Re: [Qemu-devel] [PATCH v4 13/30] stellaris_enet: avoid buffer overrun on incoming migration

[Peter Maydell]

On 31 March 2014 21:49, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, Mar 31, 2014 at 06:11:22PM +0100, Dr. David Alan Gilbert wrote:
>> * Michael S. Tsirkin (address@hidden) wrote:
>> > CVE-2013-4532
>> >
>> > s->next_packet is read from wire as an index into s->rx[]. If
>> > s->next_packet exceeds the length of s->rx[], the buffer can be
>> > subsequently overrun with arbitrary data from the wire.
>> >
>> > Fix this by failing migration if s->next_packet we read from
>> > the wire exceeds this.
>> >
>> > Similarly, validate rx_fifo against sizeof(s->rx[].data).
>> >
>> > Finally, constrain rx len to a sensibly small positive
>> > value, to avoid integer overruns when data model
>> > later uses this value.
>> >
>> > Reported-by: Michael Roth <address@hidden>
>> > Reported-by: Peter Maydell <address@hidden>
>> > Signed-off-by: Michael S. Tsirkin <address@hidden>
>> > ---
>> >  hw/net/stellaris_enet.c | 24 ++++++++++++++++++++----
>> >  1 file changed, 20 insertions(+), 4 deletions(-)
>> >
>> > diff --git a/hw/net/stellaris_enet.c b/hw/net/stellaris_enet.c
>> > index d04e6a4..182fd3e 100644
>> > --- a/hw/net/stellaris_enet.c
>> > +++ b/hw/net/stellaris_enet.c
>> > @@ -358,7 +358,7 @@ static void stellaris_enet_save(QEMUFile *f, void 
>> > *opaque)
>> >  static int stellaris_enet_load(QEMUFile *f, void *opaque, int version_id)
>> >  {
>> >      stellaris_enet_state *s = (stellaris_enet_state *)opaque;
>> > -    int i;
>> > +    int i, v;
>> >
>> >      if (version_id != 1)
>> >          return -EINVAL;
>> > @@ -381,9 +381,25 @@ static int stellaris_enet_load(QEMUFile *f, void 
>> > *opaque, int version_id)
>> >          qemu_get_buffer(f, s->rx[i].data, sizeof(s->rx[i].data));
>> >
>> >      }
>>
>> The loop that's just off the top here is:
>>     for (i = 0; i < 31; i++) {
>>         s->rx[i].len = qemu_get_be32(f);
>>         qemu_get_buffer(f, s->rx[i].data, sizeof(s->rx[i].data));
>>
>>     }
>>
>> Doesn't that 'len' need validating? I assume it's the size of the
>> packet in the fixed sized buffer? (??)
>
> Not that I see where it's used as such.

In the DATA case of stellaris_enet_read() -- when the current
rx_fifo_len goes to zero we will uncritically set rx_fifo_len to
s->rx[s->next_packet].len. So we must validate that it's between
0 and 2048 (the size of the rx[].data array), otherwise further
reads from DATA will be able to run off the end of the data array
for the following packet.

>> > -    s->next_packet = qemu_get_be32(f);
>> > -    s->rx_fifo = s->rx[s->next_packet].data + qemu_get_be32(f);
>> > -    s->rx_fifo_len = qemu_get_be32(f);
>> > +    v = qemu_get_be32(f);
>> > +    if (v < 0 || v >= ARRAY_SIZE(s->rx)) {
>> > +        return -EINVAL;
>> > +    }
>> > +    s->next_packet = v;
>> > +    v = qemu_get_be32(f);
>> > +    if (v < 0 || v >= sizeof(s->rx[i].data)) {
>> > +        return -EINVAL;
>> > +    }
>> > +    s->rx_fifo = s->rx[s->next_packet].data + v;
>> > +    v = qemu_get_be32(f);
>> > +    /* Set limit low enough to avoid integer overflow when
>> > +     * we do math on len later, but high enough to avoid
>> > +     * truncating any packets.
>> > +     */
>> > +    if (v < 0 || v >= 0x100000) {
>> > +        return -EINVAL;
>> > +    }
>> > +    s->rx_fifo_len = v;
>>
>> I don't understand this - isn't the requirement that rx_fifo+rx_fifo_len be 
>> within
>> the buffer (or I think it might be legal for the sum to point to the byte 
>> after the
>> buffer)?
>> My (quick first ever look at this code) reading is that rx_fifo and 
>> rx_fifo_len
>> related to the current packet in-flight; although I've not quite convinced 
>> myself
>> about what is supposed to happen at the end of the packet (which is why
>> I say rx_fifo might point just at? the end.

> Actually I forgot why I wrote this last check.
> Peter said we should and I thought I see the issue ...
> But I no longer see what kind of damage can rx_fifo_len cause
> unless validated.

Again, look at the DATA read logic. Every time the guest does a
DATA read, we read from the four bytes at s->rx_fifo, increment
rx_fifo by 4 and decrement rx_fifo_len by 4. When rx_fifo_len
eventually goes to zero we will (on the subsequent read) reset
both rx_fifo and rx_fifo_len from the next packet in the rx queue.
So if the incoming data sets rx_fifo_len to (let us say) 0x10000,
then the guest can cause us to read well off the end of the rx data
array. This means your check isn't tight enough -- we need to
ensure that rx_fifo and rx_fifo_len between them define a window
into the rx data and nowhere else. As David says this means you
need:

    v1 = qemu_get_be32(f);
    if (v1 < 0 || v1 > sizeof(s->rx[i].data)) {
        return -EINVAL;
    }
    s->rx_fifo = s->rx[s->next_packet].data + v1;
    v2 = qemu_get_be32(f);
    if (v2 < 0 || v1 + v2 > sizeof(s->rx[i].data)) {
        return -EINVAL;
    }
    s->rx_fifo_len = v2;

The max check on v1 is actually only there to ensure that we
don't have to think about integer overflow when we do the
upper-bound check on v1 + v2. Note that v1 == sizeof(array)
is OK if (and only if) v2 == 0.

An assert in stellaris_enet_receive() that the net code never
hands us a packet we can't fit in the array wouldn't go amiss
either, but that's a separate issue.

thanks
-- PMM