Re: [PATCH v2 2/3] tun: Pad virtio header with zero

Akihiko Odaki
2025-01-09T07:41:56Z

On 2025/01/09 16:31, Michael S. Tsirkin wrote:
> On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote:
>> tun used to simply advance iov_iter when it needs to pad virtio header,
>> which leaves the garbage in the buffer as is. This is especially
>> problematic when tun starts to allow enabling the hash reporting
>> feature; even if the feature is enabled, the packet may lack a hash
>> value and may contain a hole in the virtio header because the packet
>> arrived before the feature gets enabled or does not contain the
>> header fields to be hashed. If the hole is not filled with zero, it is
>> impossible to tell if the packet lacks a hash value.
>>
>> In theory, a user of tun can fill the buffer with zero before calling
>> read() to avoid such a problem, but leaving the garbage in the buffer is
>> awkward anyway so fill the buffer in tun.
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>
> But if the user did it, you have just overwritten his value,
> did you not?

Yes. but that means the user expects some part of buffer is not filled
after read() or recvmsg(). I'm a bit worried that not filling the buffer
may break assumptions others (especially the filesystem and socket
infrastructures in the kernel) may have.

If we are really confident that it will not cause problems, this
behavior can be opt-in based on a flag or we can just write some
documentation warning userspace programmers to initialize the buffer.

>
>> ---
>> drivers/net/tun_vnet.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c
>> index fe842df9e9ef..ffb2186facd3 100644
>> --- a/drivers/net/tun_vnet.c
>> +++ b/drivers/net/tun_vnet.c
>> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>> if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr))
>> return -EFAULT;
>>
>> - iov_iter_advance(iter, sz - sizeof(*hdr));
>> + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
>> + return -EFAULT;
>>
>> return 0;
>> }
>>
>> --
>> 2.47.1
>