Re: [PATCH v6 02/13] iov_iter: add iterator type for dmabuf maps

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] iov_iter: silent fallback to iterate_discard() for dmabuf maps
- [Medium] iov_iter: bounds check bypass in iov_iter_revert() for dmabuf maps
--

commit 8e47b333dea94c1451175bac8e284adfbbf175cc
Author: Pavel Begunkov <asml.silence@gmail.com>
Subject: iov_iter: add iterator type for dmabuf maps

Introduce a new ITER_DMABUF_MAP iterator type for dmabuf maps. The patch 
adds the new iterator type, updates struct iov_iter to hold dmabuf maps, 
and wires up basic iterator operations like advance, revert, and alignment.

> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index fe2e985d74d24..638c1116e912e 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
[ ... ]
> @@ -29,6 +30,7 @@ enum iter_type {
>  	ITER_FOLIOQ,
>  	ITER_XARRAY,
>  	ITER_DISCARD,
> +	ITER_DMABUF_MAP,
>  };

[Severity: Medium]
With the addition of ITER_DMABUF_MAP here, do iterate_and_advance2() and 
iterate_and_advance_kernel() in include/linux/iov_iter.h also need to be 
updated?

It appears they currently lack a type check for ITER_DMABUF_MAP, which 
might cause operations like copy_to_iter() or copy_from_iter() to silently 
fall through to iterate_discard(). If any middle layer attempts to read or 
write using this iterator type, the operation would drop data and return 
success without reporting an error.

[ ... ]
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 2072c04e99d05..6831a5d9396e1 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
[ ... ]
> @@ -775,6 +777,20 @@ void iov_iter_xarray(struct iov_iter *i, unsigned int direction,
>  }
>  EXPORT_SYMBOL(iov_iter_xarray);
>  
> +void iov_iter_dmabuf_map(struct iov_iter *i, unsigned int direction,
> +			 struct dma_buf_io_map *map,
> +			 loff_t off, size_t count)
> +{
> +	WARN_ON(direction & ~(READ | WRITE));
> +	*i = (struct iov_iter){
> +		.iter_type = ITER_DMABUF_MAP,
> +		.data_source = direction,
> +		.dmabuf_map = map,
> +		.count = count,
> +		.iov_offset = off,
> +	};
> +}

[Severity: Medium]
Since iov_offset is initialized here to the absolute starting offset (off) 
rather than a relative zero, does this cause an issue with the tracking 
logic in iov_iter_revert()?

In iov_iter_revert(), the unroll size is checked against i->iov_offset 
like this:

	if (unroll <= i->iov_offset) {
		i->iov_offset -= unroll;
		return;
	}

Because iov_offset starts at the absolute offset rather than 0, couldn't a 
caller successfully revert an ITER_DMABUF_MAP iterator beyond its initial 
starting offset? This would bypass the BUG() safety net intended to catch 
invalid unrolls, potentially leading to an out-of-bounds DMA access if the 
reverted iterator is used again.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1789997898.git.asml.silence@gmail.com?part=2