Re: [Qemu-devel] [PATCH] net: e1000e: fix an infinite loop issue

[Dmitry Fleytman]

Hello,

Thanks for the patch!

The problem of infinite loop indeed exists in e1000e, however it is different 
from the one fixed in e1000.
Please find my comments inline.

~Dmitry

> On 7 Feb 2017, at 11:43 AM, Li Qiang <address@hidden> wrote:
> 
> From: Li Qiang <address@hidden>
> 
> This issue is the same as e1000 network card in this commit:
> e1000: eliminate infinite loops on out-of-bounds transfer start.
> 
> Signed-off-by: Li Qiang <address@hidden>
> ---
> hw/net/e1000e_core.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/net/e1000e_core.c b/hw/net/e1000e_core.c
> index 2b11499..53f2b1d 100644
> --- a/hw/net/e1000e_core.c
> +++ b/hw/net/e1000e_core.c
> @@ -914,7 +914,8 @@ e1000e_start_xmit(E1000ECore *core, const E1000E_TxRing 
> *txr)
>     struct e1000_tx_desc desc;
>     bool ide = false;
>     const E1000E_RingInfo *txi = txr->i;
> -    uint32_t cause = E1000_ICS_TXQE;
> +    uint32_t tdh_start = core->mac[txi->dh], cause = E1000_ICS_TXQE;
> +
> 
>     if (!(core->mac[TCTL] & E1000_TCTL_EN)) {
>         trace_e1000e_tx_disabled();
> @@ -933,6 +934,14 @@ e1000e_start_xmit(E1000ECore *core, const E1000E_TxRing 
> *txr)
>         cause |= e1000e_txdesc_writeback(core, base, &desc, &ide, txi->idx);
> 
>         e1000e_ring_advance(core, txi, 1);
> +
> +        /*
> +         * The following avoid infinite loop, just as the e1000
> +         */
> +        if (core->mac[txi->dh] == tdh_start ||
> +            tdh_start >= core->mac[txi->dlen] / E1000_RING_DESC_LEN) {
> +            break;
> +            }

Part of this validity check, namely
tdh_start >= core->mac[txi->dlen] / E1000_RING_DESC_LEN)
already done by e1000e_ring_advance(), therefore not needed here.

The second part - full wraparound protection is relevant, but it fixes more 
symptoms than the cause.
The only possible cause for full wraparound is r->dt (tail) value bigger or 
equal to r->dlen (length),
so I would suggest to check for this in e1000e_ring_empty() and cover both TX 
and RX cases at once.

>     }
> 
>     if (!ide || !e1000e_intrmgr_delay_tx_causes(core, &cause)) {
> @@ -1500,6 +1509,7 @@ e1000e_write_packet_to_guest(E1000ECore *core, struct 
> NetRxPkt *pkt,
>     size_t desc_size;
>     size_t desc_offset = 0;
>     size_t iov_ofs = 0;
> +    uint32_t rdh_start;
> 
>     struct iovec *iov = net_rx_pkt_get_iovec(pkt);
>     size_t size = net_rx_pkt_get_total_len(pkt);
> @@ -1509,6 +1519,7 @@ e1000e_write_packet_to_guest(E1000ECore *core, struct 
> NetRxPkt *pkt,
>     bool do_ps = e1000e_do_ps(core, pkt, &ps_hdr_len);
> 
>     rxi = rxr->i;
> +    rdh_start = core->mac[rxi->dh];
> 
>     do {
>         hwaddr ba[MAX_PS_BUFFERS];
> @@ -1605,6 +1616,10 @@ e1000e_write_packet_to_guest(E1000ECore *core, struct 
> NetRxPkt *pkt,
>         e1000e_ring_advance(core, rxi,
>                             core->rx_desc_len / E1000_MIN_RX_DESC_LEN);
> 
> +        if (core->mac[rxi->dh] == rdh_start ||
> +            rdh_start >= core->mac[rxi->dlen] / E1000_RING_DESC_LEN) {
> +            break;
> +            }
>     } while (desc_offset < total_size);
> 
>     e1000e_update_rx_stats(core, size, total_size);
> -- 
> 1.8.3.1
>