From e6331a321aafcc291a60ceedf4d6b0051a6117ca Mon Sep 17 00:00:00 2001
From: Paul Burton <paul.burton@mips.com>
Date: Fri, 22 Mar 2019 18:04:04 +0000
Subject: [PATCH] MIPS: KVM: Use prandom_u32_max() to generate tlbwr index
Emulation of the tlbwr instruction, which writes a TLB entry to a random
index in the TLB, currently uses get_random_bytes() to generate a 4 byte
random number which we then mask to form the index. This is overkill in
a couple of ways:
- We don't need 4 bytes here since we mask the value to form a 6 bit
number anyway, so we waste /dev/random entropy generating 3 random
bytes that are unused.
- We don't need crypto-grade randomness here - the architecture spec
allows implementations to use any algorithm & merely encourages that
some pseudo-randomness be used rather than a simple counter. The
fast prandom_u32() function fits that criteria well.
So rather than using get_random_bytes() & consuming /dev/random entropy,
switch to using the faster prandom_u32_max() which provides what we need
here whilst also performing the masking/modulo for us.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Reported-by: George Spelvin <lkml@sdf.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
---
arch/mips/kvm/emulate.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/arch/mips/kvm/emulate.c b/arch/mips/kvm/emulate.c
index 0074427b04fbf..e5de6bac81979 100644
--- a/arch/mips/kvm/emulate.c
+++ b/arch/mips/kvm/emulate.c
@@ -1141,9 +1141,7 @@ enum emulation_result kvm_mips_emul_tlbwr(struct kvm_vcpu *vcpu)
unsigned long pc = vcpu->arch.pc;
int index;
- get_random_bytes(&index, sizeof(index));
- index &= (KVM_MIPS_GUEST_TLB_SIZE - 1);
-
+ index = prandom_u32_max(KVM_MIPS_GUEST_TLB_SIZE);
tlb = &vcpu->arch.guest_tlb[index];
kvm_mips_invalidate_guest_tlb(vcpu, tlb);
--
GitLab