Re: [PATCH v14 22/23] LSM: Add /proc attr entry for full LSM context

[Stephen Smalley <sds@tycho.nsa.gov>]

On 1/23/20 7:23 PM, Casey Schaufler wrote:
> Add an entry /proc/.../attr/context which displays the full
> process security "context" in compound format:'
>          lsm1\0value\0lsm2\0value\0...
> This entry is not writable.
> 
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> Cc: linux-api@vger.kernel.org

As previously discussed, there are issues with AppArmor's implementation 
of getprocattr() particularly around the trailing newline that 
dbus-daemon and perhaps others would like to see go away in any new 
interface.  Hence, I don't think we should implement this new API using 
the existing getprocattr() hook lest it also be locked into the current 
behavior forever.

> ---
>   Documentation/security/lsm.rst | 14 ++++++++
>   fs/proc/base.c                 |  1 +
>   security/security.c            | 63 ++++++++++++++++++++++++++++++++++
>   3 files changed, 78 insertions(+)
> 
> diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
> index aadf47c808c0..a4979060f5d3 100644
> --- a/Documentation/security/lsm.rst
> +++ b/Documentation/security/lsm.rst
> @@ -199,3 +199,17 @@ capability-related fields:
>   -  ``fs/nfsd/auth.c``::c:func:`nfsd_setuser()`
>   
>   -  ``fs/proc/array.c``::c:func:`task_cap()`
> +
> +LSM External Interfaces
> +=======================
> +
> +The LSM infrastructure does not generally provide external interfaces.
> +The individual security modules provide what external interfaces they
> +require. The infrastructure does provide an interface for the special
> +case where multiple security modules provide a process context. This
> +is provided in compound context format.
> +
> +-  `lsm1\0value\0lsm2\0value\0`
> +
> +The special file ``/proc/pid/attr/context`` provides the security
> +context of the identified process.
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 950c200cb9ad..d13c2cf50e4b 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2653,6 +2653,7 @@ static const struct pid_entry attr_dir_stuff[] = {
>   	ATTR(NULL, "keycreate",		0666),
>   	ATTR(NULL, "sockcreate",	0666),
>   	ATTR(NULL, "display",		0666),
> +	ATTR(NULL, "context",		0666),
>   #ifdef CONFIG_SECURITY_SMACK
>   	DIR("smack",			0555,
>   	    proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
> diff --git a/security/security.c b/security/security.c
> index 6a77c8b2ffbc..fdd0c85df89e 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -722,6 +722,42 @@ static void __init lsm_early_task(struct task_struct *task)
>   		panic("%s: Early task alloc failed.\n", __func__);
>   }
>   
> +/**
> + * append_ctx - append a lsm/context pair to a compound context
> + * @ctx: the existing compound context
> + * @ctxlen: size of the old context, including terminating nul byte
> + * @lsm: new lsm name, nul terminated
> + * @new: new context, possibly nul terminated
> + * @newlen: maximum size of @new
> + *
> + * replace @ctx with a new compound context, appending @newlsm and @new
> + * to @ctx. On exit the new data replaces the old, which is freed.
> + * @ctxlen is set to the new size, which includes a trailing nul byte.
> + *
> + * Returns 0 on success, -ENOMEM if no memory is available.
> + */
> +static int append_ctx(char **ctx, int *ctxlen, const char *lsm, char *new,
> +		      int newlen)
> +{
> +	char *final;
> +	int llen;
> +
> +	llen = strlen(lsm) + 1;
> +	newlen = strnlen(new, newlen) + 1;
> +
> +	final = kzalloc(*ctxlen + llen + newlen, GFP_KERNEL);
> +	if (final == NULL)
> +		return -ENOMEM;
> +	if (*ctxlen)
> +		memcpy(final, *ctx, *ctxlen);
> +	memcpy(final + *ctxlen, lsm, llen);
> +	memcpy(final + *ctxlen + llen, new, newlen);
> +	kfree(*ctx);
> +	*ctx = final;
> +	*ctxlen = *ctxlen + llen + newlen;
> +	return 0;
> +}
> +
>   /*
>    * Hook list operation macros.
>    *
> @@ -2041,6 +2077,10 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
>   				char **value)
>   {
>   	struct security_hook_list *hp;
> +	char *final = NULL;
> +	char *cp;
> +	int rc = 0;
> +	int finallen = 0;
>   	int display = lsm_task_display(current);
>   	int slot = 0;
>   
> @@ -2068,6 +2108,29 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
>   		return -ENOMEM;
>   	}
>   
> +	if (!strcmp(name, "context")) {
> +		hlist_for_each_entry(hp, &security_hook_heads.getprocattr,
> +				     list) {
> +			rc = hp->hook.getprocattr(p, "current", &cp);
> +			if (rc == -EINVAL || rc == -ENOPROTOOPT)
> +				continue;
> +			if (rc < 0) {
> +				kfree(final);
> +				return rc;
> +			}
> +			rc = append_ctx(&final, &finallen, hp->lsmid->lsm,
> +					cp, rc);
> +			if (rc < 0) {
> +				kfree(final);
> +				return rc;
> +			}
> +		}
> +		if (final == NULL)
> +			return -EINVAL;
> +		*value = final;
> +		return finallen;
> +	}
> +
>   	hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
>   		if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
>   			continue;
>