[PATCH RFC 0/5] LSM: Add and use a hook for side-channel safety checks

[PATCH RFC 0/5] LSM: Add and use a hook for side-channel safety checks

[Casey Schaufler]

This patchset provide a mechanism by which a security module
can advise the system about potential side-channel vulnerabilities.
If security_task_safe_sidechannel() returns 0 the security modules do
not know of any data that would be subject to a side-channel
attack. If the security module maintains data that it believes
may be susceptible to a side-channel attack it will return -EACCES.

Simple hooks are provided for SELinux and Smack. A new security
module is provided to make determinations regarding traditional
task attributes, including user IDs, capability sets and namespaces.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>

---
 MAINTAINERS                        |   6 ++
 arch/x86/mm/tlb.c                  |  12 ++-
 include/linux/lsm_hooks.h          |  12 +++
 include/linux/security.h           |   1 +
 security/Kconfig                   |   1 +
 security/Makefile                  |   2 +
 security/security.c                |   6 ++
 security/selinux/hooks.c           |   9 +++
 security/sidechannel/Kconfig       |  60 ++++++++++++++
 security/sidechannel/Makefile      |   1 +
 security/sidechannel/sidechannel.c | 156 +++++++++++++++++++++++++++++++++++++
 security/smack/smack_lsm.c         |  18 +++++
 12 files changed, 280 insertions(+), 4 deletions(-)

[PATCH RFC 1/5] LSM: Introduce a hook for side-channel danger

[Casey Schaufler]

From: Casey Schaufler <cschaufler@localhost.localdomain>

There may be cases where the data maintained for
security controls is more sensitive than general
process information and that may be subjected to
side-channel attacks. An LSM hook is provided so
that this can be check for where the system would
take action should the current task have potential
access to the passed task.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
 include/linux/lsm_hooks.h | 7 +++++++
 include/linux/security.h  | 1 +
 security/security.c       | 5 +++++
 3 files changed, 13 insertions(+)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index a08bc2587b96..fd2a7e6beb01 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -698,6 +698,11 @@
  *	security attributes, e.g. for /proc/pid inodes.
  *	@p contains the task_struct for the task.
  *	@inode contains the inode structure for the inode.
+ * @task_safe_sidechannel:
+ *	Check if a side channel attack is harmless for the current task and @p.
+ *	The caller may have determined that no attack is possible, in which
+ *	case this hook won't get called.
+ *	@p contains the task_struct for the task.
  *
  * Security hooks for Netlink messaging.
  *
@@ -1611,6 +1616,7 @@ union security_list_options {
 	int (*task_prctl)(int option, unsigned long arg2, unsigned long arg3,
 				unsigned long arg4, unsigned long arg5);
 	void (*task_to_inode)(struct task_struct *p, struct inode *inode);
+	int (*task_safe_sidechannel)(struct task_struct *p);
 
 	int (*ipc_permission)(struct kern_ipc_perm *ipcp, short flag);
 	void (*ipc_getsecid)(struct kern_ipc_perm *ipcp, u32 *secid);
@@ -1897,6 +1903,7 @@ struct security_hook_heads {
 	struct hlist_head task_kill;
 	struct hlist_head task_prctl;
 	struct hlist_head task_to_inode;
+	struct hlist_head task_safe_sidechannel;
 	struct hlist_head ipc_permission;
 	struct hlist_head ipc_getsecid;
 	struct hlist_head msg_msg_alloc_security;
diff --git a/include/linux/security.h b/include/linux/security.h
index 3410acfe139c..69a5526f789f 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -366,6 +366,7 @@ int security_task_kill(struct task_struct *p, struct siginfo *info,
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			unsigned long arg4, unsigned long arg5);
 void security_task_to_inode(struct task_struct *p, struct inode *inode);
+int security_task_safe_sidechannel(struct task_struct *p);
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
 void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
 int security_msg_msg_alloc(struct msg_msg *msg);
diff --git a/security/security.c b/security/security.c
index 4927e7cc7d96..353b711e635a 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1165,6 +1165,11 @@ void security_task_to_inode(struct task_struct *p, struct inode *inode)
 	call_void_hook(task_to_inode, p, inode);
 }
 
