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Date: Wed, 12 Dec 2018 18:09:35 +0100
From: Jann Horn <jannh@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: kernel list <linux-kernel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>,
James Morris <jmorris@...ei.org>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>,
Matthew Garrett <mjg59@...gle.com>, Michael Kerrisk-manpages <mtk.manpages@...il.com>, mickael.salaun@....gouv.fr,
zohar@...ux.ibm.com, philippe.trebuchet@....gouv.fr, shuah@...nel.org,
thibaut.sautereau@....gouv.fr, vincent.strubel@....gouv.fr,
yves-alexis.perez@....gouv.fr,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file
executability through O_MAYEXEC
On Wed, Dec 12, 2018 at 9:18 AM Mickaël Salaün <mic@...ikod.net> wrote:
> Enable to either propagate the mount options from the underlying VFS
> mount to prevent execution, or to propagate the file execute permission.
> This may allow a script interpreter to check execution permissions
> before reading commands from a file.
>
> The main goal is to be able 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).
>
> Add a new sysctl kernel.yama.open_mayexec_enforce to control this
> behavior. A following patch adds documentation.
>
> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@....gouv.fr>
> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@....gouv.fr>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Mickaël Salaün <mickael.salaun@....gouv.fr>
> ---
[...]
> +/**
> + * yama_inode_permission - check O_MAYEXEC permission before accessing an inode
> + * @inode: inode structure to check
> + * @mask: permission mask
> + *
> + * Return 0 if access is permitted, -EACCES otherwise.
> + */
> +int yama_inode_permission(struct inode *inode, int mask)
This should be static, no?
> +{
> + if (!(mask & MAY_OPENEXEC))
> + return 0;
> + /*
> + * Match regular files and directories to make it easier to
> + * modify script interpreters.
> + */
> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> + return 0;
So files are subject to checks, but loading code from things like
sockets is always fine?
> + if ((open_mayexec_enforce & YAMA_OMAYEXEC_ENFORCE_MOUNT) &&
> + !(mask & MAY_EXECMOUNT))
> + return -EACCES;
> +
> + /*
> + * May prefer acl_permission_check() instead of generic_permission(),
> + * to not be bypassable with CAP_DAC_READ_SEARCH.
> + */
> + if (open_mayexec_enforce & YAMA_OMAYEXEC_ENFORCE_FILE)
> + return generic_permission(inode, MAY_EXEC);
> +
> + return 0;
> +}
> +
> static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
> + LSM_HOOK_INIT(inode_permission, yama_inode_permission),
> LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
> LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
> LSM_HOOK_INIT(task_prctl, yama_task_prctl),
> @@ -447,6 +489,37 @@ static int yama_dointvec_minmax(struct ctl_table *table, int write,
> return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
> }
>
> +static int yama_dointvec_bitmask_macadmin(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;
Don't put capable() checks in sysctls, it doesn't work.
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