From e6331a321aafcc291a60ceedf4d6b0051a6117ca Mon Sep 17 00:00:00 2001 From: Paul Burton 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 Reported-by: George Spelvin Cc: James Hogan 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 0074427b04fb..e5de6bac8197 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);