+int security_task_safe_sidechannel(struct task_struct *p)
+{
+	return call_int_hook(task_safe_sidechannel, 0, p);
+}
+
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag)
 {
 	return call_int_hook(ipc_permission, 0, ipcp, flag);
-- 
2.17.1

[PATCH RFC 2/5] X86: Support LSM determination of side-channel vulnerability

[Casey Schaufler]

From: Casey Schaufler <cschaufler@localhost.localdomain>

When switching between tasks it may be necessary
to set an indirect branch prediction barrier if the
tasks are potentially vulnerable to side-channel
attacks. This adds a call to security_task_safe_sidechannel
so that security modules can weigh in on the decision.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
 arch/x86/mm/tlb.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 6eb1f34c3c85..8714d4af06aa 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -7,6 +7,7 @@
 #include <linux/export.h>
 #include <linux/cpu.h>
 #include <linux/debugfs.h>
+#include <linux/security.h>
 
 #include <asm/tlbflush.h>
 #include <asm/mmu_context.h>
@@ -270,11 +271,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		 * threads. It will also not flush if we switch to idle
 		 * thread and back to the same process. It will flush if we
 		 * switch to a different non-dumpable process.
+		 * If a security module thinks that the transition
+		 * is unsafe do the flush.
 		 */
-		if (tsk && tsk->mm &&
-		    tsk->mm->context.ctx_id != last_ctx_id &&
-		    get_dumpable(tsk->mm) != SUID_DUMP_USER)
-			indirect_branch_prediction_barrier();
+		if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id) {
+			if (get_dumpable(tsk->mm) != SUID_DUMP_USER ||
+			    security_task_safe_sidechannel(tsk) != 0)
+				indirect_branch_prediction_barrier();
+		}
 
 		if (IS_ENABLED(CONFIG_VMAP_STACK)) {
 			/*
-- 
2.17.1

[PATCH RFC 3/5] LSM: Security module checking for side-channel dangers

[Casey Schaufler]

From: Casey Schaufler <cschaufler@localhost.localdomain>

The sidechannel LSM checks for cases where a side-channel
attack may be dangerous based on security attributes of tasks.
This includes:
	Effective UID of the tasks is different
	Capablity sets are different
	Tasks are in different namespaces
An option is also provided to assert that task are never
to be considered safe. This is high paranoia, and expensive
as well.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
 MAINTAINERS                        |   6 ++
 include/linux/lsm_hooks.h          |   5 +
 security/Kconfig                   |   1 +
 security/Makefile                  |   2 +
 security/security.c                |   1 +
 security/sidechannel/Kconfig       |  60 +++++++++++
 security/sidechannel/Makefile      |   1 +
 security/sidechannel/sidechannel.c | 156 +++++++++++++++++++++++++++++
 8 files changed, 232 insertions(+)
 create mode 100644 security/sidechannel/Kconfig
 create mode 100644 security/sidechannel/Makefile
 create mode 100644 security/sidechannel/sidechannel.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 3119bba7971c..d078d6a5b471 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13066,6 +13066,12 @@ F:	drivers/slimbus/
 F:	Documentation/devicetree/bindings/slimbus/
 F:	include/linux/slimbus.h
 
+SIDECHANNEL SECURITY MODULE
+M:	Casey Schaufler <casey.schaufler@intel.com>
+L:	linux-security-module@vger.kernel.org
+S:	Maintained
+F:	security/sidechannel/
+
 SMACK SECURITY MODULE
 M:	Casey Schaufler <casey@schaufler-ca.com>
 L:	linux-security-module@vger.kernel.org
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index fd2a7e6beb01..d48e4a085fe2 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -2088,5 +2088,10 @@ void __init loadpin_add_hooks(void);
 #else
 static inline void loadpin_add_hooks(void) { };
 #endif
+#ifdef CONFIG_SECURITY_SIDECHANNEL
+void __init sidechannel_add_hooks(void);
+#else
+static inline void sidechannel_add_hooks(void) { };
+#endif
 
