Re: [PATCH bpf 2/2] libbpf: fix possible use after free in xsk_socket__delete

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Nov 3, 2020 at 1:42 AM Magnus Karlsson
<magnus.karlsson@gmail.com> wrote:
>
> From: Magnus Karlsson <magnus.karlsson@intel.com>
>
> Fix a possible use after free in xsk_socket__delete that will happen
> if xsk_put_ctx() frees the ctx. To fix, save the umem reference taken
> from the context and just use that instead.
>
> Fixes: 2f6324a3937f ("libbpf: Support shared umems between queues and devices")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
> ---
>  tools/lib/bpf/xsk.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 504b7a8..9bc537d 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -892,6 +892,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>  {
>         size_t desc_sz = sizeof(struct xdp_desc);
>         struct xdp_mmap_offsets off;
> +       struct xsk_umem *umem;
>         struct xsk_ctx *ctx;
>         int err;
>
> @@ -899,6 +900,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>                 return;
>
>         ctx = xsk->ctx;
> +       umem = ctx->umem;
>         if (ctx->prog_fd != -1) {
>                 xsk_delete_bpf_maps(xsk);
>                 close(ctx->prog_fd);
> @@ -918,11 +920,11 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>
>         xsk_put_ctx(ctx);
>
> -       ctx->umem->refcount--;
> +       umem->refcount--;

if you moved ctx->umem->refcount--; to before xdk_put_ctx(ctx), would
that also work?

>         /* Do not close an fd that also has an associated umem connected
>          * to it.
>          */
> -       if (xsk->fd != ctx->umem->fd)
> +       if (xsk->fd != umem->fd)
>                 close(xsk->fd);
>         free(xsk);
>  }
> --
> 2.7.4
>