[PATCH][v2] powerpc: Set right value of Speculation_Store_Bypass in /proc/<pid>/status

The issue has showed the value of status of Speculation_Store_Bypass in the
/proc/<pid>/status as `unknown` for PowerPC systems.

The patch fix the checking of the mitigation status of Speculation, and
can be reported as "not vulnerable", "globally mitigated" or "vulnerable".

Link: https://github.com/linuxppc/issues/issues/255

Changelog:
Rebase on v5.4-rc8

Signed-off-by: Gustavo Walbon <redacted>
---
 arch/powerpc/kernel/security.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
index 7d4b2080a658..04e566026bbc 100644
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c

@@ -14,7 +14,7 @@
 #include <asm/debugfs.h>
 #include <asm/security_features.h>
 #include <asm/setup.h>
-
+#include <linux/prctl.h>
 
 u64 powerpc_security_features __read_mostly = SEC_FTR_DEFAULT;
 

@@ -344,6 +344,29 @@ ssize_t cpu_show_spec_store_bypass(struct device *dev, struct device_attribute *
 	return sprintf(buf, "Vulnerable\n");
 }
 
+static int ssb_prctl_get(struct task_struct *task)
+{
+	if (stf_barrier) {
+		if (stf_enabled_flush_types == STF_BARRIER_NONE)
+			return PR_SPEC_NOT_AFFECTED;
+		else
+			return PR_SPEC_DISABLE;
+	} else
+		return PR_SPEC_DISABLE_NOEXEC;
+
+	return -EINVAL;
+}
+
+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);
+	default:
+		return -ENODEV;
+	}
+}
+
 #ifdef CONFIG_DEBUG_FS
 static int stf_barrier_set(void *data, u64 val)
 {

-- 
2.19.1

Gustavo Walbon [off-list ref] writes:

The issue has showed the value of status of Speculation_Store_Bypass in the
/proc/<pid>/status as `unknown` for PowerPC systems.

The patch fix the checking of the mitigation status of Speculation, and
can be reported as "not vulnerable", "globally mitigated" or "vulnerable".

Link: https://github.com/linuxppc/issues/issues/255

Changelog:
Rebase on v5.4-rc8

Signed-off-by: Gustavo Walbon <redacted>
---
 arch/powerpc/kernel/security.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

On further thoughts I don't think this logic (which I suggested) is
right >:(

I commented on the issue:

  I think my original suggestion on this was wrong.
  
  Our mitigation is not global, ie. it's a barrier that must be used in
  the right location. We have kernel code to insert the barrier on
  kernel entry/exit, but that doesn't protect userspace against itself
  (ie. sandboxes).
  
  There's no way to express that with the current values as far as I can
  see.
  
  I think all we can do for now is:
  
  if stf_enabled_flush_types == STF_BARRIER_NONE:
    return PR_SPEC_NOT_AFFECTED // "not vulnerable"
  else
    return PR_SPEC_ENABLE // "vulnerable"
  
  To express the situation properly we'd need another value, something
  like PR_SPEC_MITIGATION_AVAILABLE (??) which says that there is a
  mitigation available but it must be used. That still has the problem
  that it doesn't tell userspace what the mitigation is, userspace would
  have to know.

cheers