Re: [PATCH 5.15 015/110] vhost: fix hung thread due to erroneous iotlb entries

[Anirudh Rayabharam <mail@anirudhrb.com>]

Mon, Mar 14, 2022 at 12:53:17PM +0100, Greg Kroah-Hartman wrote:
> From: Anirudh Rayabharam <mail@anirudhrb.com>
> 
> [ Upstream commit e2ae38cf3d91837a493cb2093c87700ff3cbe667 ]

This breaks batching of IOTLB messages. [1] fixes it but hasn't landed in
Linus' tree yet.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/commit/?h=linux-next&id=95932ab2ea07b79cdb33121e2f40ccda9e6a73b5

	- Anirudh.
> 
> In vhost_iotlb_add_range_ctx(), range size can overflow to 0 when
> start is 0 and last is ULONG_MAX. One instance where it can happen
> is when userspace sends an IOTLB message with iova=size=uaddr=0
> (vhost_process_iotlb_msg). So, an entry with size = 0, start = 0,
> last = ULONG_MAX ends up in the iotlb. Next time a packet is sent,
> iotlb_access_ok() loops indefinitely due to that erroneous entry.
> 
> 	Call Trace:
> 	 <TASK>
> 	 iotlb_access_ok+0x21b/0x3e0 drivers/vhost/vhost.c:1340
> 	 vq_meta_prefetch+0xbc/0x280 drivers/vhost/vhost.c:1366
> 	 vhost_transport_do_send_pkt+0xe0/0xfd0 drivers/vhost/vsock.c:104
> 	 vhost_worker+0x23d/0x3d0 drivers/vhost/vhost.c:372
> 	 kthread+0x2e9/0x3a0 kernel/kthread.c:377
> 	 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> 	 </TASK>
> 
> Reported by syzbot at:
> 	https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87
> 
> To fix this, do two things:
> 
> 1. Return -EINVAL in vhost_chr_write_iter() when userspace asks to map
>    a range with size 0.
> 2. Fix vhost_iotlb_add_range_ctx() to handle the range [0, ULONG_MAX]
>    by splitting it into two entries.
> 
> Fixes: 0bbe30668d89e ("vhost: factor out IOTLB")
> Reported-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> Tested-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
> Link: https://lore.kernel.org/r/20220305095525.5145-1-mail@anirudhrb.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  drivers/vhost/iotlb.c | 11 +++++++++++
>  drivers/vhost/vhost.c |  5 +++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/drivers/vhost/iotlb.c b/drivers/vhost/iotlb.c
> index 670d56c879e5..40b098320b2a 100644
> --- a/drivers/vhost/iotlb.c
> +++ b/drivers/vhost/iotlb.c
> @@ -57,6 +57,17 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
>  	if (last < start)
>  		return -EFAULT;
>  
> +	/* If the range being mapped is [0, ULONG_MAX], split it into two entries
> +	 * otherwise its size would overflow u64.
> +	 */
> +	if (start == 0 && last == ULONG_MAX) {
> +		u64 mid = last / 2;
> +
> +		vhost_iotlb_add_range_ctx(iotlb, start, mid, addr, perm, opaque);
> +		addr += mid + 1;
> +		start = mid + 1;
> +	}
> +
>  	if (iotlb->limit &&
>  	    iotlb->nmaps == iotlb->limit &&
>  	    iotlb->flags & VHOST_IOTLB_FLAG_RETIRE) {
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 59edb5a1ffe2..55475fd59fb7 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1170,6 +1170,11 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
>  		goto done;
>  	}
>  
> +	if (msg.size == 0) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +
>  	if (dev->msg_handler)
>  		ret = dev->msg_handler(dev, &msg);
>  	else
> -- 
> 2.34.1
> 
> 
>