Re: [PATCH v5 6/6] x86/arch_prctl Add ARCH_[GET|SET]_CPUID

On Thu, Sep 22, 2016 at 3:22 PM, Andy Lutomirski [off-list ref] wrote:

On Wed, Sep 21, 2016 at 11:58 AM, Kyle Huey [off-list ref] wrote:

quoted

Intel supports faulting on the CPUID instruction beginning with Ivy Bridge.
When enabled, the processor will fault on attempts to execute the CPUID
instruction with CPL>0. Exposing this feature to userspace will allow a
ptracer to trap and emulate the CPUID instruction.

When supported, this feature is controlled by toggling bit 0 of
MSR_MISC_FEATURES_ENABLES. It is documented in detail in Section 2.3.2 of
http://www.intel.com/content/dam/www/public/us/en/documents/application-notes/virtualization-technology-flexmigration-application-note.pdf

quoted

+       if (!test_and_set_thread_flag(TIF_NOCPUID))
+               /*
+                * Must flip the CPU state synchronously with
+                * TIF_NOCPUID in the current running context.
+                */
+               switch_cpuid_faulting(true);

If statements with multiple indented lines should have braces in my
book.  See, for example, "goto fail".


static?

quoted

+int set_cpuid_mode(struct task_struct *task, unsigned long val)

static?

quoted

+ * Called immediately after a successful exec.
+ */
+void arch_post_exec(void)
+{
+       /* If cpuid was previously disabled for this task, re-enable it. */
+       if (test_thread_flag(TIF_NOCPUID))
+               enable_cpuid();
+}

quoted

diff --git a/fs/exec.c b/fs/exec.c
index 6fcfb3f..0272b7e 100644
--- a/fs/exec.c
+++ b/fs/exec.c

@@ -1287,6 +1287,7 @@ void setup_new_exec(struct linux_binprm * bprm)
        else
                set_dumpable(current->mm, suid_dumpable);

+       arch_post_exec();

Let's call this arch_setup_new_exec().

quoted

        perf_event_exec();
        __set_task_comm(current, kbasename(bprm->filename), true);

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 2b5b10e..22a3f61 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h

@@ -130,6 +130,10 @@ static inline void check_object_size(const void *ptr, unsigned long n,
 { }
 #endif /* CONFIG_HARDENED_USERCOPY */

+#ifndef arch_post_exec
+static inline void arch_post_exec(void) {}
+#endif
+

thread_info.h is a weird place for this IMO.  Admittedly, elf.h isn't
a whole lot better.

Maybe fs.h?

- Kyle