linux-kernel - Re: [PATCH 2/2] KVM: x86: map/unmap private slots in __x86_set_memory

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Message-ID: <20151013153957.GA9987@potion.brq.redhat.com>
Date:	Tue, 13 Oct 2015 17:39:57 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	aderumier@...so.com, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: map/unmap private slots in
 __x86_set_memory_region

2015-10-12 14:09+0200, Paolo Bonzini:
> Otherwise, two copies (one of them never used and thus bogus) are
> allocated for the regular and SMM address spaces.  This breaks
> SMM with EPT but without unrestricted guest support, because the
> SMM copy of the identity page map is all zeros.

(Have you found out why EPT+unrestricted didn't use the alternative SMM
 mapping as well?)

> By moving the allocation to the caller we also remove the last
> vestiges of kernel-allocated memory regions (not accessible anymore
> in userspace since commit b74a07beed0e, "KVM: Remove kernel-allocated
> memory regions", 2010-06-21); that is a nice bonus.
> 
> Reported-by: Alexandre DERUMIER <aderumier@...so.com>
> Cc: stable@...r.kernel.org
> Fixes: 9da0e4d5ac969909f6b435ce28ea28135a9cbd69
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---

vm_mmap() leaks if __kvm_set_memory_region() fails.  It's nothing new
and following process termination should take care of it,

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -7717,23 +7717,53 @@ void kvm_arch_sync_events(struct kvm *kvm)
>  int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
>  {
>  	int i, r;
> +	u64 hva;
> +	struct kvm_memslots *slots = kvm_memslots(kvm);
> +	struct kvm_memory_slot *slot, old;
| [...]
> +	slot = &slots->memslots[slots->id_to_index[id]];

This seems better written as

   	slot = id_to_memslot(slots, id);

(Made me remember that I want to refactor the memslot API ...)

| [...]
> +	} else {
> +		if (!slot->npages)
> +			return 0;
> +
> +		hva = 0;
> +	}
> +
> +	old = *slot;

(Assignment could be in the 'else' == !size branch, GCC would have fun.)

| [...]
> +	if (!size) {
> +		r = vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE);
> +		WARN_ON(r < 0);
> +	}
--
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