Re: [PATCH net v2] net: devmem: reject dma-buf bind with non-page-aligned size or SG length

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Tue, May 19, 2026 at 09:35:30PM +0100, David Carlier wrote:
> net_devmem_bind_dmabuf() trusts dmabuf->size and sg_dma_len() to be
> PAGE_SIZE multiples without checking:
> 
>   - tx_vec is sized dmabuf->size / PAGE_SIZE, and
>     net_devmem_get_niov_at() only bounds-checks virt_addr < dmabuf->size
>     before indexing tx_vec[virt_addr / PAGE_SIZE]. With size =
>     N*PAGE_SIZE + r (1 <= r < PAGE_SIZE), sendmsg() at iov_base =
>     N*PAGE_SIZE passes the bound check and reads tx_vec[N] -- one past.
> 
>   - owner->area.num_niovs = len / PAGE_SIZE while gen_pool_add_owner()
>     covers the full byte len, so a non-page-multiple non-final sg
>     desyncs num_niovs from the gen_pool region for every later sg, on
>     both RX and TX.
> 
> dma-buf does not require page-aligned sizes, so the bind path has to
> enforce what its own indexing assumes. Reject both with -EINVAL.
> 
> The size check is TX-only (only tx_vec is sized off dmabuf->size); the
> SG-length check covers both directions.
> 
> Fixes: bd61848900bf ("net: devmem: Implement TX path")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
> Changes in v2:
>   - Reframe commit message around the kernel-side OOB instead of
>     "real exporters already page-align", which read as the OOB being
>     unreachable and undercut Cc: stable (Stanislav Fomichev).
>   - Hoist the SG-length check out of the if (TX) branch so it covers
>     RX too; RX has the same num_niovs / gen_pool desync on a
>     contract-violating exporter, just without an OOB. Keep the
>     size-multiple check TX-only (Stanislav Fomichev).
>   - Drop bool todevice; compare direction == DMA_TO_DEVICE inline to
>     match the existing call site at the tx_vec[] assignment
>     (Bobby Eshleman).
> 
>  net/core/devmem.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 468344739db2..4f71de44c0fb 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -241,6 +241,11 @@ net_devmem_bind_dmabuf(struct net_device *dev,
>  	}
>  
>  	if (direction == DMA_TO_DEVICE) {
> +		if (!IS_ALIGNED(dmabuf->size, PAGE_SIZE)) {
> +			err = -EINVAL;
> +			NL_SET_ERR_MSG(extack, "TX dma-buf size must be a multiple of PAGE_SIZE");
> +			goto err_unmap;
> +		}
>  		binding->tx_vec = kvmalloc_objs(struct net_iov *,
>  						dmabuf->size / PAGE_SIZE);
>  		if (!binding->tx_vec) {
> @@ -267,6 +272,12 @@ net_devmem_bind_dmabuf(struct net_device *dev,
>  		size_t len = sg_dma_len(sg);
>  		struct net_iov *niov;
>  
> +		if (!IS_ALIGNED(len, PAGE_SIZE)) {
> +			err = -EINVAL;
> +			NL_SET_ERR_MSG(extack, "dma-buf SG length must be PAGE_SIZE aligned");
> +			goto err_free_chunks;
> +		}
> +
>  		owner = kzalloc_node(sizeof(*owner), GFP_KERNEL,
>  				     dev_to_node(&dev->dev));
>  		if (!owner) {
> -- 
> 2.53.0

LGTM.

Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>