Re: [RFC] Make Yama pid_ns aware

[Serge Hallyn]

Quoting Vasiliy Kulikov (segoon@xxxxxxxxxxxx):
> Hi,
> 
> As Yama's sysctls are about defining a security policy for the system,
> it is reasonable to define it per container in case of LXC containers
> (or out-of-tree alternatives like OpenVZ).  In my opinion they belong
> to pid namespace.  With per-pid_ns sysctls it is possible to create
> multiple containers with different ptrace, /tmp, etc. policies.

tying the ptrace policy to pidns makes some sense, but is it definately
what we want?

Is the idea that the container should never be able to bypass the
restrictions, or should root in the container eventually be able to
bypass it as he can on the host?

Would a securebits interface be more or less suitable?  It would allow
per-process setting, inherited from parent on fork.  It would however
not allow a root shell on the desktop, after the fact, saying that
a running gdb should be allowed to access firefox.  But, it would be
able to say "I, from now on, am exempt, so that I can debug the
running firefox", without the rest of the system having its setting
changed.

> As Yama is not merged yet, I post the patch in this thread.
> 
> The patch is straightforward:
> 
> 1) all sysctl variables are moved from global vars to pid_namespace
> fields.
> 
> 2) each cloned pid ns gets the settings of the parent.
> 
> 3) the variables of current pid ns are visible through sysctl interface.
> 
> proc_pid_dointvec_minmax() is stolen from its ipc_ns equivalent in
> ipc/ipc_sysctl.c.
> 
> (The patch is not tested.)
> 
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 38d1032..46edaf8 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -30,6 +30,11 @@ struct pid_namespace {
>  #ifdef CONFIG_BSD_PROCESS_ACCT
>  	struct bsd_acct_struct *bacct;
>  #endif
> +#ifdef CONFIG_SECURITY_YAMA
> +	int ptrace_scope;
> +	int protected_sticky_symlinks;
> +	int protected_nonaccess_hardlinks;
> +#endif
>  };
>  
>  extern struct pid_namespace init_pid_ns;
> diff --git a/kernel/pid.c b/kernel/pid.c
> index fa5f722..0cd8926 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -78,6 +78,11 @@ struct pid_namespace init_pid_ns = {
>  	.last_pid = 0,
>  	.level = 0,
>  	.child_reaper = &init_task,
> +#ifdef CONFIG_SECURITY_YAMA
> +	.ptrace_scope = 1,
> +	.protected_sticky_symlinks = 1,
> +	.protected_nonaccess_hardlinks = 1,
> +#endif
>  };
>  EXPORT_SYMBOL_GPL(init_pid_ns);
>  
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index e9c9adc..73d47c4 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -101,6 +101,14 @@ static struct pid_namespace *create_pid_namespace(struct pid_namespace *parent_p
>  	if (err)
>  		goto out_put_parent_pid_ns;
>  
> +#ifdef CONFIG_SECURITY_YAMA
> +	ns->ptrace_scope = parent_pid_ns->ptrace_scope;
> +	ns->protected_sticky_symlinks =
> +		parent_pid_ns->protected_sticky_symlinks;
> +	ns->protected_nonaccess_hardlinks =
> +		parent_pid_ns->protected_nonaccess_hardlinks;
> +#endif
> +
>  	return ns;
>  
>  out_put_parent_pid_ns:
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index a92538c..cf99a8c 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -16,10 +16,7 @@
>  #include <linux/ptrace.h>
>  #include <linux/prctl.h>
>  #include <linux/ratelimit.h>
> -
> -static int ptrace_scope = 1;
> -static int protected_sticky_symlinks = 1;
