[Patch v4 18/18] x86/speculation: Create PRCTL interface to restrict indirect branch speculation

From: Tim Chen
Date: Tue Oct 30 2018 - 15:22:58 EST

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Create PRCTL interface to restrict an application's indirect branch
speculation. This will protect the application against spectre v2 attack
from another application.

Invocations:
Check indirect branch speculation status with
- prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, 0, 0, 0);

Enable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_ENABLE, 0, 0);

Disable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_DISABLE, 0, 0);

Force disable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);

See Documentation/userspace-api/spec_ctrl.rst.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
Documentation/admin-guide/kernel-parameters.txt | 3 ++
Documentation/userspace-api/spec_ctrl.rst | 9 ++++
arch/x86/kernel/cpu/bugs.c | 72 ++++++++++++++++++++++++-
include/linux/sched.h | 9 ++++
include/uapi/linux/prctl.h | 1 +
tools/include/uapi/linux/prctl.h | 1 +
6 files changed, 93 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5e7028e..3c91805 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4193,6 +4193,9 @@

off - Unconditionally disable mitigations
lite - Protect processes which are marked non-dumpable
+ and processes which have requested restricted
+ indirect branch specuation via the
+ PR_SET_SPECULATION_CTRL prctl().
strict - Protect all processes
auto - Kernel selects the mode

diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
index 32f3d55..8a4e268 100644
--- a/Documentation/userspace-api/spec_ctrl.rst
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -92,3 +92,12 @@ Speculation misfeature controls
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
+
+- PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes
+ (Mitigate Spectre V2 style attacks against user processes)
+
+ Invocations:
+ * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, 0, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_ENABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_DISABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 1ba9cb5..3834338 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -154,7 +154,7 @@ static const char *spectre_v2_strings[] = {

static const char *spectre_v2_app2app_strings[] = {
[SPECTRE_V2_APP2APP_NONE] = "App-App Vulnerable",
- [SPECTRE_V2_APP2APP_LITE] = "App-App Mitigation: Protect non-dumpable process",
+ [SPECTRE_V2_APP2APP_LITE] = "App-App Mitigation: Protect non-dumpable and branch speculation restricted tasks",
[SPECTRE_V2_APP2APP_STRICT] = "App-App Mitigation: Full app to app attack protection",
};

@@ -781,7 +781,7 @@ static void set_task_stibp(struct task_struct *tsk, bool stibp_on)

if (stibp_on)
update = !test_and_set_tsk_thread_flag(tsk, TIF_STIBP);
- else
+ else if (!task_spec_indir_branch_disable(tsk))
update = test_and_clear_tsk_thread_flag(tsk, TIF_STIBP);

if (!update)
@@ -805,12 +805,57 @@ void arch_set_security(struct task_struct *tsk, unsigned int value)
set_task_stibp(tsk, false);
}

+static int indir_branch_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
+ switch (ctrl) {
+ case PR_SPEC_ENABLE:
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
+ return 0;
+ /*
+ * Indirect branch speculation is always disabled in
+ * strict mode.
+ */
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_STRICT)
+ return -EPERM;
+ task_clear_spec_indir_branch_disable(task);
+ set_task_stibp(task, false);
+ break;
+ case PR_SPEC_DISABLE:
+ /*
+ * Indirect branch speculation is always allowed when
+ * mitigation is force disabled.
+ */
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
+ return -EPERM;
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_STRICT)
+ return 0;
+ task_set_spec_indir_branch_disable(task);
+ set_task_stibp(task, true);
+ break;
+ case PR_SPEC_FORCE_DISABLE:
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
+ return -EPERM;
+ if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_STRICT)
+ return 0;
+ task_set_spec_indir_branch_disable(task);
+ task_set_spec_indir_branch_force_disable(task);
+ set_task_stibp(task, true);
+ break;
+ default:
+ return -ERANGE;
+ }
+
+ return 0;
+}
+
int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
unsigned long ctrl)
{
switch (which) {
case PR_SPEC_STORE_BYPASS:
return ssb_prctl_set(task, ctrl);
+ case PR_SPEC_INDIR_BRANCH:
+ return indir_branch_prctl_set(task, ctrl);
default:
return -ENODEV;
}
@@ -843,11 +888,34 @@ static int ssb_prctl_get(struct task_struct *task)
}
}

