[PATCH v4 07/11] x86: use fallback for random_get_entropy() instead of zero

Jason A. Donenfeld
2022-04-13T11:59:44Z

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/tsc.h | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 01a300a9700b..194dc1e3f77c 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -28,6 +28,16 @@ static inline cycles_t get_cycles(void)
return rdtsc();
}

+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
+
extern struct system_counterval_t convert_art_to_tsc(u64 art);
extern struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns);

--
2.35.1