Re: [PATCH] xsk: Add missing overflow check in xdp_umem_reg

[Alexander Lobakin]

From: Kal Conley <kal.conley@xxxxxxxxxxx>
Date: Tue,  7 Mar 2023 18:23:06 +0100

> The number of chunks can overflow u32. Make sure to return -EINVAL on
> overflow.
> 
> Fixes: bbff2f321a86 ("xsk: new descriptor addressing scheme")
> Signed-off-by: Kal Conley <kal.conley@xxxxxxxxxxx>
> ---
>  net/xdp/xdp_umem.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index 4681e8e8ad94..f1aa79018ce8 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -150,10 +150,11 @@ 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;
> +	u32 chunk_size = mr->chunk_size, headroom = mr->headroom;
>  	bool unaligned_chunks = mr->flags & XDP_UMEM_UNALIGNED_CHUNK_FLAG;
> -	u64 npgs, addr = mr->addr, size = mr->len;
> -	unsigned int chunks, chunks_rem;
> +	u64 addr = mr->addr, size = mr->len;
> +	u64 chunks, npgs;
> +	u32 chunks_rem, npgs_rem;

The RCT declaration style is messed up in the whole block. Please move
lines around, there's nothing wrong in that.

>  	int err;
>  
>  	if (chunk_size < XDP_UMEM_MIN_CHUNK_SIZE || chunk_size > PAGE_SIZE) {
> @@ -188,8 +189,8 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
>  	if (npgs > U32_MAX)
>  		return -EINVAL;
>  
> -	chunks = (unsigned int)div_u64_rem(size, chunk_size, &chunks_rem);
> -	if (chunks == 0)
> +	chunks = div_u64_rem(size, chunk_size, &chunks_rem);
> +	if (chunks == 0 || chunks > U32_MAX)

You can change the first cond to `!chunks` while at it, it's more
preferred than `== 0`.

>  		return -EINVAL;

Do you have any particular bugs that the current code leads to? Or it's
just something that might hypothetically happen?

>  
>  	if (!unaligned_chunks && chunks_rem)
> @@ -201,7 +202,7 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
>  	umem->size = size;
>  	umem->headroom = headroom;
>  	umem->chunk_size = chunk_size;
> -	umem->chunks = chunks;
> +	umem->chunks = (u32)chunks;

You already checked @chunks fits into 32 bits, so the cast can be
omitted here, it's redundant.

>  	umem->npgs = (u32)npgs;
>  	umem->pgs = NULL;
>  	umem->user = NULL;

Thanks,
Olek