The put lowers the reference count to 0 and frees ctx, reading it
afterwards is invalid. Move the put after the uses and determine the
last use by the reference count being 1.

Fixes: 39e940d4abfa ("selftests/xsk: Destroy BPF resources only when ctx refcount drops to 0")
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/testing/selftests/bpf/xsk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/xsk.c b/tools/testing/selftests/bpf/xsk.c
index f2721a4ae7c5..0b3ff49c740d 100644
--- a/tools/testing/selftests/bpf/xsk.c
+++ b/tools/testing/selftests/bpf/xsk.c
@@ -1237,15 +1237,15 @@ void xsk_socket__delete(struct xsk_socket *xsk)
 	ctx = xsk->ctx;
 	umem = ctx->umem;
 
-	xsk_put_ctx(ctx, true);
-
-	if (!ctx->refcount) {
+	if (ctx->refcount == 1) {
 		xsk_delete_bpf_maps(xsk);
 		close(ctx->prog_fd);
 		if (ctx->has_bpf_link)
 			close(ctx->link_fd);
 	}
 
+	xsk_put_ctx(ctx, true);
+
 	err = xsk_get_mmap_offsets(xsk->fd, &off);
 	if (!err) {
 		if (xsk->rx) {
-- 
2.37.2.789.g6183377224-goog

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Thu,  1 Sep 2022 13:26:45 -0700 you wrote:
> The put lowers the reference count to 0 and frees ctx, reading it
> afterwards is invalid. Move the put after the uses and determine the
> last use by the reference count being 1.
> 
> Fixes: 39e940d4abfa ("selftests/xsk: Destroy BPF resources only when ctx refcount drops to 0")
> Signed-off-by: Ian Rogers <irogers@google.com>
> 
> [...]

Here is the summary with links:
  - [v1] selftests/xsk: Avoid use-after-free on ctx
    https://git.kernel.org/bpf/bpf-next/c/af515a5587b8

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

On Thu, Sep 1, 2022 at 10:56 PM Ian Rogers <irogers@google.com> wrote:
>
> The put lowers the reference count to 0 and frees ctx, reading it
> afterwards is invalid. Move the put after the uses and determine the
> last use by the reference count being 1.

Thanks for spotting and fixing this Ian.

Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>

> Fixes: 39e940d4abfa ("selftests/xsk: Destroy BPF resources only when ctx refcount drops to 0")
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/testing/selftests/bpf/xsk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/xsk.c b/tools/testing/selftests/bpf/xsk.c
> index f2721a4ae7c5..0b3ff49c740d 100644
> --- a/tools/testing/selftests/bpf/xsk.c
> +++ b/tools/testing/selftests/bpf/xsk.c
> @@ -1237,15 +1237,15 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>         ctx = xsk->ctx;
>         umem = ctx->umem;
>
> -       xsk_put_ctx(ctx, true);
> -
> -       if (!ctx->refcount) {
> +       if (ctx->refcount == 1) {
>                 xsk_delete_bpf_maps(xsk);
>                 close(ctx->prog_fd);
>                 if (ctx->has_bpf_link)
>                         close(ctx->link_fd);
>         }
>
> +       xsk_put_ctx(ctx, true);
> +
>         err = xsk_get_mmap_offsets(xsk->fd, &off);
>         if (!err) {
>                 if (xsk->rx) {
> --
> 2.37.2.789.g6183377224-goog
>