 #endif /* ! __LINUX_LSM_HOOKS_H */
diff --git a/security/Kconfig b/security/Kconfig
index c4302067a3ad..28cb7b2939ee 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -237,6 +237,7 @@ source security/tomoyo/Kconfig
 source security/apparmor/Kconfig
 source security/loadpin/Kconfig
 source security/yama/Kconfig
+source security/sidechannel/Kconfig
 
 source security/integrity/Kconfig
 
diff --git a/security/Makefile b/security/Makefile
index 4d2d3782ddef..d0c9e1b227f9 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -10,6 +10,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO)        += tomoyo
 subdir-$(CONFIG_SECURITY_APPARMOR)	+= apparmor
 subdir-$(CONFIG_SECURITY_YAMA)		+= yama
 subdir-$(CONFIG_SECURITY_LOADPIN)	+= loadpin
+subdir-$(CONFIG_SECURITY_SIDECHANNEL)	+= sidechannel
 
 # always enable default capabilities
 obj-y					+= commoncap.o
@@ -25,6 +26,7 @@ obj-$(CONFIG_SECURITY_TOMOYO)		+= tomoyo/
 obj-$(CONFIG_SECURITY_APPARMOR)		+= apparmor/
 obj-$(CONFIG_SECURITY_YAMA)		+= yama/
 obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
+obj-$(CONFIG_SECURITY_SIDECHANNEL)	+= sidechannel/
 obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
 
 # Object integrity file lists
diff --git a/security/security.c b/security/security.c
index 353b711e635a..777919349751 100644
--- a/security/security.c
+++ b/security/security.c
@@ -80,6 +80,7 @@ int __init security_init(void)
 	capability_add_hooks();
 	yama_add_hooks();
 	loadpin_add_hooks();
+	sidechannel_add_hooks();
 
