Re: [Xen-devel] [PATCH] x86/HVM: avoid pointer wraparound in bufioreq handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.06.15 at 11:15, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Ian Campbell [mailto:ian.campbell@xxxxxxxxxx]
>> Sent: 16 June 2015 10:00
>> To: Jan Beulich
>> Cc: Andrew Cooper; Wei Liu; Ian Jackson; Stefano Stabellini; xen-devel; Keir
>> (Xen.org); Paul Durrant
>> Subject: Re: [Xen-devel] [PATCH] x86/HVM: avoid pointer wraparound in
>> bufioreq handling
>> 
>> On Tue, 2015-06-16 at 09:37 +0100, Jan Beulich wrote:
>> > >>> On 16.06.15 at 10:20, <ian.campbell@xxxxxxxxxx> wrote:
>> > > On Tue, 2015-06-16 at 07:44 +0100, Jan Beulich wrote:
>> > >> >>> On 15.06.15 at 16:30, <JBeulich@xxxxxxxx> wrote:
>> > >> > The number of slots per page being 511 (i.e. not a power of two)
>> means
>> > >> > that the (32-bit) read and write indexes going beyond 2^32 will likely
>> > >> > disturb operation. Extend I/O req server creation so the caller can
>> > >> > indicate that it is using suitable atomic accesses where needed (not
>> > >> > all accesses to the two pointers really need to be atomic), allowing
>> > >> > the hypervisor to atomically canonicalize both pointers when both
>> have
>> > >> > gone through at least one cycle.
>> > >> >
>> > >> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> > >>
>> > >> No matter that it's just a single line change, I realized that I
>> > >> forgot to Cc the tools maintainers. While a v2 will be needed (see
>> > >> the reply just sent to Andrew) I'd still appreciate input (if any) to
>> > >> limit the number of revisions needed.
>> > >
>> > > For such a simple toolstack side change which just reflects the
>> > > underlying hcall interface I have no real opinion so far as the tools
>> > > side goes, but it would be good to update the comments in xenctrl.h too.
>> > > With that done for the tools change:
>> > >         Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> >
>> > Thanks. The request for feedback went beyond the request for
>> > an ack though, namely
>> >
>> > TBD: Do we need to be worried about non-libxc users of the changed
>> >      (tools only) interface?
>> 
>> It's (currently at least) a declared non-stable API, so in principal no.
>> It would be polite to give a heads up to the expected potential users
>> though, which you've done by CCing the QEMU maintainers I think. Adding
>> Paul D for completeness though.
> 
> From my reading, both QEMU upstream and trad are safe. They use a loop of 
> the form:
> 
> while (read_ptr != write_ptr)
> {
>    do stuff
> 
>   read_ptr += (handled a qword) ? 2 : 1;
> }
> 
> So, since the only test is for equality I think overflow should be handled 
> correctly. So, does anything actually need to be fixed?

Of course this needs to be fixed: When either pointer crosses the
2^32 boundary, the slot referenced goes from 0x1f to 0 (due to the
"modulo 511" operation determining the slot to be used), introducing
a discontinuity and potentially corrupting data in slots not consumed
yet.

Jan


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