Commit 11925185 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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x86/retpoline: Simplify retpolines 



Due to:

  c9c324dc ("objtool: Support stack layout changes in alternatives")

it is now possible to simplify the retpolines.

Currently our retpolines consist of 2 symbols:

 - __x86_indirect_thunk_\reg: the compiler target
 - __x86_retpoline_\reg:  the actual retpoline.

Both are consecutive in code and aligned such that for any one register
they both live in the same cacheline:

  0000000000000000 <__x86_indirect_thunk_rax>:
   0:   ff e0                   jmpq   *%rax
   2:   90                      nop
   3:   90                      nop
   4:   90                      nop

  0000000000000005 <__x86_retpoline_rax>:
   5:   e8 07 00 00 00          callq  11 <__x86_retpoline_rax+0xc>
   a:   f3 90                   pause
   c:   0f ae e8                lfence
   f:   eb f9                   jmp    a <__x86_retpoline_rax+0x5>
  11:   48 89 04 24             mov    %rax,(%rsp)
  15:   c3                      retq
  16:   66 2e 0f 1f 84 00 00 00 00 00   nopw   %cs:0x0(%rax,%rax,1)

The thunk is an alternative_2, where one option is a JMP to the
retpoline. This was done so that objtool didn't need to deal with
alternatives with stack ops. But that problem has been solved, so now
it is possible to fold the entire retpoline into the alternative to
simplify and consolidate unused bytes:

  0000000000000000 <__x86_indirect_thunk_rax>:
   0:   ff e0                   jmpq   *%rax
   2:   90                      nop
   3:   90                      nop
   4:   90                      nop
   5:   90                      nop
   6:   90                      nop
   7:   90                      nop
   8:   90                      nop
   9:   90                      nop
   a:   90                      nop
   b:   90                      nop
   c:   90                      nop
   d:   90                      nop
   e:   90                      nop
   f:   90                      nop
  10:   90                      nop
  11:   66 66 2e 0f 1f 84 00 00 00 00 00        data16 nopw %cs:0x0(%rax,%rax,1)
  1c:   0f 1f 40 00             nopl   0x0(%rax)

Notice that since the longest alternative sequence is now:

   0:   e8 07 00 00 00          callq  c <.altinstr_replacement+0xc>
   5:   f3 90                   pause
   7:   0f ae e8                lfence
   a:   eb f9                   jmp    5 <.altinstr_replacement+0x5>
   c:   48 89 04 24             mov    %rax,(%rsp)
  10:   c3                      retq

17 bytes, we have 15 bytes NOP at the end of our 32 byte slot. (IOW, if
we can shrink the retpoline by 1 byte we can pack it more densely).

 [ bp: Massage commit message. ]

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210326151259.506071949@infradead.org
parent 23c1ad53

arch/x86/include/asm/asm-prototypes.h

+0 −7
Original line number Diff line number Diff line
@@ -22,15 +22,8 @@ extern void cmpxchg8b_emu(void); 
#define DECL_INDIRECT_THUNK(reg) \

	extern asmlinkage void __x86_indirect_thunk_ ## reg (void);



#define DECL_RETPOLINE(reg) \

	extern asmlinkage void __x86_retpoline_ ## reg (void);



#undef GEN

#define GEN(reg) DECL_INDIRECT_THUNK(reg)

#include <asm/GEN-for-each-reg.h>



#undef GEN

#define GEN(reg) DECL_RETPOLINE(reg)

#include <asm/GEN-for-each-reg.h>



#endif /* CONFIG_RETPOLINE */

arch/x86/include/asm/nospec-branch.h

+3 −3
Original line number Diff line number Diff line
@@ -80,7 +80,7 @@ 
.macro JMP_NOSPEC reg:req

