[PATCH] crypto: xor - avoid division by zero crash

On some of my parisc machines, this patch c055e3eae0f1 ("crypto: xor -
use ktime for template benchmarking") triggers a dividy-by-zero
exception because "min" becomes zero, which then leads to a kernel
crash.

It's not clear yet, why I see the issue only on some machines. At least
on those the measured time to run the xor speed tests becomes zero
nanoseconds. Maybe it's because gcc-10 optimizes the speed test out, or
because of some other changes in the time keeping routines.

In either case, the kernel should not crash.

This patch adds a workaround by reporting such cases with a kernel
warning and continues as if the xor tests would have run in 1 ns.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # 5.10+

---

diff --git a/crypto/xor.c b/crypto/xor.c
index eacbf4f93990..3639341bac7e 100644
--- a/crypto/xor.c
+++ b/crypto/xor.c

@@ -100,6 +100,8 @@ do_xor_speed(struct xor_block_template *tmpl, void *b1, void *b2)
 			mb();
 		}
 		diff = ktime_sub(ktime_get(), start);
+		if (WARN_ON(diff == 0))
+			diff = 1;
 		if (diff < min)
 			min = diff;
 	}