linux-kernel - Re: [PATCH] KVM: check userspace

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Date:   Thu, 11 Jun 2020 17:44:02 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] KVM: check userspace_addr for all memslots

On Mon, 2020-06-01 at 04:21 -0400, Paolo Bonzini wrote:
> The userspace_addr alignment and range checks are not performed for private
> memory slots that are prepared by KVM itself.  This is unnecessary and makes
> it questionable to use __*_user functions to access memory later on.  We also
> rely on the userspace address being aligned since we have an entire family
> of functions to map gfn to pfn.
> 
> Fortunately skipping the check is completely unnecessary.  Only x86 uses
> private memslots and their userspace_addr is obtained from vm_mmap,
> therefore it must be below PAGE_OFFSET.  In fact, any attempt to pass
> an address above PAGE_OFFSET would have failed because such an address
> would return true for kvm_is_error_hva.
> 
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

I bisected this patch to break a VM on my AMD system (3970X)

The reason it happens, is because I have avic enabled (which uses
a private KVM memslot), but it is permanently disabled for that VM,
since I enabled nesting for that VM (+svm) and that triggers the code
in __x86_set_memory_region to set userspace_addr of the disabled
memslot to non canonical address (0xdeadull << 48) which is later rejected in __kvm_set_memory_region
after that patch, and that makes it silently not disable the memslot, which hangs the guest.

The call is from avic_update_access_page, which is called from svm_pre_update_apicv_exec_ctrl
which discards the return value.

I think that the fix for this would be to either make access_ok always return
true for size==0, or __kvm_set_memory_region should treat size==0 specially
and skip that check for it.

Best regards,
	Maxim Levitsky