> -static int protected_nonaccess_hardlinks = 1;
> +#include <linux/pid_namespace.h>
>  
>  /* describe a PTRACE relationship for potential exception */
>  struct ptrace_relation {
> @@ -250,7 +247,7 @@ static int yama_ptrace_access_check(struct task_struct *child,
>  
>  	/* require ptrace target be a child of ptracer on attach */
>  	if (mode == PTRACE_MODE_ATTACH &&
> -	    ptrace_scope &&
> +	    current->nsproxy->pid_ns->ptrace_scope &&
>  	    !capable(CAP_SYS_PTRACE) &&
>  	    !task_is_descendant(current, child) &&
>  	    !ptracer_exception_found(current, child))
> @@ -292,7 +289,7 @@ static int yama_inode_follow_link(struct dentry *dentry,
>  	const struct inode *inode;
>  	const struct cred *cred;
>  
> -	if (!protected_sticky_symlinks)
> +	if (!current->nsproxy->pid_ns->protected_sticky_symlinks)
>  		return 0;
>  
>  	/* if inode isn't a symlink, don't try to evaluate blocking it */
> @@ -362,7 +359,7 @@ static int yama_path_link(struct dentry *old_dentry, struct path *new_dir,
>  	const int mode = inode->i_mode;
>  	const struct cred *cred = current_cred();
>  
> -	if (!protected_nonaccess_hardlinks)
> +	if (!current->nsproxy->pid_ns->protected_nonaccess_hardlinks)
>  		return 0;
>  
>  	if (cred->fsuid != inode->i_uid &&
> @@ -395,6 +392,26 @@ static struct security_operations yama_ops = {
>  static int zero;
>  static int one = 1;
>  
> +static void *get_pid_data(ctl_table *table)
> +{
> +	char *which = table->data;
> +	struct pid_namespace *pid_ns = current->nsproxy->pid_ns;
> +	which = (which - (char *)&init_pid_ns) + (char *)pid_ns;
> +	return which;
> +}
> +
> +static int proc_pid_dointvec_minmax(ctl_table *table, int write,
> +	void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	struct ctl_table pid_table;
> +
> +	memcpy(&pid_table, table, sizeof(pid_table));
> +	pid_table.data = get_pid_data(table);
> +
> +	return proc_dointvec_minmax(&pid_table, write, buffer, lenp, ppos);
> +}
> +
> +
>  struct ctl_path yama_sysctl_path[] = {
>  	{ .procname = "kernel", },
>  	{ .procname = "yama", },
> @@ -404,28 +421,28 @@ struct ctl_path yama_sysctl_path[] = {
>  static struct ctl_table yama_sysctl_table[] = {
>  	{
>  		.procname       = "protected_sticky_symlinks",
> -		.data           = &protected_sticky_symlinks,
> +		.data           = &init_pid_ns.protected_sticky_symlinks,
>  		.maxlen         = sizeof(int),
>  		.mode           = 0644,
> -		.proc_handler   = proc_dointvec_minmax,
> +		.proc_handler   = proc_pid_dointvec_minmax,
>  		.extra1         = &zero,
>  		.extra2         = &one,
>  	},
>  	{
>  		.procname       = "protected_nonaccess_hardlinks",
> -		.data           = &protected_nonaccess_hardlinks,
> +		.data           = &init_pid_ns.protected_nonaccess_hardlinks,
>  		.maxlen         = sizeof(int),
>  		.mode           = 0644,
> -		.proc_handler   = proc_dointvec_minmax,
> +		.proc_handler   = proc_pid_dointvec_minmax,
>  		.extra1         = &zero,
>  		.extra2         = &one,
>  	},
>  	{
>  		.procname       = "ptrace_scope",
> -		.data           = &ptrace_scope,
> +		.data           = &init_pid_ns.ptrace_scope,
>  		.maxlen         = sizeof(int),
>  		.mode           = 0644,
> -		.proc_handler   = proc_dointvec_minmax,
> +		.proc_handler   = proc_pid_dointvec_minmax,
>  		.extra1         = &zero,
>  		.extra2         = &one,
>  	},
> ---
> --
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