Commit af2e8c68 authored by Michael Ellerman's avatar Michael Ellerman

KVM: PPC: Book3S HV: Flush link stack on guest exit to host kernel

On some systems that are vulnerable to Spectre v2, it is up to
software to flush the link stack (return address stack), in order to
protect against Spectre-RSB.

When exiting from a guest we do some house keeping and then
potentially exit to C code which is several stack frames deep in the
host kernel. We will then execute a series of returns without
preceeding calls, opening up the possiblity that the guest could have
poisoned the link stack, and direct speculative execution of the host
to a gadget of some sort.

To prevent this we add a flush of the link stack on exit from a guest.
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent 39e72bf9
......@@ -153,9 +153,11 @@ void _kvmppc_save_tm_pr(struct kvm_vcpu *vcpu, u64 guest_msr);
extern s32 patch__call_flush_count_cache;
extern s32 patch__flush_count_cache_return;
extern s32 patch__flush_link_stack_return;
extern s32 patch__call_kvm_flush_link_stack;
extern s32 patch__memset_nocache, patch__memcpy_nocache;
extern long flush_count_cache;
extern long kvm_flush_link_stack;
#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
void kvmppc_save_tm_hv(struct kvm_vcpu *vcpu, u64 msr, bool preserve_nv);
......
......@@ -400,6 +400,9 @@ static void toggle_count_cache_flush(bool enable)
if (!enable) {
patch_instruction_site(&patch__call_flush_count_cache, PPC_INST_NOP);
#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
patch_instruction_site(&patch__call_kvm_flush_link_stack, PPC_INST_NOP);
#endif
pr_info("link-stack-flush: software flush disabled.\n");
link_stack_flush_enabled = false;
no_count_cache_flush();
......@@ -410,6 +413,12 @@ static void toggle_count_cache_flush(bool enable)
patch_branch_site(&patch__call_flush_count_cache,
(u64)&flush_count_cache, BRANCH_SET_LINK);
#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
// This enables the branch from guest_exit_cont to kvm_flush_link_stack
patch_branch_site(&patch__call_kvm_flush_link_stack,
(u64)&kvm_flush_link_stack, BRANCH_SET_LINK);
#endif
pr_info("link-stack-flush: software flush enabled.\n");
link_stack_flush_enabled = true;
......
......@@ -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
......
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