libbpf: Fix possible use after free in xsk_socket__delete

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: 2f6324a393 ("libbpf: Support shared umems between queues and devices")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1604396490-12129-3-git-send-email-magnus.karlsson@gmail.com

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--;
 	/* 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);
 }