Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the CPUID instruction.

On Sep 12, 2016 10:57 AM, "Jann Horn" [off-list ref] wrote:

On Mon, Sep 12, 2016 at 09:56:11AM -0700, Andy Lutomirski wrote:

quoted

On Sep 11, 2016 5:29 PM, "Kyle Huey" [off-list ref] wrote:

quoted

rr (http://rr-project.org/), a userspace record-and-replay reverse-
execution debugger, would like to trap and emulate the CPUID instruction.
This would allow us to a) mask away certain hardware features that rr does
not support (e.g. RDRAND) and b) enable trace portability across machines
by providing constant results.

Intel supports faulting on the CPUID instruction in newer processors. Bit
31 of MSR_PLATFORM_INFO advertises support for this feature. 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 this bit is preserved on fork(), then no_new_privs must be checked
(or it must be cleared on "unsafe" exec, but that's nasty).

I think you mean "preserved on execve()"?

Indeed.

So it should have defined and tested behavior on fork() and execve().
Maybe fork() should preserve the flag after all.

--Andy