x86/retpoline: Simplify retpolines

Due to:

  c9c324dc22aa ("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: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210326151259.506071949@infradead.org

1 files changed, 17 insertions, 17 deletions

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 6bb74b5c238c..d2c0d14c1e22 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -10,27 +10,31 @@
 #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_\@
+	call    .Ldo_rop_\@
 .Lspec_trap_\@:
 	UNWIND_HINT_EMPTY
 	pause
 	lfence
-	jmp	.Lspec_trap_\@
+	jmp .Lspec_trap_\@
 .Ldo_rop_\@:
-	mov	%\reg, (%_ASM_SP)
+	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>