linux-kernel - [PATCH 5.10 129/163] riscv: use fallback for random_get

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Date:   Fri, 27 May 2022 10:50:09 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH 5.10 129/163] riscv: use fallback for random_get_entropy() instead of zero

From: "Jason A. Donenfeld" <Jason@...c4.com>

commit 6d01238623faa9425f820353d2066baf6c9dc872 upstream.

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.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Paul Walmsley <paul.walmsley@...ive.com>
Acked-by: Palmer Dabbelt <palmer@...osinc.com>
Reviewed-by: Palmer Dabbelt <palmer@...osinc.com>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/riscv/include/asm/timex.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/riscv/include/asm/timex.h
+++ b/arch/riscv/include/asm/timex.h
@@ -41,7 +41,7 @@ static inline u32 get_cycles_hi(void)
 static inline unsigned long random_get_entropy(void)
 {
 	if (unlikely(clint_time_val == NULL))
-		return 0;
+		return random_get_entropy_fallback();
 	return get_cycles();
 }
 #define random_get_entropy()	random_get_entropy()