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SubjectRe: [PATCH net] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ
Hi Michael,

On Mon, Dec 23, 2019 at 6:09 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> The only way for guest to control offloads (as enabled by
> VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) is by sending commands
> through CTRL_VQ. So it does not make sense to
> acknowledge VIRTIO_NET_F_CTRL_GUEST_OFFLOADS without
> VIRTIO_NET_F_CTRL_VQ.
>
> The spec does not outlaw devices with such a configuration,
> but Linux assumed that with VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> control vq is always there, resulting in the following crash
> when configuring LRO:
>
> kernel BUG at drivers/net/virtio_net.c:1591!
> invalid opcode: 0000 [#1] PREEMPT SMP PTI
> CPU: 1 PID: 483 Comm: Binder:330_1 Not tainted 5.4.5-01326-g19463e9acaac #1
> Hardware name: ChromiumOS crosvm, BIOS 0
> RIP: 0010:virtnet_send_command+0x15d/0x170 [virtio_net]
> Code: d8 00 00 00 80 78 02 00 0f 94 c0 65 48 8b 0c 25 28 00 00 00 48 3b 4c 24 70 75 11 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d
> +c3 <0f> 0b e8 ec a4 12 c8 66 90 66 2e 0f 1f 84 00 00 00 00 00 55 48 89
> RSP: 0018:ffffb97940e7bb50 EFLAGS: 00010246
> RAX: ffffffffc0596020 RBX: ffffa0e1fc8ea840 RCX: 0000000000000017
> RDX: ffffffffc0596110 RSI: 0000000000000011 RDI: 000000000000000d
> RBP: ffffb97940e7bbf8 R08: ffffa0e1fc8ea0b0 R09: ffffa0e1fc8ea0b0
> R10: ffffffffffffffff R11: ffffffffc0590940 R12: 0000000000000005
> R13: ffffa0e1ffad2c00 R14: ffffb97940e7bc08 R15: 0000000000000000
> FS: 0000000000000000(0000) GS:ffffa0e1fd100000(006b) knlGS:00000000e5ef7494
> CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
> CR2: 00000000e5eeb82c CR3: 0000000079b06001 CR4: 0000000000360ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> ? preempt_count_add+0x58/0xb0
> ? _raw_spin_lock_irqsave+0x36/0x70
> ? _raw_spin_unlock_irqrestore+0x1a/0x40
> ? __wake_up+0x70/0x190
> virtnet_set_features+0x90/0xf0 [virtio_net]
> __netdev_update_features+0x271/0x980
> ? nlmsg_notify+0x5b/0xa0
> dev_disable_lro+0x2b/0x190
> ? inet_netconf_notify_devconf+0xe2/0x120
> devinet_sysctl_forward+0x176/0x1e0
> proc_sys_call_handler+0x1f0/0x250
> proc_sys_write+0xf/0x20
> __vfs_write+0x3e/0x190
> ? __sb_start_write+0x6d/0xd0
> vfs_write+0xd3/0x190
> ksys_write+0x68/0xd0
> __ia32_sys_write+0x14/0x20
> do_fast_syscall_32+0x86/0xe0
> entry_SYSENTER_compat+0x7c/0x8e
>
> A similar crash will likely trigger when enabling XDP.
>
> Reported-by: Alistair Delva <adelva@google.com>
> Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> Lightly tested.
>
> Alistair, could you please test and confirm that this resolves the
> crash for you?

This patch doesn't work. The reason is that NETIF_F_LRO is also turned
on by TSO4/TSO6, which your patch didn't check for. So it ends up
going through the same path and crashing in the same way.

if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
dev->features |= NETIF_F_LRO;

It sounds like this patch is fixing something slightly differently to
my patch fixed. virtnet_set_features() doesn't care about
GUEST_OFFLOADS, it only tests against NETIF_F_LRO. Even if "offloads"
is zero, it will call virtnet_set_guest_offloads(), which triggers the
crash.

So either we need to ensure NETIF_F_LRO is never set, or
virtnet_set_features needs to be updated to check for GUEST_OFFLOADS,
right?

> Dave, after testing confirms the fix, pls queue up for stable.
>
>
> drivers/net/virtio_net.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4d7d5434cc5d..7b8805b47f0d 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2971,6 +2971,15 @@ static int virtnet_validate(struct virtio_device *vdev)
> if (!virtnet_validate_features(vdev))
> return -EINVAL;
>
> + /* VIRTIO_NET_F_CTRL_GUEST_OFFLOADS does not work without
> + * VIRTIO_NET_F_CTRL_VQ. However the virtio spec does not
> + * specify that VIRTIO_NET_F_CTRL_GUEST_OFFLOADS depends
> + * on VIRTIO_NET_F_CTRL_VQ so devices can set the later but
> + * not the former.
> + */
> + if (!virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> + __virtio_clear_bit(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS);
> +
> if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> int mtu = virtio_cread16(vdev,
> offsetof(struct virtio_net_config,
> --
> MST
>

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