[PATCH 3.14 72/73] x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only

Greg Kroah-Hartman
2014-12-05T23:00:10Z

From: Paolo Bonzini <pbonzini@redhat.com>

commit c1118b3602c2329671ad5ec8bdf8e374323d6343 upstream.

On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA.
In that case, KVM will fail to patch VMCALL instructions to VMMCALL
as required on AMD processors.

The failure mode is currently a divide-by-zero exception, which obviously
is a KVM bug that has to be fixed. However, picking the right instruction
between VMCALL and VMMCALL will be faster and will help if you cannot upgrade
the hypervisor.

Reported-by: Chris Webb <chris@arachsys.com>
Tested-by: Chris Webb <chris@arachsys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/include/asm/kvm_para.h | 10 ++++++++--
arch/x86/kernel/cpu/amd.c | 7 +++++++
3 files changed, 16 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -203,6 +203,7 @@
#define X86_FEATURE_DECODEASSISTS (8*32+12) /* AMD Decode Assists support */
#define X86_FEATURE_PAUSEFILTER (8*32+13) /* AMD filtered pause intercept */
#define X86_FEATURE_PFTHRESHOLD (8*32+14) /* AMD pause filter threshold */
+#define X86_FEATURE_VMMCALL (8*32+15) /* Prefer vmmcall to vmcall */

/* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -2,6 +2,7 @@
#define _ASM_X86_KVM_PARA_H

#include <asm/processor.h>
+#include <asm/alternative.h>
#include <uapi/asm/kvm_para.h>

extern void kvmclock_init(void);
@@ -16,10 +17,15 @@ static inline bool kvm_check_and_clear_g
}
#endif /* CONFIG_KVM_GUEST */

-/* This instruction is vmcall. On non-VT architectures, it will generate a
- * trap that we will then rewrite to the appropriate instruction.
+#ifdef CONFIG_DEBUG_RODATA
+#define KVM_HYPERCALL \
+ ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL)
+#else
+/* On AMD processors, vmcall will generate a trap that we will
+ * then rewrite to the appropriate instruction.
*/
#define KVM_HYPERCALL ".byte 0x0f,0x01,0xc1"
+#endif

/* For KVM hypercalls, a three-byte sequence of either the vmcall or the vmmcall
* instruction. The hypervisor may replace it with something else but only the
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -508,6 +508,13 @@ static void early_init_amd(struct cpuinf
}
#endif

+ /*
+ * This is only needed to tell the kernel whether to use VMCALL
+ * and VMMCALL. VMMCALL is never executed except under virt, so
+ * we can set it unconditionally.
+ */
+ set_cpu_cap(c, X86_FEATURE_VMMCALL);
+
/* F16h erratum 793, CVE-2013-6885 */
if (c->x86 == 0x16 && c->x86_model <= 0xf) {
u64 val;