Re: [PATCH] random: Add "initialized" variable to proc

[Andy Lutomirski]

On Thu, May 1, 2014 at 8:35 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, May 1, 2014 at 8:05 AM,  <tytso@xxxxxxx> wrote:
>> On Wed, Apr 30, 2014 at 09:05:00PM -0700, H. Peter Anvin wrote:
>>>
>>> Giving the guest a seed would be highly useful, though.  There are a
>>> number of ways to do that; changing the boot protocol is probably
>>> only useful if Qemu itself bouts the kernel as opposed to an in-VM
>>> bootloader.
>>
>> So how about simply passing a memory address and an optional offset on
>> the boot command line?  That way the hypervisor can drop the seed in
>> some convenient real memory location, and the kernel can just copy it
>> someplace safe, or in the case of kernel ASLR, the relocator can use
>> it to seed its CRNG, and then after it relocates the kernel, it can
>> crank the CRNG to pass a seed to the kernel's urandom driver.
>>
>> That way, we don't have to do something which is ACPI or DT dependent.
>> Maybe there will be embedded architectures where using DT might be
>> more convenient, but this would probably be simplest for KVM/qumu-based
>> VM's, I would think.
>
> One problem with passing a seed in memory like this is that it
> provides no benefit if the guest reboots without restarting the
> hypervisor.  Using an MSR or something avoids that issue.
>
> Passing an address in I/O space that can be read to synchronously
> obtain a seed would work, but it could still be messy to get the
> address to propagate through the booatloader and the reboot process.
>

A CPUID leaf or an MSR advertised by a CPUID leaf has another
advantage: it's easy to use in the ASLR code -- I don't think there's
a real IDT, so there's nothing like rdmsr_safe available.  It also
avoids doing anything complicated with the boot process to allow the
same seed to be used for ASLR and random.c; it can just be invoked
twice on boot.

Here are two easyish ways to do it:

a. Add a new CPUID leaf KVM_CPUID_URANDOM = 0x40000002.  The existence
of the leaf is signaled by KVM_CPUID_SIGNATURE.eax >= 0x40000002.
Reading the leaf either gives all zeros to indicate that it's
unsupported or disabled or it gives 256 bits of urandom-style data in
rax,rbx,rcx,edx.  32-bit callers will have trouble extracting more
than 128 of those 256 bits, but that should be fine.

b. Add a new MSR_KVM_URANDOM and indicate support using
KVM_FEATURE_URANDOM.  The is cleaner, since it matches existing
practice, but it's awkward to return more than 64 bits at a time from
rdmsr.  128 bits is straightforward by cheating and using the high
bits in rax and rdx, but that's kind of gross.  Clobbering any more
registers is awful, and passing a pointer into wrmsr seems
overcomplicated.

There's also the hypercall interface, but it looks like hyperv support
can interfere with it, and I'm not sure whether the guest needs to
cooperate with whatever the magical vmcall patching code is doing.

What's the right forum for this?  This thread is probably not it.

--Andy
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