Re: [PATCH 1/2] hw/net/virtio-net.c: Don't assume IP length field is aligned

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 7 Nov 2024 at 16:32, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> In virtio-net.c we assume that the IP length field in the packet is
> aligned, and we copy its address into a uint16_t* in the
> VirtioNetRscUnit struct which we then dereference later.  This isn't
> a safe assumption; it will also result in compilation failures if we
> mark the ip_header struct as QEMU_PACKED because the compiler will
> not let you take the address of an unaligned struct field.
>
> Make the ip_plen field in VirtioNetRscUnit a void*, and make all the
> places where we read or write through that pointer instead use some
> new accessor functions read_unit_ip_len() and write_unit_ip_len()
> which account for the pointer being potentially unaligned and also do
> the network-byte-order conversion we were previously using htons() to
> perform.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  include/hw/virtio/virtio-net.h |  2 +-
>  hw/net/virtio-net.c            | 23 +++++++++++++++++++----
>  2 files changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index 060c23c04d2..b9ea9e824e3 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -102,7 +102,7 @@ typedef struct VirtioNetRscStat {
>  /* Rsc unit general info used to checking if can coalescing */
>  typedef struct VirtioNetRscUnit {
>      void *ip;   /* ip header */
> -    uint16_t *ip_plen;      /* data len pointer in ip header field */
> +    void *ip_plen; /* pointer to unaligned uint16_t data len in ip header */
>      struct tcp_header *tcp; /* tcp header */
>      uint16_t tcp_hdrlen;    /* tcp header len */
>      uint16_t payload;       /* pure payload without virtio/eth/ip/tcp */
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index f2104ed364a..11cf462180d 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -2049,6 +2049,21 @@ static ssize_t virtio_net_do_receive(NetClientState *nc, const uint8_t *buf,
>      return virtio_net_receive_rcu(nc, buf, size, false);
>  }
>
> +/*
> + * Accessors to read and write the IP packet data length field. This
> + * is a potentially unaligned network-byte-order 16 bit unsigned integer
> + * pointed to by unit->ip_len.
> + */
> +static uint16_t read_unit_ip_len(VirtioNetRscUnit *unit)
> +{
> +    return ldl_be_p(unit->ip_plen);
> +}
> +
> +static void write_unit_ip_len(VirtioNetRscUnit *unit, uint16_t l)
> +{
> +    stl_be_p(unit->ip_plen, l);
> +}

These should of course be lduw_be_p() and stw_be_p(), since
it's a 16 bit field.

Interestingly nothing fails in "make check" or "make check-functional"
if this breaks, suggesting we aren't exercising virtio-net very hard.

-- PMM