 	/*
 	 * Load all the remaining security modules.
diff --git a/security/sidechannel/Kconfig b/security/sidechannel/Kconfig
new file mode 100644
index 000000000000..af9396534128
--- /dev/null
+++ b/security/sidechannel/Kconfig
@@ -0,0 +1,60 @@
+config SECURITY_SIDECHANNEL
+	bool "Sidechannel attack safety extra checks"
+	depends on SECURITY
+	default n
+	help
+	  Look for a variety of cases where a side-channel attack
+	  could potentially be exploited. Instruct the switching
+	  code to use the indirect_branch_prediction_barrier in
+	  cases where the passed task and the current task may be
+	  at risk.
+
+          If you are unsure how to answer this question, answer N.
+
+config SECURITY_SIDECHANNEL_UIDS
+	bool "Sidechannel check on UID"
+	depends on SECURITY_SIDECHANNEL
+	default n
+	help
+	  Assume that tasks with different effective UIDs may be
+	  subject to side-channel attacks. As most task switching
+	  occurs between tasks with different effective UIDs this
+	  can have a significant performance impact.
+
+          If you are unsure how to answer this question, answer N.
+
+
+config SECURITY_SIDECHANNEL_CAPABILITIES
+	bool "Sidechannel check on capability sets"
+	depends on SECURITY_SIDECHANNEL
+	default n
+	help
+	  Assume that tasks with different sets of privilege may be
+	  subject to side-channel attacks. Potential interactions
+	  where the attacker lacks capabilities the attacked has
+	  are blocked.
+
+          If you are unsure how to answer this question, answer N.
+
+config SECURITY_SIDECHANNEL_NAMESPACES
+	bool "Sidechannel check on namespaces"
+	depends on SECURITY_SIDECHANNEL
+	depends on NAMESPACES
+	default n
+	help
+	  Assume that tasks in different namespaces may be
+	  subject to side-channel attacks. User, PID and cgroup
+	  namespaces are checked.
+
+          If you are unsure how to answer this question, answer N.
+
+config SECURITY_SIDECHANNEL_ALWAYS
+	bool "Sidechannel assumed to always be possible"
+	depends on SECURITY_SIDECHANNEL
+	default n
+	help
+	  Assume that all tasks may be subject to side-channel attacks.
+	  Always instruct the system to use countermeasures regardless
+	  of the potential impact.
+
+          If you are unsure how to answer this question, answer N.
diff --git a/security/sidechannel/Makefile b/security/sidechannel/Makefile
new file mode 100644
index 000000000000..f61d83f28035
--- /dev/null
+++ b/security/sidechannel/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_SECURITY_SIDECHANNEL) += sidechannel.o
diff --git a/security/sidechannel/sidechannel.c b/security/sidechannel/sidechannel.c
new file mode 100644
index 000000000000..9dc875611bd8
--- /dev/null
+++ b/security/sidechannel/sidechannel.c
@@ -0,0 +1,156 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Side Channel Safety Security Module
+ *
+ * Copyright (C) 2018 Intel Corporation.
+ *
+ */
+
+#define pr_fmt(fmt) "SideChannel: " fmt
+
+#include <linux/types.h>
+#include <linux/lsm_hooks.h>
+#include <linux/capability.h>
+#include <linux/cred.h>
+#include <linux/sched.h>
+#include <linux/string_helpers.h>
+#include <linux/nsproxy.h>
+#include <linux/pid_namespace.h>
+
+#ifdef CONFIG_SECURITY_SIDECHANNEL_ALWAYS
+static int sidechannel_task_safe_sidechannel(struct task_struct *p)
+{
+	return -EACCES;
+}
+#else
+/*
+ * safe_by_uid - Are task and current sidechannel safe?
+ * @p: task to check on
+ *
+ * Returns 0 if the tasks are sidechannel safe, -EACCES otherwise.
+ */
+#ifdef CONFIG_SECURITY_SIDECHANNEL_UIDS
+static int safe_by_uid(struct task_struct *p)
+{
+	const struct cred *ccred = current->cred;
+	const struct cred *pcred = p->cred;
+
+	/*
+	 * Credential checks. Considered safe if:
+	 *	UIDs are the same
+	 */
+	if (ccred != pcred && ccred->euid.val != pcred->euid.val)
+		return -EACCES;
+	return 0;
+}
+#else
+static inline int safe_by_uid(struct task_struct *p)
+{
+	return 0;
+}
+#endif
+
+/*
+ * safe_by_capability - Are task and current sidechannel safe?
+ * @p: task to check on
+ *
+ * Returns 0 if the tasks are sidechannel safe, -EACCES otherwise.
+ */
+#ifdef CONFIG_SECURITY_SIDECHANNEL_CAPABILITIES
+static int safe_by_capability(struct task_struct *p)
+{
+	const struct cred *ccred = current->cred;
+	const struct cred *pcred = p->cred;
+
+	/*
+	 * Capabilities checks. Considered safe if:
+	 *	current has all the capabilities p does
+	 */
+	if (ccred != pcred &&
+	    !cap_issubset(pcred->cap_effective, ccred->cap_effective))
+		return -EACCES;
+	return 0;
+}
+#else
+static inline int safe_by_capability(struct task_struct *p)
+{
+	return 0;
+}
+#endif
+
+#ifdef CONFIG_SECURITY_SIDECHANNEL_NAMESPACES
+/**
+ * safe_by_namespace - Are task and current sidechannel safe?
+ * @p: task to check on
+ *
+ * Returns 0 if the tasks are sidechannel safe, -EACCES otherwise.
+ */
+static int safe_by_namespace(struct task_struct *p)
+{
+	struct cgroup_namespace *ccgn = NULL;
+	struct cgroup_namespace *pcgn = NULL;
+
+	/*
+	 * Namespace checks. Considered safe if:
+	 *	cgroup namespace is the same
+	 *	User namespace is the same
+	 *	PID namespace is the same
+	 */
+	if (current->nsproxy)
+		ccgn = current->nsproxy->cgroup_ns;
+	if (p->nsproxy)
+		pcgn = p->nsproxy->cgroup_ns;
+	if (ccgn != pcgn)
+		return -EACCES;
+	if (current->cred->user_ns != p->cred->user_ns)
+		return -EACCES;
+	if (task_active_pid_ns(current) != task_active_pid_ns(p))
+		return -EACCES;
+	return 0;
+}
+#else
+static inline int safe_by_namespace(struct task_struct *p)
+{
+	return 0;
+}
+#endif
+
+/**
+ * sidechannel_task_safe_sidechannel - Are task and current sidechannel safe?
+ * @p: task to check on
+ *
+ * Returns 0 if the tasks are sidechannel safe, -EACCES otherwise.
+ */
+static int sidechannel_task_safe_sidechannel(struct task_struct *p)
+{
+	int rc;
+
+	/*
+	 * Easy optimizations
+	 */
+	if (p == current || p->pid == current->pid)
+		return 0;
+
+	rc = safe_by_uid(p);
+	if (rc)
+		return rc;
+	rc = safe_by_capability(p);
+	if (rc)
+		return rc;
+	rc = safe_by_namespace(p);
+	if (rc)
+		return rc;
+	return 0;
+}
+#endif /* CONFIG_SECURITY_SIDECHANNEL_ALWAYS */
+
+static struct security_hook_list sidechannel_hooks[] __lsm_ro_after_init = {
+	LSM_HOOK_INIT(task_safe_sidechannel, sidechannel_task_safe_sidechannel),
+};
+
+void __init sidechannel_add_hooks(void)
+{
+	pr_info("Extra sidechannel checks enabled\n");
+	security_add_hooks(sidechannel_hooks, ARRAY_SIZE(sidechannel_hooks),
+			   "sidechannel");
+}
-- 
2.17.1

