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

Jan Kara
2025-01-09T10:37:52Z

On Thu 09-01-25 18:36:52, Akihiko Odaki wrote:
> On 2025/01/09 16:43, Michael S. Tsirkin wrote:
> > On Thu, Jan 09, 2025 at 04:41:50PM +0900, Akihiko Odaki wrote:
> > > 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.
> >
> > It's been like this for years, I'd say we wait until we know there's a problem?
>
> Perhaps we can just leave it as is. Let me ask filesystem and networking
> people:
>
> Is it OK to leave some part of buffer uninitialized with read_iter() or
> recvmsg()?

I think that leaving part of the IO buffer within returned IO length
uninitialized is a very bad practice and I'm not aware of any place in
filesystem area that would do that. It makes life unnecessarily harder
for userspace and also it is invitation for subtle information leaks
(depending on who allocates the buffer and who then gets to read the
results). So I think the patch makes sense.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR