Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On Tue, May 5, 2020 at 11:33 AM Mickaël Salaün <mic@digikod.net> wrote:
>
> Enable to forbid access to files open with O_MAYEXEC.  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.
>
> For tailored Linux distributions, it is possible to enforce such
> restriction at build time thanks to the CONFIG_OMAYEXEC_STATIC option.
> The policy can then be configured with CONFIG_OMAYEXEC_ENFORCE_MOUNT and
> CONFIG_OMAYEXEC_ENFORCE_FILE.
>
> 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).
>
> 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: Kees Cook <keescook@chromium.org>
> ---

> diff --git a/fs/namei.c b/fs/namei.c
> index 33b6d372e74a..70f179f6bc6c 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -411,10 +412,90 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
<snip>
> +#if defined(CONFIG_SYSCTL) && !defined(CONFIG_OMAYEXEC_STATIC)
> +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer,
> +               size_t *lenp, loff_t *ppos)
> +{
> +       int error;
> +
> +       if (write) {
> +               struct ctl_table table_copy;
> +               int tmp_mayexec_enforce;
> +
> +               if (!capable(CAP_MAC_ADMIN))
> +                       return -EPERM;

Not fond of using CAP_MAC_ADMIN here (or elsewhere outside of security
modules).  The ability to set this sysctl is not equivalent to being
able to load a MAC policy, set arbitrary MAC labels on
processes/files, etc.

> + * omayexec_inode_permission - Check O_MAYEXEC before accessing an inode
> + *
> + * @inode: Inode to check permission on
> + * @mask: Right to check for (%MAY_OPENEXEC, %MAY_EXECMOUNT, %MAY_EXEC)
> + *
> + * Returns 0 if access is permitted, -EACCES otherwise.
> + */
> +static inline int omayexec_inode_permission(struct inode *inode, int mask)
> +{
> +       if (!(mask & MAY_OPENEXEC))
> +               return 0;
> +
> +       if ((sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_MOUNT) &&
> +                       !(mask & MAY_EXECMOUNT))
> +               return -EACCES;
> +
> +       if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_FILE)
> +               return generic_permission(inode, MAY_EXEC);
> +
> +       return 0;
> +}

I'm wondering if this is being done at the wrong level.  I would think
that OMAYEXEC_ENFORCE_FILE would mean to check file execute permission
with respect to all mechanisms/policies, including DAC,
filesystem-specific checking (inode->i_op->permission), security
modules, etc.  That requires more than just calling
generic_permission() with MAY_EXEC, which only covers the default
DAC/ACL logic; you'd need to take the handling up a level to
inode_permission() and re-map MAY_OPENEXEC to MAY_EXEC for
do_inode_permission() and security_inode_permission() at least.
Alternatively, we can modify each individual filesystem (that
implements its own i_op->permission) and security module to start
handling MAY_OPENEXEC and have them choose to remap it to a file
execute check (or not) independent of the sysctl.  Not sure of your
intent.  As it stands, selinux_inode_permission() will ignore the new
MAY_OPENEXEC flag until someone updates it.  Likewise for Smack.
AppArmor/TOMOYO would probably need to check and handle FMODE_EXEC in
their file_open hooks since they don't implement inode_permission().