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

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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: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Michael Ellerman 2019-11-13 21:05:44 +11:00
parent 39e72bf96f
commit af2e8c68b9
3 changed files with 41 additions and 0 deletions
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2
arch/powerpc/include/asm/asm-prototypes.h
View File

@ -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);

9
arch/powerpc/kernel/security.c
View File

@ -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;

30
arch/powerpc/kvm/book3s_hv_rmhandlers.S
View File

@ -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