[PATCH RFC 4/5] Smack: Support determination of side-channel vulnerability

[Casey Schaufler]

Smack considers its private task data safe if the current task
has read access to the passed task.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
 security/smack/smack_lsm.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 91750205a5de..85dc053e610c 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -2299,6 +2299,23 @@ static void smack_task_to_inode(struct task_struct *p, struct inode *inode)
 	isp->smk_inode = skp;
 }
 
+/**
+ * smack_task_safe_sidechannel - Are the task and current sidechannel safe?
+ * @p: task to check on
+ *
+ * A crude value for sidechannel safety is that the current task is
+ * already allowed to read from the other.
+ *
+ * Returns 0 if the tasks are sidechannel safe, -EACCES otherwise.
+ */
+static int smack_task_safe_sidechannel(struct task_struct *p)
+{
+	struct smack_known *skp = smk_of_task_struct(p);
+	struct smack_known *ckp = smk_of_task_struct(current);
+
+	return smk_access(ckp, skp, MAY_READ, NULL);
+}
+
 /*
  * Socket hooks.
  */
@@ -4718,6 +4735,7 @@ static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(task_movememory, smack_task_movememory),
 	LSM_HOOK_INIT(task_kill, smack_task_kill),
 	LSM_HOOK_INIT(task_to_inode, smack_task_to_inode),
+	LSM_HOOK_INIT(task_safe_sidechannel, smack_task_safe_sidechannel),
 
 	LSM_HOOK_INIT(ipc_permission, smack_ipc_permission),
 	LSM_HOOK_INIT(ipc_getsecid, smack_ipc_getsecid),
-- 
2.17.1

[PATCH RFC 5/5] SELinux: Support SELinux determination of side-channel vulnerability

[Casey Schaufler]

SELinux considers tasks to be side-channel safe if they
have PROCESS_SHARE access.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
 security/selinux/hooks.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index a8bf324130f5..7fbd7d7ac1cb 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4219,6 +4219,14 @@ static void selinux_task_to_inode(struct task_struct *p,
 	spin_unlock(&isec->lock);
 }
 
+static int selinux_task_safe_sidechannel(struct task_struct *p)
+{
+	struct av_decision avd;
+
+	return avc_has_perm_noaudit(&selinux_state, current_sid(), task_sid(p),
+				    SECCLASS_PROCESS, PROCESS__SHARE, 0, &avd);
+}
+
 /* Returns error only if unable to parse addresses */
 static int selinux_parse_skb_ipv4(struct sk_buff *skb,
 			struct common_audit_data *ad, u8 *proto)
@@ -7002,6 +7010,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
 	LSM_HOOK_INIT(task_kill, selinux_task_kill),
 	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
+	LSM_HOOK_INIT(task_safe_sidechannel, selinux_task_safe_sidechannel),
 
