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

From: Magnus Karlsson <magnus.karlsson@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	Networking <netdev@vger.kernel.org>,
	"Jonathan Lemon" <jonathan.lemon@gmail.com>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf 2/2] libbpf: fix possible use after free in xsk_socket__delete
Date: Wed, 4 Nov 2020 09:26:57 +0100	[thread overview]
Message-ID: <CAJ8uoz2Cqtw0gPpuyk79z4Rt8dYLmxd9DsSeAB4fQFJWMHLHVw@mail.gmail.com> (raw)
In-Reply-To: <CAEf4Bzah-7akFkjUAJR=ovXLAnLd6EvLMMOy+GBbc4R28TY-eg@mail.gmail.com>

On Tue, Nov 3, 2020 at 8:05 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> 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?

Yes, it would for that statement, but I still need the umem pointer
for the statement below. And this statement of potentially closing the
fd needs to be after xsk_put_ctx(). So we might as well keep
ujmem->refcount-- where it is, if that is ok with you?

> >         /* 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
> >