vhost-vdpa: don't install the eventfd_ctx_fdget() error in config_ctx

vhost_vdpa_set_config_call() swaps the eventfd_ctx_fdget() return value
into v->config_ctx before checking it, so on failure the field briefly
holds an ERR_PTR:

	ctx = fd == VHOST_FILE_UNBIND ? NULL : eventfd_ctx_fdget(fd);
	swap(ctx, v->config_ctx);

	if (!IS_ERR_OR_NULL(ctx))
		eventfd_ctx_put(ctx);

	if (IS_ERR(v->config_ctx)) {
		long ret = PTR_ERR(v->config_ctx);

		v->config_ctx = NULL;
		return ret;
	}

Commit 0bde59c1723a ("vhost-vdpa: set v->config_ctx to NULL if
eventfd_ctx_fdget() fails") added that clearing, and spelled out the
invariant the rest of the file relies on: "we consider 'v->config_ctx'
valid if it is not NULL".  The window between the swap and the clearing
still breaks it.  vhost_vdpa_config_cb() only tests for NULL, so a config
interrupt delivered inside the window hands the ERR_PTR to
eventfd_signal().

Check the fd before installing it instead.  That closes the window and
matches how vhost_vring_ioctl() handles the same failure for the vq call
fd.

It also stops a rejected fd from tearing down a config interrupt that was
working: until now the swap replaced the live context and put it, so
after an EBADF the device silently stopped delivering config interrupts
until userspace installed a new fd.

Fixes: 776f395004d8 ("vhost_vdpa: Support config interrupt in vdpa")
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260807100025.19750-2-yuz08559@gmail.com>

1 files changed, 4 insertions, 8 deletions

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 4eb1eb5e5c79..3e5165b7c094 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -546,18 +546,14 @@ static long vhost_vdpa_set_config_call(struct vhost_vdpa *v, u32 __user *argp)
 		return  -EFAULT;
 
 	ctx = fd == VHOST_FILE_UNBIND ? NULL : eventfd_ctx_fdget(fd);
+	if (IS_ERR(ctx))
+		return PTR_ERR(ctx);
+
 	swap(ctx, v->config_ctx);
 
-	if (!IS_ERR_OR_NULL(ctx))
+	if (ctx)
 		eventfd_ctx_put(ctx);
 
-	if (IS_ERR(v->config_ctx)) {
-		long ret = PTR_ERR(v->config_ctx);
-
-		v->config_ctx = NULL;
-		return ret;
-	}
-
 	v->vdpa->config->set_config_cb(v->vdpa, &cb);
 
 	return 0;