Re: [PATCH v26 14/25] LSM: Specify which LSM to display

[Casey Schaufler]

On 5/14/2021 12:23 PM, Kees Cook wrote:
> On Thu, May 13, 2021 at 01:07:56PM -0700, Casey Schaufler wrote:
>> Create a new entry "interface_lsm" in the procfs attr directory for
>> controlling which LSM security information is displayed for a
>> process. A process can only read or write its own display value.
>>
>> The name of an active LSM that supplies hooks for
>> human readable data may be written to "interface_lsm" to set the
>> value. The name of the LSM currently in use can be read from
>> "interface_lsm". At this point there can only be one LSM capable
>> of display active. A helper function lsm_task_ilsm() is
>> provided to get the interface lsm slot for a task_struct.
>>
>> Setting the "interface_lsm" requires that all security modules using
>> setprocattr hooks allow the action. Each security module is
>> responsible for defining its policy.
>>
>> AppArmor hook provided by John Johansen <john.johansen@xxxxxxxxxxxxx>
>> SELinux hook provided by Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
>>
>> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
>> Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
>> Cc: John Johansen <john.johansen@xxxxxxxxxxxxx>
>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> Cc: linux-api@xxxxxxxxxxxxxxx
>> Cc: linux-doc@xxxxxxxxxxxxxxx
>> ---
>>  .../ABI/testing/procfs-attr-lsm_display       |  22 +++
>>  Documentation/security/lsm.rst                |  14 ++
>>  fs/proc/base.c                                |   1 +
>>  include/linux/lsm_hooks.h                     |  17 ++
>>  security/apparmor/include/apparmor.h          |   3 +-
>>  security/apparmor/lsm.c                       |  32 ++++
>>  security/security.c                           | 166 ++++++++++++++++--
>>  security/selinux/hooks.c                      |  11 ++
>>  security/selinux/include/classmap.h           |   2 +-
>>  security/smack/smack_lsm.c                    |   7 +
>>  10 files changed, 256 insertions(+), 19 deletions(-)
>>  create mode 100644 Documentation/ABI/testing/procfs-attr-lsm_display
>>
>> diff --git a/Documentation/ABI/testing/procfs-attr-lsm_display b/Documentation/ABI/testing/procfs-attr-lsm_display
>> new file mode 100644
>> index 000000000000..0f60005c235c
>> --- /dev/null
>> +++ b/Documentation/ABI/testing/procfs-attr-lsm_display
>> @@ -0,0 +1,22 @@
>> +What:		/proc/*/attr/lsm_display
>> +Contact:	linux-security-module@xxxxxxxxxxxxxxx,
>> +Description:	The name of the Linux security module (LSM) that will
>> +		provide information in the /proc/*/attr/current,
>> +		/proc/*/attr/prev and /proc/*/attr/exec interfaces.
>> +		The details of permissions required to read from
>> +		this interface are dependent on the LSMs active on the
>> +		system.
>> +		A process cannot write to this interface unless it
>> +		refers to itself.
>> +		The other details of permissions required to write to
>> +		this interface are dependent on the LSMs active on the
>> +		system.
>> +		The format of the data used by this interface is a
>> +		text string identifying the name of an LSM. The values
>> +		accepted are:
>> +			selinux		- the SELinux LSM
>> +			smack		- the Smack LSM
>> +			apparmor	- The AppArmor LSM
>> +		By convention the LSM names are lower case and do not
>> +		contain special characters.
>> +Users:		LSM user-space
>> diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
>> index 6a2a2e973080..b77b4a540391 100644
>> --- a/Documentation/security/lsm.rst
>> +++ b/Documentation/security/lsm.rst
>> @@ -129,3 +129,17 @@ to identify it as the first security module to be registered.
>>  The capabilities security module does not use the general security
>>  blobs, unlike other modules. The reasons are historical and are
>>  based on overhead, complexity and performance concerns.
>> +
>> +LSM External Interfaces
>> +=======================
>> +
>> +The LSM infrastructure does not generally provide external interfaces.
>> +The individual security modules provide what external interfaces they
>> +require.
>> +
>> +The file ``/sys/kernel/security/lsm`` provides a comma
>> +separated list of the active security modules.
>> +
>> +The file ``/proc/pid/attr/interface_lsm`` contains the name of the security
>> +module for which the ``/proc/pid/attr/current`` interface will
>> +apply. This interface can be written to.
