[PATCH] MIPS: KVM: Use prandom_u32_max() to generate tlbwr index

[PATCH] MIPS: KVM: Use prandom_u32_max() to generate tlbwr index

[Paul Burton]

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>
---

 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);
-- 
2.21.0

Re: [PATCH] MIPS: KVM: Use prandom_u32_max() to generate tlbwr index

[Paul Burton]

Hello,

Paul Burton wrote:
> 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>

Applied to mips-next.

Thanks,
    Paul

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