On Tue, Nov 3, 2020 at 8:05 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Nov 3, 2020 at 1:42 AM Magnus Karlsson
> <magnus.karlsson@xxxxxxxxx> wrote:
> >
> > From: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > ---
> > 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
> >