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 3851bfcdba56..10de522f3112 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -2807,6 +2807,7 @@ static const struct pid_entry attr_dir_stuff[] = {
>>  	ATTR(NULL, "fscreate",		0666),
>>  	ATTR(NULL, "keycreate",		0666),
>>  	ATTR(NULL, "sockcreate",	0666),
>> +	ATTR(NULL, "interface_lsm",	0666),
> Anyone can open this file... (continued below)
>
>>  #ifdef CONFIG_SECURITY_SMACK
>>  	DIR("smack",			0555,
>>  	    proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>> index c61a16f0a5bc..d2c4bc94d47f 100644
>> --- a/include/linux/lsm_hooks.h
>> +++ b/include/linux/lsm_hooks.h
>> @@ -1686,4 +1686,21 @@ static inline void security_delete_hooks(struct security_hook_list *hooks,
>>  
>>  extern int lsm_inode_alloc(struct inode *inode);
>>  
>> +/**
>> + * lsm_task_ilsm - the "interface_lsm" for this task
>> + * @task: The task to report on
>> + *
>> + * Returns the task's interface LSM slot.
>> + */
>> +static inline int lsm_task_ilsm(struct task_struct *task)
>> +{
>> +#ifdef CONFIG_SECURITY
>> +	int *ilsm = task->security;
>> +
>> +	if (ilsm)
>> +		return *ilsm;
>> +#endif
>> +	return LSMBLOB_INVALID;
>> +}
>> +
>>  #endif /* ! __LINUX_LSM_HOOKS_H */
>> diff --git a/security/apparmor/include/apparmor.h b/security/apparmor/include/apparmor.h
>> index 1fbabdb565a8..b1622fcb4394 100644
>> --- a/security/apparmor/include/apparmor.h
>> +++ b/security/apparmor/include/apparmor.h
>> @@ -28,8 +28,9 @@
>>  #define AA_CLASS_SIGNAL		10
>>  #define AA_CLASS_NET		14
>>  #define AA_CLASS_LABEL		16
>> +#define AA_CLASS_DISPLAY_LSM	17
>>  
>> -#define AA_CLASS_LAST		AA_CLASS_LABEL
>> +#define AA_CLASS_LAST		AA_CLASS_DISPLAY_LSM
>>  
>>  /* Control parameters settable through module/boot flags */
>>  extern enum audit_mode aa_g_audit;
>> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
>> index 392e25940d1f..4237536106aa 100644
>> --- a/security/apparmor/lsm.c
>> +++ b/security/apparmor/lsm.c
>> @@ -621,6 +621,25 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
>>  	return error;
>>  }
>>  
>> +
>> +static int profile_interface_lsm(struct aa_profile *profile,
>> +				 struct common_audit_data *sa)
>> +{
>> +	struct aa_perms perms = { };
>> +	unsigned int state;
>> +
>> +	state = PROFILE_MEDIATES(profile, AA_CLASS_DISPLAY_LSM);
>> +	if (state) {
>> +		aa_compute_perms(profile->policy.dfa, state, &perms);
>> +		aa_apply_modes_to_perms(profile, &perms);
>> +		aad(sa)->label = &profile->label;
>> +
>> +		return aa_check_perms(profile, &perms, AA_MAY_WRITE, sa, NULL);
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>>  static int apparmor_setprocattr(const char *name, void *value,
>>  				size_t size)
>>  {
>> @@ -632,6 +651,19 @@ static int apparmor_setprocattr(const char *name, void *value,
>>  	if (size == 0)
>>  		return -EINVAL;
>>  
>> +	/* LSM infrastructure does actual setting of interface_lsm if allowed */
>> +	if (!strcmp(name, "interface_lsm")) {
>> +		struct aa_profile *profile;
>> +		struct aa_label *label;
>> +
>> +		aad(&sa)->info = "set interface lsm";
>> +		label = begin_current_label_crit_section();
>> +		error = fn_for_each_confined(label, profile,
>> +					profile_interface_lsm(profile, &sa));
>> +		end_current_label_crit_section(label);
>> +		return error;
>> +	}
>> +
>>  	/* AppArmor requires that the buffer must be null terminated atm */
>>  	if (args[size - 1] != '\0') {
>>  		/* null terminate */
>> diff --git a/security/security.c b/security/security.c
>> index f5407a85641e..1ce125c01782 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -77,7 +77,16 @@ static struct kmem_cache *lsm_file_cache;
>>  static struct kmem_cache *lsm_inode_cache;
>>  
>>  char *lsm_names;
>> -static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init;
>> +
>> +/*
>> + * The task blob includes the "interface_lsm" slot used for
>> + * chosing which module presents contexts.
