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>
[dja: straightforward backport to v4.14]
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/powerpc/include/asm/asm-prototypes.h b/arch/powerpc/include/asm/asm-prototypes.h
index cb3da44..c06cfdf 100644 (file)
--- a/arch/powerpc/include/asm/asm-prototypes.h
+++ b/arch/powerpc/include/asm/asm-prototypes.h
@@ -125,7 +125,9 @@ extern int __ucmpdi2(u64, u64);
 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 long flush_count_cache;
+extern long kvm_flush_link_stack;
 
 #endif /* _ASM_POWERPC_ASM_PROTOTYPES_H */

diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
index 07e066a..11fff96 100644 (file)
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c
@@ -393,6 +393,9 @@ 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();
@@ -403,6 +406,12 @@ 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 79a180c..4b60bec 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -18,6 +18,7 @@
  */
 
 #include <asm/ppc_asm.h>
+#include <asm/code-patching-asm.h>
 #include <asm/kvm_asm.h>
 #include <asm/reg.h>
 #include <asm/mmu.h>
@@ -1266,6 +1267,10 @@ mc_cont:
        bl      kvmhv_accumulate_time
 #endif
 
+       /* Possibly flush the link stack here. */
+1:     nop
+       patch_site 1b patch__call_kvm_flush_link_stack
+
        stw     r12, STACK_SLOT_TRAP(r1)
        mr      r3, r12
        /* Increment exit count, poke other threads to exit */
@@ -1685,6 +1690,28 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
        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
+
 /*
  * Check whether an HDSI is an HPTE not found fault or something else.
  * If it is an HPTE not found fault that is due to the guest accessing