| From 28d437d550e1e39f805d99f9f8ac399c778827b7 Mon Sep 17 00:00:00 2001 |
| From: Tom Lendacky <thomas.lendacky@amd.com> |
| Date: Sat, 13 Jan 2018 17:27:30 -0600 |
| Subject: x86/retpoline: Add LFENCE to the retpoline/RSB filling RSB macros |
| |
| From: Tom Lendacky <thomas.lendacky@amd.com> |
| |
| commit 28d437d550e1e39f805d99f9f8ac399c778827b7 upstream. |
| |
| The PAUSE instruction is currently used in the retpoline and RSB filling |
| macros as a speculation trap. The use of PAUSE was originally suggested |
| because it showed a very, very small difference in the amount of |
| cycles/time used to execute the retpoline as compared to LFENCE. On AMD, |
| the PAUSE instruction is not a serializing instruction, so the pause/jmp |
| loop will use excess power as it is speculated over waiting for return |
| to mispredict to the correct target. |
| |
| The RSB filling macro is applicable to AMD, and, if software is unable to |
| verify that LFENCE is serializing on AMD (possible when running under a |
| hypervisor), the generic retpoline support will be used and, so, is also |
| applicable to AMD. Keep the current usage of PAUSE for Intel, but add an |
| LFENCE instruction to the speculation trap for AMD. |
| |
| The same sequence has been adopted by GCC for the GCC generated retpolines. |
| |
| Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> |
| Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
| Reviewed-by: Borislav Petkov <bp@alien8.de> |
| Acked-by: David Woodhouse <dwmw@amazon.co.uk> |
| Acked-by: Arjan van de Ven <arjan@linux.intel.com> |
| Cc: Rik van Riel <riel@redhat.com> |
| Cc: Andi Kleen <ak@linux.intel.com> |
| Cc: Paul Turner <pjt@google.com> |
| Cc: Peter Zijlstra <peterz@infradead.org> |
| Cc: Tim Chen <tim.c.chen@linux.intel.com> |
| Cc: Jiri Kosina <jikos@kernel.org> |
| Cc: Dave Hansen <dave.hansen@intel.com> |
| Cc: Andy Lutomirski <luto@kernel.org> |
| Cc: Josh Poimboeuf <jpoimboe@redhat.com> |
| Cc: Dan Williams <dan.j.williams@intel.com> |
| Cc: Linus Torvalds <torvalds@linux-foundation.org> |
| Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org> |
| Cc: Kees Cook <keescook@google.com> |
| Link: https://lkml.kernel.org/r/20180113232730.31060.36287.stgit@tlendack-t1.amdoffice.net |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| arch/x86/include/asm/nospec-branch.h | 6 +++++- |
| 1 file changed, 5 insertions(+), 1 deletion(-) |
| |
| --- a/arch/x86/include/asm/nospec-branch.h |
| +++ b/arch/x86/include/asm/nospec-branch.h |
| @@ -11,7 +11,7 @@ |
| * Fill the CPU return stack buffer. |
| * |
| * Each entry in the RSB, if used for a speculative 'ret', contains an |
| - * infinite 'pause; jmp' loop to capture speculative execution. |
| + * infinite 'pause; lfence; jmp' loop to capture speculative execution. |
| * |
| * This is required in various cases for retpoline and IBRS-based |
| * mitigations for the Spectre variant 2 vulnerability. Sometimes to |
| @@ -38,11 +38,13 @@ |
| call 772f; \ |
| 773: /* speculation trap */ \ |
| pause; \ |
| + lfence; \ |
| jmp 773b; \ |
| 772: \ |
| call 774f; \ |
| 775: /* speculation trap */ \ |
| pause; \ |
| + lfence; \ |
| jmp 775b; \ |
| 774: \ |
| dec reg; \ |
| @@ -73,6 +75,7 @@ |
| call .Ldo_rop_\@ |
| .Lspec_trap_\@: |
| pause |
| + lfence |
| jmp .Lspec_trap_\@ |
| .Ldo_rop_\@: |
| mov \reg, (%_ASM_SP) |
| @@ -165,6 +168,7 @@ |
| " .align 16\n" \ |
| "901: call 903f;\n" \ |
| "902: pause;\n" \ |
| + " lfence;\n" \ |
| " jmp 902b;\n" \ |
| " .align 16\n" \ |
| "903: addl $4, %%esp;\n" \ |