Xen devel - Jan 2013 - SMAP feature causing kernel alignment faults due to setting EFLAGS.AC

This appears to be the case in the recent messages in Debian bug #660425
[0] (thread starts at [1] and continues at [2]).

SMAP uses EFLAGS.AC in kernel mode to indicate whether accesses to
usermode pages should fault or not, which gets confused because under
64-bit Xen kernel == ring3. The obvious fix would be to clear SMAP from
the cpuid features (probably in kernel as well as hypervosr) but might
we want to do something more advanced? Not sure what, actually
implementing PV SMAP would likely require a third set of pagetables with
only the kernel mappings...

Ian.

[0] http://bugs.debian.org/660425
[1] http://lists.debian.org/debian-kernel/2012/12/msg00520.html 
[2] http://lists.debian.org/debian-kernel/2013/01/msg00464.html

>>> On 28.01.13 at 11:51, Ian Campbell <Ian.Campbell@citrix.com>
wrote:
> This appears to be the case in the recent messages in Debian bug #660425
> [0] (thread starts at [1] and continues at [2]).
> 
> SMAP uses EFLAGS.AC in kernel mode to indicate whether accesses to
> usermode pages should fault or not, which gets confused because under
> 64-bit Xen kernel == ring3. The obvious fix would be to clear SMAP from
> the cpuid features (probably in kernel as well as hypervosr) but might
> we want to do something more advanced? Not sure what, actually
> implementing PV SMAP would likely require a third set of pagetables with
> only the kernel mappings...
Clearing the feature bit in the (64-bit) kernel is certainly
appropriate; whether doing so in the hypervisor is too I''m not
sure - that would particularly depend on whether we want to
implement a PV variant of it (which I''d expect to suck
performance-wise, since the instructions [CLAC/STAC] would
need emulation too, yet they necessarily sit in fast paths).

For 32-bit PV guests the situation is slightly better in that at
least we wouldn''t need another set of page tables. And maybe
(if we think the feature is worthwhile) we could even talk Intel
into allowing CLAC/STAC in rings 1 and 2.

Jan