From: Ard Biesheuvel <ardb@kernel.org>
Date: Mon, 13 Apr 2020 16:06:56 +0200
Subject: efi/libstub/random: Increase random alloc granularity
Patch-mainline: v5.8-rc1
Git-commit: c37c9162a2fae026dc8570ad7c5ade0422b0a267
References: jsc#SLE-16407

The implementation of efi_random_alloc() arbitrarily truncates the
provided random seed to 16 bits, which limits the granularity of the
randomly chosen allocation offset in memory. This is currently only
an issue if the size of physical memory exceeds 128 GB, but going
forward, we will reduce the allocation alignment to 64 KB, and this
means we need to increase the granularity to ensure that the random
memory allocations are distributed evenly.

We will need to switch to 64-bit arithmetic for the multiplication,
but this does not result in 64-bit integer intrinsic calls on ARM or
on i386.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Lee, Chun-Yi <jlee@suse.com>
---
 drivers/firmware/efi/libstub/randomalloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/firmware/efi/libstub/randomalloc.c
+++ b/drivers/firmware/efi/libstub/randomalloc.c
@@ -87,7 +87,7 @@ efi_status_t efi_random_alloc(unsigned l
 	}
 
 	/* find a random number between 0 and total_slots */
-	target_slot = (total_slots * (u16)random_seed) >> 16;
+	target_slot = (total_slots * (u64)(random_seed & U32_MAX)) >> 32;
 
 	/*
 	 * target_slot is now a value in the range [0, total_slots), and so