Re: [PATCH 0/5] Rust support for `struct iov_iter`

[Greg Kroah-Hartman]

On Tue, Mar 18, 2025 at 09:57:55PM +0100, Andreas Hindborg wrote:
> "Greg Kroah-Hartman" <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Tue, Mar 11, 2025 at 02:25:11PM +0000, Alice Ryhl wrote:
> >> This series adds support for the `struct iov_iter` type. This type
> >> represents an IO buffer for reading or writing, and can be configured
> >> for either direction of communication.
> >>
> >> In Rust, we define separate types for reading and writing. This will
> >> ensure that you cannot mix them up and e.g. call copy_from_iter in a
> >> read_iter syscall.
> >>
> >> To use the new abstractions, miscdevices are given new methods read_iter
> >> and write_iter that can be used to implement the read/write syscalls on
> >> a miscdevice. The miscdevice sample is updated to provide read/write
> >> operations.
> >
> > Nice, this is good to have, but what's the odds of tieing in the
> > "untrusted buffer" logic here so that all misc drivers HAVE to properly
> > validate the data sent to them before they can touch it:
> > 	https://lore.kernel.org/r/20240925205244.873020-1-benno.lossin@xxxxxxxxx
> >
> > I'd like to force drivers to do this, otherwise it's just going to force
> > us to audit all paths from userspace->kernel that happen.
> >
> 
> I think that for user backed iterators (`user_backed_iter(iter) != 0`)
> we will have the same problems as discussed in [1]. To validate, we
> would have to copy the data to another buffer and then validate it
> there, in a race free place. But the copying is apparently a problem.

We already copy all data first, that's not an issue.  Validate it after
it has been copied before you do something with it, just like we do
today for normal ioctl C code.  Same goes for data coming from hardware,
it's already been copied into a buffer that you can use, no need to copy
it again, just "validate" it before using it.

thanks,

greg k-h