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Subject[tip: x86/pti] prctl: Hook L1D flushing in via prctl
The following commit has been merged into the x86/pti branch of tip:

Commit-ID: b6724f118d44606fddde391ba7527526b3cad211
Gitweb: https://git.kernel.org/tip/b6724f118d44606fddde391ba7527526b3cad211
Author: Balbir Singh <sblbir@amazon.com>
AuthorDate: Wed, 29 Jul 2020 10:11:02 +10:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 16 Sep 2020 15:08:03 +02:00

prctl: Hook L1D flushing in via prctl

Use the existing PR_GET/SET_SPECULATION_CTRL API to expose the L1D
flush capability. For L1D flushing PR_SPEC_FORCE_DISABLE and
PR_SPEC_DISABLE_NOEXEC are not supported.

There is also no seccomp integration for the feature.

Signed-off-by: Balbir Singh <sblbir@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200729001103.6450-5-sblbir@amazon.com

---
arch/x86/kernel/cpu/bugs.c | 54 +++++++++++++++++++++++++++++++++++++-
arch/x86/mm/tlb.c | 25 ++++++++++++++++-
include/uapi/linux/prctl.h | 1 +-
3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d3f0db4..3923e48 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -296,6 +296,13 @@ enum taa_mitigations {
TAA_MITIGATION_TSX_DISABLED,
};

+enum l1d_flush_out_mitigations {
+ L1D_FLUSH_OUT_OFF,
+ L1D_FLUSH_OUT_ON,
+};
+
+static enum l1d_flush_out_mitigations l1d_flush_out_mitigation __ro_after_init = L1D_FLUSH_OUT_ON;
+
/* Default mitigation for TAA-affected CPUs */
static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
static bool taa_nosmt __ro_after_init;
@@ -379,6 +386,18 @@ out:
pr_info("%s\n", taa_strings[taa_mitigation]);
}

+static int __init l1d_flush_out_parse_cmdline(char *str)
+{
+ if (!boot_cpu_has_bug(X86_BUG_L1TF))
+ return 0;
+
+ if (!strcmp(str, "off"))
+ l1d_flush_out_mitigation = L1D_FLUSH_OUT_OFF;
+
+ return 0;
+}
+early_param("l1d_flush_out", l1d_flush_out_parse_cmdline);
+
static int __init tsx_async_abort_parse_cmdline(char *str)
{
if (!boot_cpu_has_bug(X86_BUG_TAA))
@@ -1215,6 +1234,23 @@ static void task_update_spec_tif(struct task_struct *tsk)
speculation_ctrl_update_current();
}

+static int l1d_flush_out_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
+
+ if (l1d_flush_out_mitigation == L1D_FLUSH_OUT_OFF)
+ return -EPERM;
+
+ switch (ctrl) {
+ case PR_SPEC_ENABLE:
+ return enable_l1d_flush_for_task(task);
+ case PR_SPEC_DISABLE:
+ return disable_l1d_flush_for_task(task);
+ default:
+ return -ERANGE;
+ }
+ return 0;
+}
+
static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
{
if (ssb_mode != SPEC_STORE_BYPASS_PRCTL &&
@@ -1306,6 +1342,8 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
return ssb_prctl_set(task, ctrl);
case PR_SPEC_INDIRECT_BRANCH:
return ib_prctl_set(task, ctrl);
+ case PR_SPEC_L1D_FLUSH_OUT:
+ return l1d_flush_out_prctl_set(task, ctrl);
default:
return -ENODEV;
}
@@ -1322,6 +1360,20 @@ void arch_seccomp_spec_mitigate(struct task_struct *task)
}
#endif

+static int l1d_flush_out_prctl_get(struct task_struct *task)
+{
+ int ret;
+
+ if (l1d_flush_out_mitigation == L1D_FLUSH_OUT_OFF)
+ return PR_SPEC_FORCE_DISABLE;
+
+ ret = test_ti_thread_flag(&task->thread_info, TIF_SPEC_L1D_FLUSH);
+ if (ret)
+ return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
+ else
+ return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+}
+
static int ssb_prctl_get(struct task_struct *task)
{
switch (ssb_mode) {
@@ -1375,6 +1427,8 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
return ssb_prctl_get(task);
case PR_SPEC_INDIRECT_BRANCH:
return ib_prctl_get(task);
+ case PR_SPEC_L1D_FLUSH_OUT:
+ return l1d_flush_out_prctl_get(task);
default:
return -ENODEV;
}
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 6369a54..6b0f4c8 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -316,8 +316,31 @@ EXPORT_SYMBOL_GPL(leave_mm);

int enable_l1d_flush_for_task(struct task_struct *tsk)
{
+ int cpu, ret = 0, i;
+
+ /*
+ * Do not enable L1D_FLUSH_OUT if
+ * b. The CPU is not affected by the L1TF bug
+ * c. The CPU does not have L1D FLUSH feature support
+ * c. The task's affinity is on cores with SMT on.
+ */
+
+ if (!boot_cpu_has_bug(X86_BUG_L1TF) ||
+ !static_cpu_has(X86_FEATURE_FLUSH_L1D))
+ return -EINVAL;
+
+ cpu = get_cpu();
+
+ for_each_cpu(i, &tsk->cpus_mask) {
+ if (cpu_data(i).smt_active == true) {
+ put_cpu();
+ return -EINVAL;
+ }
+ }
+
set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
- return 0;
+ put_cpu();
+ return ret;
}

int disable_l1d_flush_for_task(struct task_struct *tsk)
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f81..1e86486 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -213,6 +213,7 @@ struct prctl_mm_map {
/* Speculation control variants */
# define PR_SPEC_STORE_BYPASS 0
# define PR_SPEC_INDIRECT_BRANCH 1
+# define PR_SPEC_L1D_FLUSH_OUT 2
/* Return and control values for PR_SET/GET_SPECULATION_CTRL */
# define PR_SPEC_NOT_AFFECTED 0
# define PR_SPEC_PRCTL (1UL << 0)
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