[PATCH] x86/spectre_v1: Disable compiler optimizations over array_index_mask_nospec()

[Dan Williams]

Mark notes that gcc optimization passes have the potential to elide
necessary invocations of this instruction sequence, so include an
optimization barrier.

    > I think that either way, we have a potential problem if the compiler
    > generates a branch dependent on the result of validate_index_nospec().
    >
    > In that case, we could end up with codegen approximating:
    >
    >       bool safe = false;
    >
    >       if (idx < bound) {
    >               idx = array_index_nospec(idx, bound);
    >               safe = true;
    >       }
    >
    >       // this branch can be mispredicted
    >       if (safe) {
    >               foo = array[idx];
    >       }
    >
    > ... and thus we lose the nospec protection.

    I see GCC do this at -O0, but so far I haven't tricked it into doing
    this at -O1 or above.

    Regardless, I worry this is fragile -- GCC *can* generate code as per
    the above, even if it's unlikely to.

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: babdde2698d4 ("x86: Implement array_index_mask_nospec")
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/include/asm/barrier.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 042b5e892ed1..41f7435c84a7 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -38,10 +38,11 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 {
 	unsigned long mask;
 
-	asm ("cmp %1,%2; sbb %0,%0;"
+	asm volatile ("cmp %1,%2; sbb %0,%0;"
 			:"=r" (mask)
 			:"g"(size),"r" (index)
 			:"cc");
+	barrier();
 	return mask;
 }