Re: [PATCH v2] net/e1000e_core: adjust count if RDH exceeds RDT in e1000e_ring_advance()

[Mauro Matteo Cascella]

On Fri, Nov 27, 2020 at 6:21 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2020/11/24 上午5:30, Mauro Matteo Cascella wrote:
> > On Thu, Nov 19, 2020 at 6:57 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> On 2020/11/18 下午4:53, Mauro Matteo Cascella wrote:
> >>> On Wed, Nov 18, 2020 at 4:56 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> On 2020/11/13 下午6:31, Mauro Matteo Cascella wrote:
> >>>>> The e1000e_write_packet_to_guest() function iterates over a set of
> >>>>> receive descriptors by advancing rx descriptor head register (RDH) from
> >>>>> its initial value to rx descriptor tail register (RDT). The check in
> >>>>> e1000e_ring_empty() is responsible for detecting whether RDH has reached
> >>>>> RDT, terminating the loop if that's the case. Additional checks have
> >>>>> been added in the past to deal with bogus values submitted by the guest
> >>>>> to prevent possible infinite loop. This is done by "wrapping around" RDH
> >>>>> at some point and detecting whether it assumes the original value during
> >>>>> the loop.
> >>>>>
> >>>>> However, when e1000e is configured to use the packet split feature, RDH 
> >>>>> is
> >>>>> incremented by two instead of one, as the packet split descriptors are
> >>>>> 32 bytes while regular descriptors are 16 bytes. A malicious or buggy
> >>>>> guest may set RDT to an odd value and transmit only null RX descriptors.
> >>>>> This corner case would prevent RDH from ever matching RDT, leading to an
> >>>>> infinite loop. This patch adds a check in e1000e_ring_advance() to make 
> >>>>> sure
> >>>>> RDH does not exceed RDT in a single incremental step, adjusting the 
> >>>>> count
> >>>>> value accordingly.
> >>>> Can this patch solve this issue in another way?
> >>>>
> >>>> 20201111130636.2208620-1-ppandit@redhat.com/">https://patchew.org/QEMU/20201111130636.2208620-1-ppandit@redhat.com/
> >>>>
> >>>> Thanks
> >>>>
> >>> Yes, it does work nicely. Still, I think this patch is useful to avoid
> >>> possible inconsistent state in e1000e_ring_advance() when count > 1.
> >>
> >> So if RDT is odd, it looks to me the following codes in
> >> e1000e_write_packet_to_guest() needs to be fixed as well.
> >>
> >>
> >>           base = e1000e_ring_head_descr(core, rxi);
> >>
> >>           pci_dma_read(d, base, &desc, core->rx_desc_len);
> >>
> >> Otherwise e1000e may try to read out of descriptor ring.
> > Sorry, I'm not sure I understand what you mean. Isn't the base address
> > computed from RDH? How can e1000e read out of the descriptor ring if
> > RDT is odd?
> >
> >> Thanks
> >
> > On Thu, Nov 19, 2020 at 6:57 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> On 2020/11/18 下午4:53, Mauro Matteo Cascella wrote:
> >>> On Wed, Nov 18, 2020 at 4:56 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> On 2020/11/13 下午6:31, Mauro Matteo Cascella wrote:
> >>>>> The e1000e_write_packet_to_guest() function iterates over a set of
> >>>>> receive descriptors by advancing rx descriptor head register (RDH) from
> >>>>> its initial value to rx descriptor tail register (RDT). The check in
> >>>>> e1000e_ring_empty() is responsible for detecting whether RDH has reached
> >>>>> RDT, terminating the loop if that's the case. Additional checks have
> >>>>> been added in the past to deal with bogus values submitted by the guest
> >>>>> to prevent possible infinite loop. This is done by "wrapping around" RDH
> >>>>> at some point and detecting whether it assumes the original value during
> >>>>> the loop.
> >>>>>
> >>>>> However, when e1000e is configured to use the packet split feature, RDH 
> >>>>> is
> >>>>> incremented by two instead of one, as the packet split descriptors are
> >>>>> 32 bytes while regular descriptors are 16 bytes. A malicious or buggy
> >>>>> guest may set RDT to an odd value and transmit only null RX descriptors.
> >>>>> This corner case would prevent RDH from ever matching RDT, leading to an
> >>>>> infinite loop. This patch adds a check in e1000e_ring_advance() to make 
> >>>>> sure
> >>>>> RDH does not exceed RDT in a single incremental step, adjusting the 
> >>>>> count
> >>>>> value accordingly.
> >>>> Can this patch solve this issue in another way?
> >>>>
> >>>> 20201111130636.2208620-1-ppandit@redhat.com/">https://patchew.org/QEMU/20201111130636.2208620-1-ppandit@redhat.com/
> >>>>
> >>>> Thanks
> >>>>
> >>> Yes, it does work nicely. Still, I think this patch is useful to avoid
> >>> possible inconsistent state in e1000e_ring_advance() when count > 1.
> >>
> >> So if RDT is odd, it looks to me the following codes in
> >> e1000e_write_packet_to_guest() needs to be fixed as well.
> >>
> >>
> >>           base = e1000e_ring_head_descr(core, rxi);
> >>
> >>           pci_dma_read(d, base, &desc, core->rx_desc_len);
> >>
> >> Otherwise e1000e may try to read out of descriptor ring.
> >>
> >> Thanks
>
>
> Sorry, I meant RDH actually, when packet split descriptor is used, it
> doesn't check whether DH exceeds DLEN?
>

When the packet split feature is used (i.e., count > 1) this patch
basically sets RDH=RDT in case the increment would exceed RDT. The
next iteration should detect that RDH equals RDT in
e1000e_ring_empty(), and exit the loop right before pci_dma_read(). On
the other hand RDH is set to zero if it exceeds DLEN in
e1000e_ring_advance() so we should be fine in either case, unless I'm
missing something?


Thank you for your time,
--
Mauro Matteo Cascella
Red Hat Product Security
PGP-Key ID: BB3410B0