From bc3c133841435829ba5c0a48427e2a77633502ab Mon Sep 17 00:00:00 2001 From: Andrew Cooper Date: Mon, 25 Apr 2022 17:25:53 +0100 Subject: [PATCH] x86/nospec: Fix evaluate_nospec() code generation under Clang MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit It turns out that evaluate_nospec() code generation is not safe under Clang. Given: void eval_nospec_test(int x) { if ( evaluate_nospec(x) ) asm volatile ("nop #true" ::: "memory"); else asm volatile ("nop #false" ::: "memory"); } Clang emits: : 0f ae e8 lfence 85 ff test %edi,%edi 74 02 je 90 nop c3 ret 90 nop c3 ret which is not safe because the lfence has been hoisted above the conditional jump. Clang concludes that both barrier_nospec_true()'s have identical side effects and can safely be merged. Clang can be persuaded that the side effects are different if there are different comments in the asm blocks. This is fragile, but no more fragile that other aspects of this construct. Introduce barrier_nospec_false() with a separate internal comment to prevent Clang merging it with barrier_nospec_true() despite the otherwise-identical content. The generated code now becomes: : 85 ff test %edi,%edi 74 05 je 0f ae e8 lfence 90 nop c3 ret 0f ae e8 lfence 90 nop c3 ret which has the correct number of lfence's, and in the correct place. Link: https://github.com/llvm/llvm-project/issues/55084 Signed-off-by: Andrew Cooper Reviewed-by: Roger Pau Monné Reviewed-by: Jan Beulich --- xen/arch/x86/include/asm/nospec.h | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/xen/arch/x86/include/asm/nospec.h b/xen/arch/x86/include/asm/nospec.h index 5312ae4c6f..7150e76b87 100644 --- a/xen/arch/x86/include/asm/nospec.h +++ b/xen/arch/x86/include/asm/nospec.h @@ -10,15 +10,26 @@ static always_inline bool barrier_nospec_true(void) { #ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH - alternative("lfence", "", X86_FEATURE_SC_NO_BRANCH_HARDEN); + alternative("lfence #nospec-true", "", X86_FEATURE_SC_NO_BRANCH_HARDEN); #endif return true; } +static always_inline bool barrier_nospec_false(void) +{ +#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH + alternative("lfence #nospec-false", "", X86_FEATURE_SC_NO_BRANCH_HARDEN); +#endif + return false; +} + /* Allow to protect evaluation of conditionals with respect to speculation */ static always_inline bool evaluate_nospec(bool condition) { - return condition ? barrier_nospec_true() : !barrier_nospec_true(); + if ( condition ) + return barrier_nospec_true(); + else + return barrier_nospec_false(); } /* Allow to block speculative execution in generic code */ -- 2.39.5