From: Michael Ellerman mpe@ellerman.id.au
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: backport to v4.4, drop P9 support] Signed-off-by: Daniel Axtens dja@axtens.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/powerpc/include/asm/asm-prototypes.h | 2 ++ arch/powerpc/kernel/security.c | 9 +++++++++ arch/powerpc/kvm/book3s_hv_rmhandlers.S | 20 ++++++++++++++++++++ 3 files changed, 31 insertions(+)
--- a/arch/powerpc/include/asm/asm-prototypes.h +++ b/arch/powerpc/include/asm/asm-prototypes.h @@ -16,7 +16,9 @@ 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 */ --- a/arch/powerpc/kernel/security.c +++ b/arch/powerpc/kernel/security.c @@ -391,6 +391,9 @@ static void toggle_count_cache_flush(boo
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(); @@ -401,6 +404,12 @@ static void toggle_count_cache_flush(boo 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;
--- 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> @@ -1169,6 +1170,10 @@ mc_cont: bl kvmhv_accumulate_time #endif
+ /* Possibly flush the link stack here. */ +1: nop + patch_site 1b patch__call_kvm_flush_link_stack + mr r3, r12 /* Increment exit count, poke other threads to exit */ bl kvmhv_commence_exit @@ -1564,6 +1569,21 @@ 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 + + /* 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