linux-kernel - Re: [patch] kvm: suppress KVM_SET_GSI

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Date:   Tue, 13 Feb 2018 16:44:28 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [patch] kvm: suppress KVM_SET_GSI_ROUTING allocation failure

On Tue 13-02-18 16:03:09, Paolo Bonzini wrote:
> On 13/02/2018 15:48, Michal Hocko wrote:
> > On Thu 08-02-18 13:35:08, David Rientjes wrote:
> >> The KVM_SET_GSI_ROUTING ioctl does a vmalloc() of
> >> sizeof(struct kvm_irq_routing_entry) multiplied by a user-supplied value.
> >> This can be up to 4096 entries on architectures such as arm64 and s390
> >> (and the upper bound may be increased on s390 eventually).
> >>
> >> This can produce a vmalloc allocation failure warning:
> >>
> >> vmalloc: allocation failure: 0 bytes, mode:0x24000c2(GFP_KERNEL|__GFP_HIGHMEM)
> > 
> > I am not arguing about the kvm change but do we actaully want to warn
> > for 0 sized allocations? This just doesn't make much sense to me.
> > In other words don't we want this?
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 673942094328..c5d832510c54 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1748,7 +1748,9 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> >  	unsigned long real_size = size;
> >  
> >  	size = PAGE_ALIGN(size);
> > -	if (!size || (size >> PAGE_SHIFT) > totalram_pages)
> > +	if (!size)
> > +		return NULL;
> > +	if ((size >> PAGE_SHIFT) > totalram_pages)
> >  		goto fail;
> >  
> >  	area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNINITIALIZED |
> > 
> 
> There have been quite a few reports of this from syzkaller and generally
> we've fixed them.  It does seem like a recipe for NULL-pointer
> dereferences when the size is user-controlled (as in this case).

We do return NULL for that case regardless the above. The patch just
doesn't warn. Or do you think it is helpful to warn?
-- 
Michal Hocko
SUSE Labs