Re: [PATCH] perf/x86: Fix integer overflow when left shifting an integer more than 32 bits

[Peter Zijlstra]

On Tue, Apr 20, 2021 at 05:03:03PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 03:29:07PM +0100, Colin King wrote:
> > From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > 
> > The 64 bit value read from MSR_ARCH_PERFMON_FIXED_CTR_CTRL is being
> > bit-wise masked with the value (0x03 << i*4). However, the shifted value
> > is evaluated using 32 bit arithmetic, so will overflow when i > 8.
> > Fix this by making 0x03 a ULL so that the shift is performed using
> > 64 bit arithmetic.
> > 
> > Addresses-Coverity: ("Unintentional integer overflow")
> 
> Strange tag that, also inaccurate, wide shifts are UB and don't behave
> consistently.
> 
> As is, we've not had hardware with that many fixed counters, but yes,
> worth fixing I suppose.

Patch now reads:

---
Subject: perf/x86: Allow for 8<num_fixed_counters<16
From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Date: Tue, 20 Apr 2021 15:29:07 +0100

From: Colin Ian King <colin.king@xxxxxxxxxxxxx>

The 64 bit value read from MSR_ARCH_PERFMON_FIXED_CTR_CTRL is being
bit-wise masked with the value (0x03 << i*4). However, the shifted value
is evaluated using 32 bit arithmetic, so will UB when i > 8. Fix this
by making 0x03 a ULL so that the shift is performed using 64 bit
arithmetic.

This makes the arithmetic internally consistent and preparers for the
day when hardware provides 8<num_fixed_counters<16.

Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210420142907.382417-1-colin.king@xxxxxxxxxxxxx
---
 arch/x86/events/core.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -261,7 +261,7 @@ static bool check_hw_exists(void)
 		for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
 			if (fixed_counter_disabled(i))
 				continue;
-			if (val & (0x03 << i*4)) {
+			if (val & (0x03ULL << i*4)) {
 				bios_fail = 1;
 				val_fail = val;
 				reg_fail = reg;