From c37c9162a2fae026dc8570ad7c5ade0422b0a267 Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Mon, 13 Apr 2020 16:06:56 +0200 Subject: [PATCH] efi/libstub/random: Increase random alloc granularity 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 --- drivers/firmware/efi/libstub/randomalloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/firmware/efi/libstub/randomalloc.c b/drivers/firmware/efi/libstub/randomalloc.c index 6200dfa650f5..a408df474d83 100644 --- 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 long size, } /* 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 -- 2.11.0