[tip: x86/misc] x86/espfix: Use get_random_long() rather than archrandom

[tip-bot2 for Jason A. Donenfeld]

The following commit has been merged into the x86/misc branch of tip:

Commit-ID:     00ed1eabcc3b785aecf6f6cbb41d7b436bb54d28
Gitweb:        https://git.kernel.org/tip/00ed1eabcc3b785aecf6f6cbb41d7b436bb54d28
Author:        Jason A. Donenfeld <Jason@xxxxxxxxx>
AuthorDate:    Sat, 29 Oct 2022 02:26:13 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Mon, 31 Oct 2022 20:12:50 +01:00

x86/espfix: Use get_random_long() rather than archrandom

A call is made to arch_get_random_longs() and rdtsc(), rather than just
using get_random_long(), because this was written during a time when
very early boot would give abysmal entropy. These days, a call to
get_random_long() at early boot will incorporate RDRAND, RDTSC, and
more, without having to do anything bespoke.

In fact, the situation is now such that on the majority of x86 systems,
the pool actually is initialized at this point, even though it doesn't
need to be for get_random_long() to still return something better than
what this function currently does.

So simplify this to just call get_random_long() instead.

Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/20221029002613.143153-1-Jason@xxxxxxxxx
---
 arch/x86/kernel/espfix_64.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/arch/x86/kernel/espfix_64.c b/arch/x86/kernel/espfix_64.c
index 9417d5a..16f9814 100644
--- a/arch/x86/kernel/espfix_64.c
+++ b/arch/x86/kernel/espfix_64.c
@@ -94,17 +94,7 @@ static inline unsigned long espfix_base_addr(unsigned int cpu)
 
 static void init_espfix_random(void)
 {
-	unsigned long rand;
-
-	/*
-	 * This is run before the entropy pools are initialized,
-	 * but this is hopefully better than nothing.
-	 */
-	if (!arch_get_random_longs(&rand, 1)) {
-		/* The constant is an arbitrary large prime */
-		rand = rdtsc();
-		rand *= 0xc345c6b72fd16123UL;
-	}
+	unsigned long rand = get_random_long();
 
 	slot_random = rand % ESPFIX_STACKS_PER_PAGE;
 	page_random = (rand / ESPFIX_STACKS_PER_PAGE)