[PATCH 4.4 13/18] perf/x86: Fix uninitialized value usage

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Peter Zijlstra <peterz@infradead.org>

commit e01d8718de4170373cd7fbf5cf6f9cb61cebb1e9 upstream.

When calling intel_alt_er() with .idx != EXTRA_REG_RSP_* we will not
initialize alt_idx and then use this uninitialized value to index an
array.

When that is not fatal, it can result in an infinite loop in its
caller __intel_shared_reg_get_constraints(), with IRQs disabled.

Alternative error modes are random memory corruption due to the
cpuc->shared_regs->regs[] array overrun, which manifest in either
get_constraints or put_constraints doing weird stuff.

Only took 6 hours of painful debugging to find this. Neither GCC nor
Smatch warnings flagged this bug.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: ae3f011fc251 ("perf/x86/intel: Fix SLM MSR_OFFCORE_RSP1 valid_mask")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/x86/kernel/cpu/perf_event_intel.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -1937,7 +1937,8 @@ intel_bts_constraints(struct perf_event
 
 static int intel_alt_er(int idx, u64 config)
 {
-	int alt_idx;
+	int alt_idx = idx;
+
 	if (!(x86_pmu.flags & PMU_FL_HAS_RSP_1))
 		return idx;