Re: Sources of initialized memory in virtio?

[Jason Wang <jasowang@redhat.com>]

On 2018年09月13日 21:00, Alexander Potapenko wrote:
> Hi mighty virtio maintainers,
>
> I'm working on KMSAN, a new runtime detector of uninitialized memory
> based on compiler instrumentation (https://github.com/google/kmsan)
> KMSAN is mostly being tested on QEMU with KVM enabled, so my kernel
> interacts a lot with various virtio drivers, that's why I'm seeking
> your help.
>
> By default KMSAN treats kernel memory allocated by kmalloc() and
> alloc_page() as uninitialized. Writing a constant to memory or using
> it in copy_from_user() makes that memory initialized.
> Unfortunately a lot of writes to memory from KVM (mostly in the disk
> and network drivers) remain unnoticed by the tool, therefore we're
> seeing a lot of false positive reports (along with actual bugs, like
> CVE-2018-1118).
>
> KMSAN has an API function `kmsan_unpoison_shadow(void *buf, int len)`,
> which means "from now on, till this memory is freed or written to,
> mark it as initialized".
> I've tried playing Whack-a-Mole adding it to various places where the
> data comes from KVM, but failed to find them all. In fact, some of my
> annotations were wrong, so I ended up with the following two patches:
>
> https://github.com/google/kmsan/commit/76c671199a4de5bbe73cd13210a5e28848211bd1
> https://github.com/google/kmsan/commit/40ba1c8e2a3c6bbe8f34037413e253894251a405
>
> But I'm far from being sure this is the complete list of places where
> the memory is initialized by virtio drivers.
> May I ask you to help me find the places where we actually need to
> annotate the memory in virtio?

It looks to me another one is the used ring which device writes back the 
completed descriptor id and length. It (vr->used) was a part of a page 
which was allocated in vring_alloc_queue() through alloc_pages_exact() 
with __GFP_ZERO. So I'm not we need care about it.

Thanks

>
> Thanks in advance,

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