +mmap() and mprotect() system calls.
> +
> +To select it at boot time, add ``nax`` to ``security`` kernel command-line
> +parameter.
> +
> +The following sysctl parameters are available:
> +
> +* ``kernel.nax.check_all``:
> + - 0: Check all processes.
> + - 1: Check only privileged processes. The privileged process is a process
> +      for which any of the following is true:
> +      - ``uid  == 0``
> +      - ``euid == 0``
> +      - ``suid == 0``
> +      - ``cap_effective`` has any capability except for the ones allowed
> +        in ``kernel.nax.allowed_caps``
> +      - ``cap_permitted`` has any capability except for the ones allowed
> +        in ``kernel.nax.allowed_caps``
> +
> + Checking of uid/euid/suid is important because a process may call seteuid(0)
> + to gain privileges (if SECURE_NO_SETUID_FIXUP secure bit is not set).
> +
> +* ``kernel.nax.allowed_caps``:
> +
> + Hexadecimal number representing the set of capabilities a non-root
> + process can possess without being considered "privileged" by NAX LSM.
> +
> + For the meaning of the capabilities bits and their value, please check
> + ``include/uapi/linux/capability.h`` and ``capabilities(7)`` manual page.
> +
> + For example, ``CAP_SYS_PTRACE`` has a number 19. Therefore, to add it to
> + allowed capabilities list, we need to set 19'th bit (2^19 or 1 << 19)
> + or 80000 in hexadecimal form. Capabilities can be bitwise ORed.
> +
> +* ``kernel.nax.mode``:
> +
> + - 0: Only log errors (when enabled by ``kernel.nax.quiet``) (default mode)
> + - 1: Forbid unsafe pages mappings (and log when enabled)
> + - 2: Kill the violating process (and log when enabled)
> +
> +* ``kernel.nax.quiet``:
> +
> + - 0: Log violations (default)
> + - 1: Be quiet
> +
> +* ``kernel.nax.locked``:
> +
> + - 0: Changing of the module's sysctl parameters is allowed
> + - 1: Further changing of the module's sysctl parameters is forbidden
> +
> + Setting this parameter to ``1`` after initial setup during the system boot
> + will prevent the module disabling at the later time.
> +
> +There are matching kernel command-line parameters (with the same values):
> +
> +- ``nax_allowed_caps``
> +- ``nax_check_all``
> +- ``nax_mode``
> +- ``nax_quiet``
> +- ``nax_locked``
> +
> +The ``nax_locked`` command-line parameter must be specified last to avoid
> +premature setting locking.


Is it common to have these types of restrictions for kernel command-line 
parameters, in this case, kernel command-line parameter ordering?  Seems 
like that would be prone for a lot of avoidable troubleshooting issues 
and unnecessary usage questions.

<big snip>
.
.
.

> +
> +static void __init
> +nax_init_sysctl(void)
> +{
> +	if (!register_sysctl_paths(nax_sysctl_path, nax_sysctl_table))
> +		panic("NAX: sysctl registration failed.\n");
> +}
> +
> +#else /* !CONFIG_SYSCTL */
> +
> +static inline void
> +nax_init_sysctl(void)
> +{
> +
> +}
> +
> +#endif /* !CONFIG_SYSCTL */
> +
> +static int __init setup_allowed_caps(char *str)
> +{
> +	if (locked)
> +		return 1;
> +
> +	/* Do not allow trailing garbage or excessive length */
> +	if (strlen(str) > ALLOWED_CAPS_HEX_LEN) {

  a little nitpick, could strnlen() be used instead to define a max 
length of the input 'str'?


Regards,
Jay