On Fri, Nov 27, 2020 at 6:21 AM Jason Wang <jasow...@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 <jasow...@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 <jasow...@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? > >>>> > >>>> https://patchew.org/QEMU/20201111130636.2208620-1-ppan...@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 <jasow...@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 <jasow...@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? > >>>> > >>>> https://patchew.org/QEMU/20201111130636.2208620-1-ppan...@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
