Subject: Re: [PATCH] vhost_net: initialize rx_ring in vhost_net_open()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Mar 2018 17:33:34 +0200

On Thu, Mar 08, 2018 at 02:37:17PM +0100, Alexander Potapenko wrote:
> KMSAN reported a use of uninit memory in vhost_net_buf_unproduce()
> while trying to access n->vqs[VHOST_NET_VQ_TX].rx_ring:
> 
> ==================================================================
> BUG: KMSAN: use of uninitialized memory in vhost_net_buf_unproduce+0x7bb/0x9a0 drivers/vho
> et.c:170
> CPU: 0 PID: 3021 Comm: syz-fuzzer Not tainted 4.16.0-rc4+ #3853
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x185/0x1d0 lib/dump_stack.c:53
>  kmsan_report+0x142/0x1f0 mm/kmsan/kmsan.c:1093
>  __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:676
>  vhost_net_buf_unproduce+0x7bb/0x9a0 drivers/vhost/net.c:170
>  vhost_net_stop_vq drivers/vhost/net.c:974 [inline]
>  vhost_net_stop+0x146/0x380 drivers/vhost/net.c:982
>  vhost_net_release+0xb1/0x4f0 drivers/vhost/net.c:1015
>  __fput+0x49f/0xa00 fs/file_table.c:209
>  ____fput+0x37/0x40 fs/file_table.c:243
>  task_work_run+0x243/0x2c0 kernel/task_work.c:113
>  tracehook_notify_resume include/linux/tracehook.h:191 [inline]
>  exit_to_usermode_loop arch/x86/entry/common.c:166 [inline]
>  prepare_exit_to_usermode+0x349/0x3b0 arch/x86/entry/common.c:196
>  syscall_return_slowpath+0xf3/0x6d0 arch/x86/entry/common.c:265
>  do_syscall_64+0x34d/0x450 arch/x86/entry/common.c:292
> ...
> origin:
>  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:303 [inline]
>  kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:213
>  kmsan_kmalloc_large+0x6f/0xd0 mm/kmsan/kmsan.c:392
>  kmalloc_large_node_hook mm/slub.c:1366 [inline]
>  kmalloc_large_node mm/slub.c:3808 [inline]
>  __kmalloc_node+0x100e/0x1290 mm/slub.c:3818
>  kmalloc_node include/linux/slab.h:554 [inline]
>  kvmalloc_node+0x1a5/0x2e0 mm/util.c:419
>  kvmalloc include/linux/mm.h:541 [inline]
>  vhost_net_open+0x64/0x5f0 drivers/vhost/net.c:921
>  misc_open+0x7b5/0x8b0 drivers/char/misc.c:154
>  chrdev_open+0xc28/0xd90 fs/char_dev.c:417
>  do_dentry_open+0xccb/0x1430 fs/open.c:752
>  vfs_open+0x272/0x2e0 fs/open.c:866
>  do_last fs/namei.c:3378 [inline]
>  path_openat+0x49ad/0x6580 fs/namei.c:3519
>  do_filp_open+0x267/0x640 fs/namei.c:3553
>  do_sys_open+0x6ad/0x9c0 fs/open.c:1059
>  SYSC_openat+0xc7/0xe0 fs/open.c:1086
>  SyS_openat+0x63/0x90 fs/open.c:1080
>  do_syscall_64+0x2f1/0x450 arch/x86/entry/common.c:287
> ==================================================================
> 
> Signed-off-by: Alexander Potapenko <[email-masked]>
> ---
>  drivers/vhost/net.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 610cba276d47..60f1080bffc7 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -948,6 +948,7 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  		n->vqs[i].done_idx = 0;
>  		n->vqs[i].vhost_hlen = 0;
>  		n->vqs[i].sock_hlen = 0;
> +		n->vqs[i].rx_ring = NULL;
>  		vhost_net_buf_init(&n->vqs[i].rxq);
>  	}
>  	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
> -- 
> 2.16.2.395.g2e18187dfd-goog

I suspect that's not sufficient. rx ring is tied to the tap device.
I think we need to drop it every time we drop the device.