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Date: Wed, 4 Nov 2020 09:26:57 +0100 From: Magnus Karlsson <magnus.karlsson@...il.com> To: Andrii Nakryiko <andrii.nakryiko@...il.com> Cc: Magnus Karlsson <magnus.karlsson@...el.com>, Björn Töpel <bjorn.topel@...el.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Networking <netdev@...r.kernel.org>, Jonathan Lemon <jonathan.lemon@...il.com>, bpf <bpf@...r.kernel.org> Subject: Re: [PATCH bpf 2/2] libbpf: fix possible use after free in xsk_socket__delete On Tue, Nov 3, 2020 at 8:05 PM Andrii Nakryiko <andrii.nakryiko@...il.com> wrote: > > On Tue, Nov 3, 2020 at 1:42 AM Magnus Karlsson > <magnus.karlsson@...il.com> wrote: > > > > From: Magnus Karlsson <magnus.karlsson@...el.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@...el.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 > >
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