 	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
 	LSM_HOOK_INIT(ipc_getsecid, selinux_ipc_getsecid),
-- 
2.17.1

Re: [PATCH RFC 2/5] X86: Support LSM determination of side-channel vulnerability

[Jann Horn]

On Thu, Aug 16, 2018 at 11:51 AM Casey Schaufler
<casey.schaufler@intel.com> wrote:
>
> From: Casey Schaufler <cschaufler@localhost.localdomain>
>
> When switching between tasks it may be necessary
> to set an indirect branch prediction barrier if the
> tasks are potentially vulnerable to side-channel
> attacks. This adds a call to security_task_safe_sidechannel
> so that security modules can weigh in on the decision.
>
> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
> ---
>  arch/x86/mm/tlb.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 6eb1f34c3c85..8714d4af06aa 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -7,6 +7,7 @@
>  #include <linux/export.h>
>  #include <linux/cpu.h>
>  #include <linux/debugfs.h>
> +#include <linux/security.h>
>
>  #include <asm/tlbflush.h>
>  #include <asm/mmu_context.h>
> @@ -270,11 +271,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>                  * threads. It will also not flush if we switch to idle
>                  * thread and back to the same process. It will flush if we
>                  * switch to a different non-dumpable process.
> +                * If a security module thinks that the transition
> +                * is unsafe do the flush.
>                  */
> -               if (tsk && tsk->mm &&
> -                   tsk->mm->context.ctx_id != last_ctx_id &&
> -                   get_dumpable(tsk->mm) != SUID_DUMP_USER)
> -                       indirect_branch_prediction_barrier();
> +               if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id) {
> +                       if (get_dumpable(tsk->mm) != SUID_DUMP_USER ||
> +                           security_task_safe_sidechannel(tsk) != 0)
> +                               indirect_branch_prediction_barrier();
> +               }

Does this enforce transitivity? What happens if we first switch from
an attacker task to a task without ->mm, and immediately afterwards
from the task without ->mm to a victim task? In that case, whether a
flush happens between the attacker task and the victim task depends on
whether the LSM thinks that the mm-less task should have access to the
victim task, right?

Re: [PATCH RFC 5/5] SELinux: Support SELinux determination of side-channel vulnerability

[Stephen Smalley]

On 08/15/2018 07:53 PM, Casey Schaufler wrote:
> SELinux considers tasks to be side-channel safe if they
> have PROCESS_SHARE access.
> 
> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
> ---
>   security/selinux/hooks.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index a8bf324130f5..7fbd7d7ac1cb 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -4219,6 +4219,14 @@ static void selinux_task_to_inode(struct task_struct *p,
>   	spin_unlock(&isec->lock);
>   }
>   
> +static int selinux_task_safe_sidechannel(struct task_struct *p)
> +{
> +	struct av_decision avd;
> +
> +	return avc_has_perm_noaudit(&selinux_state, current_sid(), task_sid(p),
> +				    SECCLASS_PROCESS, PROCESS__SHARE, 0, &avd);
> +}

If you are going to apply this kind of check, is there a reason you 
wouldn't just use the ptrace checking logic?  Just call 
ptrace_may_access() with PTRACE_MODE_READ and dispense with having a 
separate hook altogether.  Then you get uids/gids, caps, dumpable, and 
security module checking for free.

Regardless, I don't think share permission is the right answer here; it 
has very different semantics and security implications, and is almost 
never allowed in Android policy (just one instance for kernel->init 
transition).

