linux-kernel - Re: [PATCH] xen/privcmd: allow fetching resource sizes

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Message-ID: <ed089f3f-4950-f3e9-6df6-07ca9148e6a6@cam.ac.uk>
Date:   Mon, 11 Jan 2021 22:39:49 +0000
From:   Andrew Cooper <amc96@....ac.uk>
To:     boris.ostrovsky@...cle.com, Roger Pau Monne <roger.pau@...rix.com>,
        linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Paul Durrant <paul.durrant@...rix.com>,
        xen-devel@...ts.xenproject.org,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Cooper <amc96@....ac.uk>
Subject: Re: [PATCH] xen/privcmd: allow fetching resource sizes

On 11/01/2021 22:09, boris.ostrovsky@...cle.com wrote:
> On 1/11/21 10:29 AM, Roger Pau Monne wrote:
>>  
>> +	xdata.domid = kdata.dom;
>> +	xdata.type = kdata.type;
>> +	xdata.id = kdata.id;
>> +
>> +	if (!kdata.addr && !kdata.num) {
>
> I think we should not allow only one of them to be zero. If it's only kdata.num then we will end up with pfns array set to ZERO_SIZE_PTR (which is 0x10). We seem to be OK in that we are not derefencing pfns (either in kernel or in hypervisor) if number of frames is zero but IMO we shouldn't be tempting the fate.
>
>
> (And if it's only kdata.addr then we will get a vma but I am not sure it will do what we want.)

Passing addr == 0 without num being 0 is already an error in Xen, and
passing num == 0 without addr being 0 is bogus and will be an error by
the time I'm finished fixing this.

FWIW, the common usecase for non-trivial examples will be:

xenforeignmem_resource_size(domid, type, id, &size);
xenforeignmem_map_resource(domid, type, id, NULL, size, ...);

which translates into:

ioctl(MAP_RESOURCE, NULL, 0) => size
mmap(NULL, size, ...) => ptr
ioctl(MAP_RESOURCE, ptr, size)

from the kernels point of view, and two hypercalls from Xen's point of
view.  The NULL's above are expected to be the common case for letting
the kernel chose the vma, but ought to be filled in by the time the
second ioctl() occurs.

See
https://lore.kernel.org/xen-devel/20200922182444.12350-1-andrew.cooper3@citrix.com/T/#u
for all the gory details.

~Andrew