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Subject[PATCH 3/4] x86/retpoline: Simplify vmexit_fill_RSB()
Date
From: Borislav Petkov <bp@suse.de>

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

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: David Woodhouse <dwmw2@infradead.org>
---
arch/x86/include/asm/nospec-branch.h | 39 +++++++++++++++++++++++++++---------
arch/x86/include/asm/processor.h | 5 +++++
arch/x86/lib/Makefile | 1 +
arch/x86/lib/retpoline.S | 5 +++++
4 files changed, 41 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 33a35daf6c4d..61d4d7033758 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -53,6 +53,8 @@

#ifdef __ASSEMBLY__

+#include <asm/bitsperlong.h>
+
/*
* This should be used immediately before a retpoline alternative. It tells
* objtool where the retpolines are so that it can make sense of the control
@@ -121,6 +123,30 @@
#endif
.endm

+/* Same as above but with alignment additionally */
+.macro ___FILL_RETURN_BUFFER reg:req nr:req sp:req
+ mov (\nr / 2), \reg
+ .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 \reg
+ jnz 771b
+ add (BITS_PER_LONG/8) * \nr, \sp
+.endm
+
/*
* A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
* monstrosity above, manually.
@@ -206,15 +232,10 @@ 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_clobber_ax",
+ X86_FEATURE_RETPOLINE,
+ ASM_NO_INPUT_CLOBBER(_ASM_AX, "memory"));
#endif
}

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index d3a67fba200a..491f6e0be66e 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -971,4 +971,9 @@ bool xen_set_default_idle(void);

void stop_this_cpu(void *dummy);
void df_debug(struct pt_regs *regs, long error_code);
+
+#ifdef CONFIG_RETPOLINE
+asmlinkage void __fill_rsb_clobber_ax(void);
+#endif
+
#endif /* _ASM_X86_PROCESSOR_H */
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index f23934bbaf4e..69a473919260 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -27,6 +27,7 @@ lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem.o
lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o insn-eval.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

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index c909961e678a..297b0fd2ad10 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -46,3 +46,8 @@ GENERATE_THUNK(r13)
GENERATE_THUNK(r14)
GENERATE_THUNK(r15)
#endif
+
+ENTRY(__fill_rsb_clobber_ax)
+ ___FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, %_ASM_SP
+END(__fill_rsb_clobber_ax)
+EXPORT_SYMBOL_GPL(__fill_rsb_clobber_ax)
--
2.13.0
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