[PATCH v5 07/11] x86: use fallback for random_get_entropy() instead of zero

Jason A. Donenfeld
2022-04-19T11:19:18Z

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling random_get_entropy_fallback() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though
random_get_entropy_fallback() is super high precision or guaranteed to
be entropic, but basically anything that's not zero all the time is
better than returning zero all the time.

If CONFIG_X86_TSC=n, then it's possible that we're running on a 486 with
no RDTSC, so we only need the fallback code for that case.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: x86@kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
arch/x86/include/asm/timex.h | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/arch/x86/include/asm/timex.h b/arch/x86/include/asm/timex.h
index a4a8b1b16c0c..fac180359693 100644
--- a/arch/x86/include/asm/timex.h
+++ b/arch/x86/include/asm/timex.h
@@ -5,6 +5,16 @@
#include <asm/processor.h>
#include <asm/tsc.h>

+static inline unsigned long random_get_entropy(void)
+{
+#ifndef CONFIG_X86_TSC
+ if (!boot_cpu_has(X86_FEATURE_TSC))
+ return random_get_entropy_fallback();
+#endif
+ return rdtsc();
+}
+#define random_get_entropy random_get_entropy
+
/* Assume we use the PIT time source for the clock tick */
#define CLOCK_TICK_RATE PIT_TICK_RATE

--
2.35.1