lguest: fix crash 'unhandled trap 13 at <native_read_msr_safe>'

Impact: fix lguest boot crash on modern Intel machines

The code in early_init_intel does:

	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
		u64 misc_enable;

		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);

And that rdmsr faults (not allowed from non-0 PL).  We can get around
this by mugging the family ID part of the cpuid.  5 seems like a good
number.

Of course, this is a hack (how very lguest!).  We could just indicate
that we don't support MSRs, or implement lguest_rdmst.

Reported-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Tested-by: Patrick McHardy <kaber@trash.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 92f1c6f3e19d..ba5c05e97f10 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -343,6 +343,11 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
                  * flush_tlb_user() for both user and kernel mappings unless
                  * the Page Global Enable (PGE) feature bit is set. */
                 *dx |= 0x00002000;
+                /* We also lie, and say we're family id 5.  6 or greater
+                 * leads to a rdmsr in early_init_intel which we can't handle.
+                 * Family ID is returned as bits 8-12 in ax. */
+                *ax &= 0xFFFFF0FF;
+                *ax |= 0x00000500;
                 break;
         case 0x80000000:
                 /* Futureproof this a little: if they ask how much extended