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authorMichael Ellerman <mpe@ellerman.id.au>2019-11-13 21:05:44 +1100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2019-11-29 10:08:31 +0100
commit0815f75f90178bc7e1933cf0d0c818b5f3f5a20c (patch)
tree8cf58f0749610a1a923535a68eb7db600e0a25db
parentd1aa3c8004692e4059e1faa30e438bec8af9b9c6 (diff)
KVM: PPC: Book3S HV: Flush link stack on guest exit to host kernel
commit af2e8c68b9c5403f77096969c516f742f5bb29e0 upstream. 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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/powerpc/include/asm/asm-prototypes.h2
-rw-r--r--arch/powerpc/kernel/security.c9
-rw-r--r--arch/powerpc/kvm/book3s_hv_rmhandlers.S30
3 files changed, 41 insertions, 0 deletions
diff --git a/arch/powerpc/include/asm/asm-prototypes.h b/arch/powerpc/include/asm/asm-prototypes.h
index aa58a7eb7eb2..baaafc9b9d88 100644
--- a/arch/powerpc/include/asm/asm-prototypes.h
+++ b/arch/powerpc/include/asm/asm-prototypes.h
@@ -141,9 +141,11 @@ 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);
diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
index a3138e7d71bb..bd91dceb7010 100644
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c
@@ -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;
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index 07181d0dfcb7..0ba1d7abb798 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>
@@ -1458,6 +1459,13 @@ 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
@@ -1933,6 +1941,28 @@ 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