>> + * Using a long to avoid potential alignment issues with
>> + * module assigned task blobs.
>> + */
>> +static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init = {
>> +	.lbs_task = sizeof(long),
>> +};
>>  
>>  /* Boot-time LSM user choice */
>>  static __initdata const char *chosen_lsm_order;
>> @@ -669,6 +678,8 @@ int lsm_inode_alloc(struct inode *inode)
>>   */
>>  static int lsm_task_alloc(struct task_struct *task)
>>  {
>> +	int *ilsm;
>> +
>>  	if (blob_sizes.lbs_task == 0) {
>>  		task->security = NULL;
>>  		return 0;
>> @@ -677,6 +688,15 @@ static int lsm_task_alloc(struct task_struct *task)
>>  	task->security = kzalloc(blob_sizes.lbs_task, GFP_KERNEL);
>>  	if (task->security == NULL)
>>  		return -ENOMEM;
>> +
>> +	/*
>> +	 * The start of the task blob contains the "interface" LSM slot number.
>> +	 * Start with it set to the invalid slot number, indicating that the
>> +	 * default first registered LSM be displayed.
>> +	 */
>> +	ilsm = task->security;
>> +	*ilsm = LSMBLOB_INVALID;
>> +
>>  	return 0;
>>  }
>>  
>> @@ -1732,14 +1752,26 @@ int security_file_open(struct file *file)
>>  
>>  int security_task_alloc(struct task_struct *task, unsigned long clone_flags)
>>  {
>> +	int *oilsm = current->security;
>> +	int *nilsm;
>>  	int rc = lsm_task_alloc(task);
>>  
>> -	if (rc)
>> +	if (unlikely(rc))
>>  		return rc;
>> +
>>  	rc = call_int_hook(task_alloc, 0, task, clone_flags);
>> -	if (unlikely(rc))
>> +	if (unlikely(rc)) {
>>  		security_task_free(task);
>> -	return rc;
>> +		return rc;
>> +	}
>> +
>> +	if (oilsm) {
>> +		nilsm = task->security;
>> +		if (nilsm)
>> +			*nilsm = *oilsm;
>> +	}
>> +
>> +	return 0;
>>  }
>>  
>>  void security_task_free(struct task_struct *task)
>> @@ -2171,23 +2203,110 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
>>  				char **value)
>>  {
>>  	struct security_hook_list *hp;
>> +	int ilsm = lsm_task_ilsm(current);
>> +	int slot = 0;
>> +
>> +	if (!strcmp(name, "interface_lsm")) {
>> +		/*
>> +		 * lsm_slot will be 0 if there are no displaying modules.
>> +		 */
>> +		if (lsm_slot == 0)
>> +			return -EINVAL;
>> +
>> +		/*
>> +		 * Only allow getting the current process' interface_lsm.
>> +		 * There are too few reasons to get another process'
>> +		 * interface_lsm and too many LSM policy issues.
>> +		 */
>> +		if (current != p)
>> +			return -EINVAL;
> ... but context isn't established by just checking "current", as this
> file handle may have been given to another process.
>
> I suspect the security_get/setprocattr needs to gain a pointer to "file"
> so that the f_cred struct can be examined[1] (i.e. compare opener
> against reader/writer).
>
> [1] https://www.kernel.org/doc/html/latest/security/credentials.html#open-file-credentials

It's not credentials being checked here. The check is whether the task that
would be affected is "current". Process A can't open /proc/B/attr/interface_lsm
with write access. The only process that can open it for write access is B.
If process B opens /proc/B/attr/interface_lsm for write access it could send
the file handle to process A, but process A can't write to the file because
(current != p) that is, (A != B).

