[PATCH 3.16 160/366] x86/spectre_v1: Disable compiler optimizations over array_index_mask_nospec()

Ben Hutchings
2018-11-11T19:49:05Z

From: Dan Williams <dan.j.williams@intel.com>

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)
do_something(idx1);
}

< some other code >

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

... 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);
do_something(idx);
}

< some other code >

if (idx < foo) {
do_something_else(idx);
}

... 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:

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
"

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: babdde2698d4 ("x86: Implement array_index_mask_nospec")
Link: https://lkml.kernel.org/lkml/152838798950.14521.4893346294059739135.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/include/asm/barrier.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -38,7 +38,7 @@ static inline unsigned long array_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");