+static int indir_branch_prctl_get(struct task_struct *task)
+{
+ if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
+ return PR_SPEC_NOT_AFFECTED;
+
+ switch (spectre_v2_app2app_enabled) {
+ case SPECTRE_V2_APP2APP_NONE:
+ return PR_SPEC_ENABLE;
+ case SPECTRE_V2_APP2APP_LITE:
+ if (task_spec_indir_branch_force_disable(task))
+ return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
+ if (test_tsk_thread_flag(task, TIF_STIBP))
+ return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+ return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
+ case SPECTRE_V2_APP2APP_STRICT:
+ return PR_SPEC_DISABLE;
+ default:
+ return PR_SPEC_NOT_AFFECTED;
+ }
+}
+
int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
{
switch (which) {
case PR_SPEC_STORE_BYPASS:
return ssb_prctl_get(task);
+ case PR_SPEC_INDIR_BRANCH:
+ return indir_branch_prctl_get(task);
default:
return -ENODEV;
}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57..bec1442 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1439,6 +1439,8 @@ static inline bool is_percpu_thread(void)
#define PFA_SPREAD_SLAB 2 /* Spread some slab caches over cpuset */
#define PFA_SPEC_SSB_DISABLE 3 /* Speculative Store Bypass disabled */
#define PFA_SPEC_SSB_FORCE_DISABLE 4 /* Speculative Store Bypass force disabled*/
+#define PFA_SPEC_INDIR_BRANCH_DISABLE 5 /* Indirect branch speculation restricted in apps */
+#define PFA_SPEC_INDIR_BRANCH_FORCE_DISABLE 6 /* Indirect branch speculation restricted in apps forced */

#define TASK_PFA_TEST(name, func) \
static inline bool task_##func(struct task_struct *p) \
@@ -1470,6 +1472,13 @@ TASK_PFA_CLEAR(SPEC_SSB_DISABLE, spec_ssb_disable)
TASK_PFA_TEST(SPEC_SSB_FORCE_DISABLE, spec_ssb_force_disable)
TASK_PFA_SET(SPEC_SSB_FORCE_DISABLE, spec_ssb_force_disable)

+TASK_PFA_TEST(SPEC_INDIR_BRANCH_DISABLE, spec_indir_branch_disable)
+TASK_PFA_SET(SPEC_INDIR_BRANCH_DISABLE, spec_indir_branch_disable)
+TASK_PFA_CLEAR(SPEC_INDIR_BRANCH_DISABLE, spec_indir_branch_disable)
+
+TASK_PFA_TEST(SPEC_INDIR_BRANCH_FORCE_DISABLE, spec_indir_branch_force_disable)
+TASK_PFA_SET(SPEC_INDIR_BRANCH_FORCE_DISABLE, spec_indir_branch_force_disable)
+
static inline void
current_restore_flags(unsigned long orig_flags, unsigned long flags)
{
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index c0d7ea0..577f2ca 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -212,6 +212,7 @@ struct prctl_mm_map {
#define PR_SET_SPECULATION_CTRL 53
/* Speculation control variants */
# define PR_SPEC_STORE_BYPASS 0
+# define PR_SPEC_INDIR_BRANCH 1
/* Return and control values for PR_SET/GET_SPECULATION_CTRL */
# define PR_SPEC_NOT_AFFECTED 0
# define PR_SPEC_PRCTL (1UL << 0)
diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
index c0d7ea0..577f2ca 100644
--- a/tools/include/uapi/linux/prctl.h
+++ b/tools/include/uapi/linux/prctl.h
@@ -212,6 +212,7 @@ struct prctl_mm_map {
#define PR_SET_SPECULATION_CTRL 53
/* Speculation control variants */
# define PR_SPEC_STORE_BYPASS 0
+# define PR_SPEC_INDIR_BRANCH 1
/* Return and control values for PR_SET/GET_SPECULATION_CTRL */
# define PR_SPEC_NOT_AFFECTED 0
# define PR_SPEC_PRCTL (1UL << 0)
--
2.9.4

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