>> +
>> +		ilsm = lsm_task_ilsm(p);
>> +		if (ilsm != LSMBLOB_INVALID)
>> +			slot = ilsm;
>> +		*value = kstrdup(lsm_slotlist[slot]->lsm, GFP_KERNEL);
>> +		if (*value)
>> +			return strlen(*value);
>> +		return -ENOMEM;
>> +	}
>>  
>>  	hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
>>  		if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
>>  			continue;
>> +		if (lsm == NULL && ilsm != LSMBLOB_INVALID &&
>> +		    ilsm != hp->lsmid->slot)
>> +			continue;
>>  		return hp->hook.getprocattr(p, name, value);
>>  	}
>>  	return LSM_RET_DEFAULT(getprocattr);
>>  }
>>  
>> +/**
>> + * security_setprocattr - Set process attributes via /proc
>> + * @lsm: name of module involved, or NULL
>> + * @name: name of the attribute
>> + * @value: value to set the attribute to
>> + * @size: size of the value
>> + *
>> + * Set the process attribute for the specified security module
>> + * to the specified value. Note that this can only be used to set
>> + * the process attributes for the current, or "self" process.
>> + * The /proc code has already done this check.
>> + *
>> + * Returns 0 on success, an appropriate code otherwise.
>> + */
>>  int security_setprocattr(const char *lsm, const char *name, void *value,
>>  			 size_t size)
>>  {
>>  	struct security_hook_list *hp;
>> +	char *termed;
>> +	char *copy;
>> +	int *ilsm = current->security;
>> +	int rc = -EINVAL;
>> +	int slot = 0;
>> +
>> +	if (!strcmp(name, "interface_lsm")) {
>> +		/*
>> +		 * Change the "interface_lsm" value only if all the security
>> +		 * modules that support setting a procattr allow it.
>> +		 * It is assumed that all such security modules will be
>> +		 * cooperative.
>> +		 */
>> +		if (size == 0)
>> +			return -EINVAL;
>> +
>> +		hlist_for_each_entry(hp, &security_hook_heads.setprocattr,
>> +				     list) {
>> +			rc = hp->hook.setprocattr(name, value, size);
>> +			if (rc < 0)
>> +				return rc;
>> +		}
> Similarly here -- how do the LSMs reason about who actually _opened_
> this file?
>
> -Kees
>
>> +
>> +		rc = -EINVAL;
>> +
>> +		copy = kmemdup_nul(value, size, GFP_KERNEL);
>> +		if (copy == NULL)
>> +			return -ENOMEM;
>> +
>> +		termed = strsep(&copy, " \n");
>> +
>> +		for (slot = 0; slot < lsm_slot; slot++)
>> +			if (!strcmp(termed, lsm_slotlist[slot]->lsm)) {
>> +				*ilsm = lsm_slotlist[slot]->slot;
>> +				rc = size;
>> +				break;
>> +			}
>> +
>> +		kfree(termed);
>> +		return rc;
>> +	}
>>  
>>  	hlist_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
>>  		if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
>>  			continue;
>> +		if (lsm == NULL && *ilsm != LSMBLOB_INVALID &&
>> +		    *ilsm != hp->lsmid->slot)
>> +			continue;
>>  		return hp->hook.setprocattr(name, value, size);
>>  	}
>>  	return LSM_RET_DEFAULT(setprocattr);
>> @@ -2207,15 +2326,15 @@ EXPORT_SYMBOL(security_ismaclabel);
>>  int security_secid_to_secctx(struct lsmblob *blob, char **secdata, u32 *seclen)
>>  {
>>  	struct security_hook_list *hp;
>> -	int rc;
>> +	int ilsm = lsm_task_ilsm(current);
>>  
>>  	hlist_for_each_entry(hp, &security_hook_heads.secid_to_secctx, list) {
>>  		if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >= lsm_slot))
>>  			continue;
>> -		rc = hp->hook.secid_to_secctx(blob->secid[hp->lsmid->slot],
>> -					      secdata, seclen);
>> -		if (rc != LSM_RET_DEFAULT(secid_to_secctx))
>> -			return rc;
>> +		if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
>> +			return hp->hook.secid_to_secctx(
>> +					blob->secid[hp->lsmid->slot],
>> +					secdata, seclen);
>>  	}
>>  
>>  	return LSM_RET_DEFAULT(secid_to_secctx);
>> @@ -2226,16 +2345,15 @@ int security_secctx_to_secid(const char *secdata, u32 seclen,
>>  			     struct lsmblob *blob)
>>  {
>>  	struct security_hook_list *hp;
>> -	int rc;
>> +	int ilsm = lsm_task_ilsm(current);
>>  
>>  	lsmblob_init(blob, 0);
