Merge tag 'powerpc-spectre-rsb' of powerpc-CVE-2019-18660.bundle

Pull powerpc Spectre-RSB fixes from Michael Ellerman:
 "We failed to activate the mitigation for Spectre-RSB (Return Stack
  Buffer, aka. ret2spec) on context switch, on CPUs prior to Power9
  DD2.3.

  That allows a process to poison the RSB (called Link Stack on Power
  CPUs) and possibly misdirect speculative execution of another process.
  If the victim process can be induced to execute a leak gadget then it
  may be possible to extract information from the victim via a side
  channel.

  The fix is to correctly activate the link stack flush mitigation on
  all CPUs that have any mitigation of Spectre v2 in userspace enabled.

  There's a second commit which adds a link stack flush in the KVM guest
  exit path. A leak via that path has not been demonstrated, but we
  believe it's at least theoretically possible.

  This is the fix for CVE-2019-18660"

* tag 'powerpc-spectre-rsb' of /home/torvalds/Downloads/powerpc-CVE-2019-18660.bundle:
  KVM: PPC: Book3S HV: Flush link stack on guest exit to host kernel
  powerpc/book3s64: Fix link stack flush on context switch

1 files changed, 30 insertions, 0 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index faebcbb8c4db..0496e66aaa56 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -11,6 +11,7 @@
  */
 
 #include <asm/ppc_asm.h>
+#include <asm/code-patching-asm.h>
 #include <asm/kvm_asm.h>
 #include <asm/reg.h>
 #include <asm/mmu.h>
@@ -1487,6 +1488,13 @@ guest_exit_cont:		/* r9 = vcpu, r12 = trap, r13 = paca */
 1:
 #endif /* CONFIG_KVM_XICS */
 
+	/*
+	 * Possibly flush the link stack here, before we do a blr in
+	 * guest_exit_short_path.
+	 */
+1:	nop
+	patch_site 1b patch__call_kvm_flush_link_stack
+
 	/* If we came in through the P9 short path, go back out to C now */
 	lwz	r0, STACK_SLOT_SHORT_PATH(r1)
 	cmpwi	r0, 0
@@ -1963,6 +1971,28 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 	mtlr	r0
 	blr
 
+.balign 32
+.global kvm_flush_link_stack
+kvm_flush_link_stack:
+	/* Save LR into r0 */
+	mflr	r0
+
+	/* Flush the link stack. On Power8 it's up to 32 entries in size. */
+	.rept 32
+	bl	.+4
+	.endr
+
+	/* And on Power9 it's up to 64. */
+BEGIN_FTR_SECTION
+	.rept 32
+	bl	.+4
+	.endr
+END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
+
+	/* Restore LR */
+	mtlr	r0
+	blr
+
 kvmppc_guest_external:
 	/* External interrupt, first check for host_ipi. If this is
 	 * set, we know the host wants us out so let's do it now