RE: [PATCH bpf] xsk: fix number of pinned pages/umem size discrepancy

["Loftus, Ciara" <ciara.loftus@intel.com>]

> 
> From: Björn Töpel <bjorn.topel@intel.com>
> 
> For AF_XDP sockets, there was a discrepancy between the number of of
> pinned pages and the size of the umem region.
> 
> The size of the umem region is used to validate the AF_XDP descriptor
> addresses. The logic that pinned the pages covered by the region only
> took whole pages into consideration, creating a mismatch between the
> size and pinned pages. A user could then pass AF_XDP addresses outside
> the range of pinned pages, but still within the size of the region,
> crashing the kernel.
> 
> This change correctly calculates the number of pages to be
> pinned. Further, the size check for the aligned mode is
> simplified. Now the code simply checks if the size is divisible by the
> chunk size.
> 
> Fixes: bbff2f321a86 ("xsk: new descriptor addressing scheme")
> Reported-by: Ciara Loftus <ciara.loftus@intel.com>
> Signed-off-by: Björn Töpel <bjorn.topel@intel.com>

Thanks for the patch Björn.

Tested-by: Ciara Loftus <ciara.loftus@intel.com>

> ---
>  net/xdp/xdp_umem.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index e97db37354e4..b010bfde0149 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -303,10 +303,10 @@ static int xdp_umem_account_pages(struct
> xdp_umem *umem)
> 
>  static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg
> *mr)
>  {
> +	u32 npgs_rem, chunk_size = mr->chunk_size, headroom = mr-
> >headroom;
>  	bool unaligned_chunks = mr->flags &
> XDP_UMEM_UNALIGNED_CHUNK_FLAG;
> -	u32 chunk_size = mr->chunk_size, headroom = mr->headroom;
>  	u64 npgs, addr = mr->addr, size = mr->len;
> -	unsigned int chunks, chunks_per_page;
> +	unsigned int chunks, chunks_rem;
>  	int err;
> 
>  	if (chunk_size < XDP_UMEM_MIN_CHUNK_SIZE || chunk_size >
> PAGE_SIZE) {
> @@ -336,19 +336,18 @@ static int xdp_umem_reg(struct xdp_umem
> *umem, struct xdp_umem_reg *mr)
>  	if ((addr + size) < addr)
>  		return -EINVAL;
> 
> -	npgs = size >> PAGE_SHIFT;
> +	npgs = div_u64_rem(size, PAGE_SIZE, &npgs_rem);
> +	if (npgs_rem)
> +		npgs++;
>  	if (npgs > U32_MAX)
>  		return -EINVAL;
> 
> -	chunks = (unsigned int)div_u64(size, chunk_size);
> +	chunks = (unsigned int)div_u64_rem(size, chunk_size,
> &chunks_rem);
>  	if (chunks == 0)
>  		return -EINVAL;
> 
> -	if (!unaligned_chunks) {
> -		chunks_per_page = PAGE_SIZE / chunk_size;
> -		if (chunks < chunks_per_page || chunks %
> chunks_per_page)
> -			return -EINVAL;
> -	}
> +	if (!unaligned_chunks && chunks_rem)
> +		return -EINVAL;
> 
>  	if (headroom >= chunk_size - XDP_PACKET_HEADROOM)
>  		return -EINVAL;
> 
> base-commit: 746f534a4809e07f427f7d13d10f3a6a9641e5c3
> --
> 2.25.1