Commit 995cddcc authored by Dan Williams's avatar Dan Williams Committed by Greg Kroah-Hartman

x86/spectre_v1: Disable compiler optimizations over array_index_mask_nospec()

commit eab6870fee877258122a042bfd99ee7908c40280 upstream.

Mark Rutland noticed that GCC optimization passes have the potential to elide
necessary invocations of the array_index_mask_nospec() instruction sequence,
so mark the asm() volatile.

Mark explains:

"The volatile will inhibit *some* cases where the compiler could lift the
 array_index_nospec() call out of a branch, e.g. where there are multiple
 invocations of array_index_nospec() with the same arguments:

        if (idx < foo) {
                idx1 = array_idx_nospec(idx, foo)

        < some other code >

        if (idx < foo) {
                idx2 = array_idx_nospec(idx, foo);

 ... since the compiler can determine that the two invocations yield the same
 result, and reuse the first result (likely the same register as idx was in
 originally) for the second branch, effectively re-writing the above as:

        if (idx < foo) {
                idx = array_idx_nospec(idx, foo);

        < some other code >

        if (idx < foo) {

 ... if we don't take the first branch, then speculatively take the second, we
 lose the nospec protection.

 There's more info on volatile asm in the GCC docs:
Reported-by: default avatarMark Rutland <>
Signed-off-by: default avatarDan Williams <>
Acked-by: default avatarMark Rutland <>
Acked-by: default avatarThomas Gleixner <>
Acked-by: default avatarLinus Torvalds <>
Cc: <>
Cc: Peter Zijlstra <>
Fixes: babdde26 ("x86: Implement array_index_mask_nospec")
Link: default avatarIngo Molnar <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent c806e085
......@@ -37,7 +37,7 @@ 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)
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