Commit ba6e31af authored by Pawan Gupta's avatar Pawan Gupta Committed by Borislav Petkov

x86/speculation: Add LFENCE to RSB fill sequence

RSB fill sequence does not have any protection for miss-prediction of
conditional branch at the end of the sequence. CPU can speculatively
execute code immediately after the sequence, while RSB filling hasn't
completed yet.

  #define __FILL_RETURN_BUFFER(reg, nr, sp)       \
          mov     $(nr/2), reg;                   \
  771:                                            \
          ANNOTATE_INTRA_FUNCTION_CALL;           \
          call    772f;                           \
  773:    /* speculation trap */                  \
          UNWIND_HINT_EMPTY;                      \
          pause;                                  \
          lfence;                                 \
          jmp     773b;                           \
  772:                                            \
          ANNOTATE_INTRA_FUNCTION_CALL;           \
          call    774f;                           \
  775:    /* speculation trap */                  \
          UNWIND_HINT_EMPTY;                      \
          pause;                                  \
          lfence;                                 \
          jmp     775b;                           \
  774:                                            \
          add     $(BITS_PER_LONG/8) * 2, sp;     \
          dec     reg;                            \
          jnz     771b;        <----- CPU can miss-predict here.

Before RSB is filled, RETs that come in program order after this macro
can be executed speculatively, making them vulnerable to RSB-based
attacks.

Mitigate it by adding an LFENCE after the conditional branch to prevent
speculation while RSB is being filled.
Suggested-by: default avatarAndrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
parent 2b129932
......@@ -60,7 +60,9 @@
774: \
add $(BITS_PER_LONG/8) * 2, sp; \
dec reg; \
jnz 771b;
jnz 771b; \
/* barrier for jnz misprediction */ \
lfence;
#ifdef __ASSEMBLY__
......
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