> +
>   /* Returns error only if unable to parse addresses */
>   static int selinux_parse_skb_ipv4(struct sk_buff *skb,
>   			struct common_audit_data *ad, u8 *proto)
> @@ -7002,6 +7010,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
>   	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
>   	LSM_HOOK_INIT(task_kill, selinux_task_kill),
>   	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
> +	LSM_HOOK_INIT(task_safe_sidechannel, selinux_task_safe_sidechannel),
>   
>   	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
>   	LSM_HOOK_INIT(ipc_getsecid, selinux_ipc_getsecid),
> 

Re: [PATCH RFC 3/5] LSM: Security module checking for side-channel dangers

[Jann Horn]

On Thu, Aug 16, 2018 at 11:51 AM Casey Schaufler
<casey.schaufler@intel.com> wrote:
>
> From: Casey Schaufler <cschaufler@localhost.localdomain>
>
> The sidechannel LSM checks for cases where a side-channel
> attack may be dangerous based on security attributes of tasks.
> This includes:
>         Effective UID of the tasks is different
>         Capablity sets are different
>         Tasks are in different namespaces
> An option is also provided to assert that task are never
> to be considered safe. This is high paranoia, and expensive
> as well.
>
> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
[...]
> +static int safe_by_uid(struct task_struct *p)
> +{
> +       const struct cred *ccred = current->cred;
> +       const struct cred *pcred = p->cred;

See below.

[...]
> +static int safe_by_capability(struct task_struct *p)
> +{
> +       const struct cred *ccred = current->cred;
> +       const struct cred *pcred = p->cred;

See below.

[...]
> +       if (current->cred->user_ns != p->cred->user_ns)
> +               return -EACCES;

Shouldn't this access be using one of the rcu_dereference_* helpers?
Something like rcu_dereference_protected(..., 1).

Also, you're looking at ->cred, and you don't want to do that. ->cred
are the *subjective* credentials of the task, meaning the credentials
that should only be used when this task actively performs an access.
Depending on what userspace is doing, an unprivileged process' ->cred
will randomly flake to something with GLOBAL_ROOT_UID because of
override_creds() calls - for example, if you access a file through
overlayfs in certain ways, your ->cred will temporarily be overwritten
by ovl_override_creds(), which mostly switches to the credentials of
the filesystem's creator (iow, normally root). coredumps can also
override the EUID with GLOBAL_ROOT_UID:

    if (__get_dumpable(cprm.mm_flags) == SUID_DUMP_ROOT) {
        /* Setuid core dump mode */
        cred->fsuid = GLOBAL_ROOT_UID;  /* Dump root private */
        need_suid_safe = true;
    }

    retval = coredump_wait(siginfo->si_signo, &core_state);
    if (retval < 0)
        goto fail_creds;

    old_cred = override_creds(cred);

Please use the objective credentials (->real_cred) instead, which are
not subject to random override_creds() effects, and are designed to be
used when looking at the privileges associated with an overall task
(as opposed to the privileges associated with the task's current
syscall context).

At least for the current-> access, you probably want
current_real_cred(), which has been defined for this kind of use.

Re: [PATCH RFC 5/5] SELinux: Support SELinux determination of side-channel vulnerability

[Jann Horn]

On Thu, Aug 16, 2018 at 11:52 AM Casey Schaufler
<casey.schaufler@intel.com> wrote:
>
> SELinux considers tasks to be side-channel safe if they
> have PROCESS_SHARE access.
>
> Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
> ---
>  security/selinux/hooks.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index a8bf324130f5..7fbd7d7ac1cb 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -4219,6 +4219,14 @@ static void selinux_task_to_inode(struct task_struct *p,
>         spin_unlock(&isec->lock);
>  }
>
> +static int selinux_task_safe_sidechannel(struct task_struct *p)
> +{
> +       struct av_decision avd;
> +
> +       return avc_has_perm_noaudit(&selinux_state, current_sid(), task_sid(p),
> +                                   SECCLASS_PROCESS, PROCESS__SHARE, 0, &avd);
> +}

current_sid() -> current_security() -> current_cred_xxx() ->
current_cred() accesses current->cred, the subjective credentials
associated with the current syscall context, affected by
override_creds(). You probably want to look at objective credentials
here, since what you're interested in is not the security context of
the current syscall, but the security context of the userspace code
running in the current address space.

task_sid() does the right thing and looks at the objective creds.