>>  	hlist_for_each_entry(hp, &security_hook_heads.secctx_to_secid, list) {
>>  		if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >= lsm_slot))
>>  			continue;
>> -		rc = hp->hook.secctx_to_secid(secdata, seclen,
>> -					      &blob->secid[hp->lsmid->slot]);
>> -		if (rc != 0)
>> -			return rc;
>> +		if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
>> +			return hp->hook.secctx_to_secid(secdata, seclen,
>> +						&blob->secid[hp->lsmid->slot]);
>>  	}
>>  	return 0;
>>  }
>> @@ -2243,7 +2361,14 @@ EXPORT_SYMBOL(security_secctx_to_secid);
>>  
>>  void security_release_secctx(char *secdata, u32 seclen)
>>  {
>> -	call_void_hook(release_secctx, secdata, seclen);
>> +	struct security_hook_list *hp;
>> +	int ilsm = lsm_task_ilsm(current);
>> +
>> +	hlist_for_each_entry(hp, &security_hook_heads.release_secctx, list)
>> +		if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot) {
>> +			hp->hook.release_secctx(secdata, seclen);
>> +			return;
>> +		}
>>  }
>>  EXPORT_SYMBOL(security_release_secctx);
>>  
>> @@ -2384,8 +2509,15 @@ EXPORT_SYMBOL(security_sock_rcv_skb);
>>  int security_socket_getpeersec_stream(struct socket *sock, char __user *optval,
>>  				      int __user *optlen, unsigned len)
>>  {
>> -	return call_int_hook(socket_getpeersec_stream, -ENOPROTOOPT, sock,
>> -				optval, optlen, len);
>> +	int ilsm = lsm_task_ilsm(current);
>> +	struct security_hook_list *hp;
>> +
>> +	hlist_for_each_entry(hp, &security_hook_heads.socket_getpeersec_stream,
>> +			     list)
>> +		if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
>> +			return hp->hook.socket_getpeersec_stream(sock, optval,
>> +								 optlen, len);
>> +	return -ENOPROTOOPT;
>>  }
>>  
>>  int security_socket_getpeersec_dgram(struct socket *sock, struct sk_buff *skb,
>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 0133b142e938..dba867721336 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -6510,6 +6510,17 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
>>  	/*
>>  	 * Basic control over ability to set these attributes at all.
>>  	 */
>> +
>> +	/*
>> +	 * For setting interface_lsm, we only perform a permission check;
>> +	 * the actual update to the interface_lsm value is handled by the
>> +	 * LSM framework.
>> +	 */
>> +	if (!strcmp(name, "interface_lsm"))
>> +		return avc_has_perm(&selinux_state,
>> +				    mysid, mysid, SECCLASS_PROCESS2,
>> +				    PROCESS2__SETDISPLAY, NULL);
>> +
>>  	if (!strcmp(name, "exec"))
>>  		error = avc_has_perm(&selinux_state,
>>  				     mysid, mysid, SECCLASS_PROCESS,
>> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
>> index 62d19bccf3de..8f4b0dd6dd78 100644
>> --- a/security/selinux/include/classmap.h
>> +++ b/security/selinux/include/classmap.h
>> @@ -53,7 +53,7 @@ struct security_class_mapping secclass_map[] = {
>>  	    "execmem", "execstack", "execheap", "setkeycreate",
>>  	    "setsockcreate", "getrlimit", NULL } },
>>  	{ "process2",
>> -	  { "nnp_transition", "nosuid_transition", NULL } },
>> +	  { "nnp_transition", "nosuid_transition", "setdisplay", NULL } },
>>  	{ "system",
>>  	  { "ipc_info", "syslog_read", "syslog_mod",
>>  	    "syslog_console", "module_request", "module_load", NULL } },
>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>> index 5c10ad27be37..7aa7ea38f627 100644
>> --- a/security/smack/smack_lsm.c
>> +++ b/security/smack/smack_lsm.c
>> @@ -3508,6 +3508,13 @@ static int smack_setprocattr(const char *name, void *value, size_t size)
>>  	struct smack_known_list_elem *sklep;
>>  	int rc;
>>  
>> +	/*
>> +	 * Allow the /proc/.../attr/current and SO_PEERSEC "interface_lsm"
>> +	 * to be reset at will.
>> +	 */
>> +	if (strcmp(name, "interface_lsm") == 0)
>> +		return 0;
>> +
>>  	if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
>>  		return -EPERM;
>>  
>> -- 
>> 2.29.2
>>