#ifdef CONFIG_RETPOLINE

	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \

		      __stringify(jmp __x86_retpoline_\reg), X86_FEATURE_RETPOLINE, \

		      __stringify(jmp __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \

		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_AMD

#else

	jmp	*%\reg
@@ -90,7 +90,7 @@ 
.macro CALL_NOSPEC reg:req

#ifdef CONFIG_RETPOLINE

	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *%\reg), \

		      __stringify(call __x86_retpoline_\reg), X86_FEATURE_RETPOLINE, \

		      __stringify(call __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \

		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *%\reg), X86_FEATURE_RETPOLINE_AMD

#else

	call	*%\reg
@@ -128,7 +128,7 @@ 
	ALTERNATIVE_2(						\

	ANNOTATE_RETPOLINE_SAFE					\

	"call *%[thunk_target]\n",				\

	"call __x86_retpoline_%V[thunk_target]\n",		\

	"call __x86_indirect_thunk_%V[thunk_target]\n",		\

	X86_FEATURE_RETPOLINE,					\

	"lfence;\n"						\

	ANNOTATE_RETPOLINE_SAFE					\

arch/x86/lib/retpoline.S

+17 −17
Original line number Diff line number Diff line
@@ -10,15 +10,7 @@ 
#include <asm/unwind_hints.h>

#include <asm/frame.h>



.macro THUNK reg

	.section .text.__x86.indirect_thunk



	.align 32

SYM_FUNC_START(__x86_indirect_thunk_\reg)

	JMP_NOSPEC \reg

SYM_FUNC_END(__x86_indirect_thunk_\reg)



SYM_FUNC_START_NOALIGN(__x86_retpoline_\reg)

.macro RETPOLINE reg

	ANNOTATE_INTRA_FUNCTION_CALL

	call    .Ldo_rop_\@

.Lspec_trap_\@:
@@ -30,7 +22,19 @@ SYM_FUNC_START_NOALIGN(__x86_retpoline_\reg) 
	mov     %\reg, (%_ASM_SP)

	UNWIND_HINT_FUNC

	ret

SYM_FUNC_END(__x86_retpoline_\reg)

.endm



.macro THUNK reg

	.section .text.__x86.indirect_thunk



	.align 32

SYM_FUNC_START(__x86_indirect_thunk_\reg)



	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \

		      __stringify(RETPOLINE \reg), X86_FEATURE_RETPOLINE, \

		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_AMD



SYM_FUNC_END(__x86_indirect_thunk_\reg)



.endm
@@ -48,7 +52,6 @@ SYM_FUNC_END(__x86_retpoline_\reg) 


#define __EXPORT_THUNK(sym)	_ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)

#define EXPORT_THUNK(reg)	__EXPORT_THUNK(__x86_indirect_thunk_ ## reg)

#define EXPORT_RETPOLINE(reg)  __EXPORT_THUNK(__x86_retpoline_ ## reg)



#undef GEN

#define GEN(reg) THUNK reg
@@ -58,6 +61,3 @@ SYM_FUNC_END(__x86_retpoline_\reg) 
#define GEN(reg) EXPORT_THUNK(reg)

#include <asm/GEN-for-each-reg.h>

#undef GEN

#define GEN(reg) EXPORT_RETPOLINE(reg)

#include <asm/GEN-for-each-reg.h>

tools/objtool/check.c

+1 −2
Original line number Diff line number Diff line
@@ -872,8 +872,7 @@ static int add_jump_destinations(struct objtool_file *file) 
		} else if (reloc->sym->type == STT_SECTION) {

			dest_sec = reloc->sym->sec;

			dest_off = arch_dest_reloc_offset(reloc->addend);

		} else if (!strncmp(reloc->sym->name, "__x86_indirect_thunk_", 21) ||

			   !strncmp(reloc->sym->name, "__x86_retpoline_", 16)) {

		} else if (!strncmp(reloc->sym->name, "__x86_indirect_thunk_", 21)) {

			/*

			 * Retpoline jumps are really dynamic jumps in

			 * disguise, so convert them accordingly.