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Subject[Patch v4 18/18] x86/speculation: Create PRCTL interface to restrict indirect branch speculation
Date
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@linux.intel.com>
---
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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