4.9-stable review patch. If anyone has any objections, please let me know.
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From: Borislav Petkov bp@alien8.de
(cherry picked from commit 1dde7415e99933bb7293d6b2843752cbdb43ec11)
Simplify it to call an asm-function instead of pasting 41 insn bytes at every call site. Also, add alignment to the macro as suggested here:
https://support.google.com/faqs/answer/7625886
[dwmw2: Clean up comments, let it clobber %ebx and just tell the compiler]
Signed-off-by: Borislav Petkov bp@suse.de Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: ak@linux.intel.com Cc: dave.hansen@intel.com Cc: karahmed@amazon.de Cc: arjan@linux.intel.com Cc: torvalds@linux-foundation.org Cc: peterz@infradead.org Cc: bp@alien8.de Cc: pbonzini@redhat.com Cc: tim.c.chen@linux.intel.com Cc: gregkh@linux-foundation.org Link: https://lkml.kernel.org/r/1517070274-12128-3-git-send-email-dwmw@amazon.co.u... Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/entry/entry_32.S | 3 - arch/x86/entry/entry_64.S | 3 - arch/x86/include/asm/asm-prototypes.h | 3 + arch/x86/include/asm/nospec-branch.h | 70 +++------------------------------- arch/x86/lib/Makefile | 1 arch/x86/lib/retpoline.S | 56 +++++++++++++++++++++++++++ 6 files changed, 71 insertions(+), 65 deletions(-)
--- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -237,7 +237,8 @@ ENTRY(__switch_to_asm) * exist, overwrite the RSB with entries which capture * speculative execution to prevent attack. */ - FILL_RETURN_BUFFER %ebx, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW + /* Clobbers %ebx */ + FILL_RETURN_BUFFER RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW #endif
/* restore callee-saved registers */ --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -435,7 +435,8 @@ ENTRY(__switch_to_asm) * exist, overwrite the RSB with entries which capture * speculative execution to prevent attack. */ - FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW + /* Clobbers %rbx */ + FILL_RETURN_BUFFER RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW #endif
/* restore callee-saved registers */ --- a/arch/x86/include/asm/asm-prototypes.h +++ b/arch/x86/include/asm/asm-prototypes.h @@ -37,4 +37,7 @@ INDIRECT_THUNK(dx) INDIRECT_THUNK(si) INDIRECT_THUNK(di) INDIRECT_THUNK(bp) +asmlinkage void __fill_rsb(void); +asmlinkage void __clear_rsb(void); + #endif /* CONFIG_RETPOLINE */ --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -7,50 +7,6 @@ #include <asm/alternative-asm.h> #include <asm/cpufeatures.h>
-/* - * Fill the CPU return stack buffer. - * - * Each entry in the RSB, if used for a speculative 'ret', contains an - * 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 - * eliminate potentially bogus entries from the RSB, and sometimes - * purely to ensure that it doesn't get empty, which on some CPUs would - * allow predictions from other (unwanted!) sources to be used. - * - * We define a CPP macro such that it can be used from both .S files and - * inline assembly. It's possible to do a .macro and then include that - * from C via asm(".include <asm/nospec-branch.h>") but let's not go there. - */ - -#define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */ -#define RSB_FILL_LOOPS 16 /* To avoid underflow */ - -/* - * Google experimented with loop-unrolling and this turned out to be - * the optimal version — two calls, each with their own speculation - * trap should their return address end up getting used, in a loop. - */ -#define __FILL_RETURN_BUFFER(reg, nr, sp) \ - mov $(nr/2), reg; \ -771: \ - call 772f; \ -773: /* speculation trap */ \ - pause; \ - lfence; \ - jmp 773b; \ -772: \ - call 774f; \ -775: /* speculation trap */ \ - pause; \ - lfence; \ - jmp 775b; \ -774: \ - dec reg; \ - jnz 771b; \ - add $(BITS_PER_LONG/8) * nr, sp; - #ifdef __ASSEMBLY__
/* @@ -121,17 +77,10 @@ #endif .endm
- /* - * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP - * monstrosity above, manually. - */ -.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req +/* This clobbers the BX register */ +.macro FILL_RETURN_BUFFER nr:req ftr:req #ifdef CONFIG_RETPOLINE - ANNOTATE_NOSPEC_ALTERNATIVE - ALTERNATIVE "jmp .Lskip_rsb_@", \ - __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)) \ - \ftr -.Lskip_rsb_@: + ALTERNATIVE "", "call __clear_rsb", \ftr #endif .endm
@@ -206,15 +155,10 @@ extern char __indirect_thunk_end[]; static inline void vmexit_fill_RSB(void) { #ifdef CONFIG_RETPOLINE - unsigned long loops; - - asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE - ALTERNATIVE("jmp 910f", - __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)), - X86_FEATURE_RETPOLINE) - "910:" - : "=r" (loops), ASM_CALL_CONSTRAINT - : : "memory" ); + alternative_input("", + "call __fill_rsb", + X86_FEATURE_RETPOLINE, + ASM_NO_INPUT_CLOBBER(_ASM_BX, "memory")); #endif }
--- a/arch/x86/lib/Makefile +++ b/arch/x86/lib/Makefile @@ -26,6 +26,7 @@ lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o lib-$(CONFIG_RANDOMIZE_BASE) += kaslr.o lib-$(CONFIG_RETPOLINE) += retpoline.o +OBJECT_FILES_NON_STANDARD_retpoline.o :=y
obj-y += msr.o msr-reg.o msr-reg-export.o hweight.o
--- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -7,6 +7,7 @@ #include <asm/alternative-asm.h> #include <asm/export.h> #include <asm/nospec-branch.h> +#include <asm/bitsperlong.h>
.macro THUNK reg .section .text.__x86.indirect_thunk @@ -46,3 +47,58 @@ GENERATE_THUNK(r13) GENERATE_THUNK(r14) GENERATE_THUNK(r15) #endif + +/* + * Fill the CPU return stack buffer. + * + * Each entry in the RSB, if used for a speculative 'ret', contains an + * 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 + * eliminate potentially bogus entries from the RSB, and sometimes + * purely to ensure that it doesn't get empty, which on some CPUs would + * allow predictions from other (unwanted!) sources to be used. + * + * Google experimented with loop-unrolling and this turned out to be + * the optimal version - two calls, each with their own speculation + * trap should their return address end up getting used, in a loop. + */ +.macro STUFF_RSB nr:req sp:req + mov $(\nr / 2), %_ASM_BX + .align 16 +771: + call 772f +773: /* speculation trap */ + pause + lfence + jmp 773b + .align 16 +772: + call 774f +775: /* speculation trap */ + pause + lfence + jmp 775b + .align 16 +774: + dec %_ASM_BX + jnz 771b + add $((BITS_PER_LONG/8) * \nr), \sp +.endm + +#define RSB_FILL_LOOPS 16 /* To avoid underflow */ + +ENTRY(__fill_rsb) + STUFF_RSB RSB_FILL_LOOPS, %_ASM_SP + ret +END(__fill_rsb) +EXPORT_SYMBOL_GPL(__fill_rsb) + +#define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */ + +ENTRY(__clear_rsb) + STUFF_RSB RSB_CLEAR_LOOPS, %_ASM_SP + ret +END(__clear_rsb) +EXPORT_SYMBOL_GPL(__clear_rsb)