[PATCH v2] tap-win32: fix multiple tap support

[PATCH v2] tap-win32: fix multiple tap support

[Gal Horowitz]

Currently when more than one tap is created on Windows, QEMU immediately
crashes with a null-deref since the code incorrectly uses a static global
for the tap state.

Instead, this patch allocates a structure for each tap at startup.

Signed-off-by: Gal Horowitz <galush.horowitz@gmail.com>
---
Changes in v2:
- Add cleanup of the structure fields
- Terminate the thread before freeing the structure
- Link to v1: https://lore.kernel.org/qemu-devel/20250920-fix-win32-multiple-taps-v1-1-bee41dcc213d@gmail.com
---
 net/tap-win32.c | 44 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/net/tap-win32.c b/net/tap-win32.c
index 38baf90e0b3f121f74eb32f1bff779c84ce03114..1b83423191c5a6f248c771d5eb5582cc80e8abcb 100644
--- a/net/tap-win32.c
+++ b/net/tap-win32.c
@@ -104,6 +104,7 @@ typedef struct tap_win32_overlapped {
     HANDLE output_queue_semaphore;
     HANDLE free_list_semaphore;
     HANDLE tap_semaphore;
+    HANDLE thread_handle;
     CRITICAL_SECTION output_queue_cs;
     CRITICAL_SECTION free_list_cs;
     OVERLAPPED read_overlapped;
@@ -114,8 +115,6 @@ typedef struct tap_win32_overlapped {
     tun_buffer_t* output_queue_back;
 } tap_win32_overlapped_t;
 
-static tap_win32_overlapped_t tap_overlapped;
-
 static tun_buffer_t* get_buffer_from_free_list(tap_win32_overlapped_t* const overlapped)
 {
     tun_buffer_t* buffer = NULL;
@@ -402,8 +401,10 @@ static int tap_win32_set_status(HANDLE handle, int status)
                 &status, sizeof (status), &len, NULL);
 }
 
-static void tap_win32_overlapped_init(tap_win32_overlapped_t* const overlapped, const HANDLE handle)
+static tap_win32_overlapped_t *tap_win32_overlapped_new(const HANDLE handle)
 {
+    tap_win32_overlapped_t *overlapped = g_new0(tap_win32_overlapped_t, 1);
+
     overlapped->handle = handle;
 
     overlapped->read_event = CreateEvent(NULL, FALSE, FALSE, NULL);
@@ -454,6 +455,8 @@ static void tap_win32_overlapped_init(tap_win32_overlapped_t* const overlapped,
     overlapped->tap_semaphore = CreateSemaphore(NULL, 0, TUN_MAX_BUFFER_COUNT, NULL);
     if(!overlapped->tap_semaphore)
         fprintf(stderr, "error creating tap_semaphore.\n");
+
+    return overlapped;
 }
 
 static int tap_win32_write(tap_win32_overlapped_t *overlapped,
@@ -604,7 +607,7 @@ static int tap_win32_open(tap_win32_overlapped_t **phandle,
         unsigned long debug;
     } version;
     DWORD version_len;
-    DWORD idThread;
+    tap_win32_overlapped_t *tap_overlapped = NULL;
 
     if (preferred_name != NULL) {
         snprintf(name_buffer, sizeof(name_buffer), "%s", preferred_name);
@@ -645,15 +648,35 @@ static int tap_win32_open(tap_win32_overlapped_t **phandle,
         return -1;
     }
 
-    tap_win32_overlapped_init(&tap_overlapped, handle);
+    tap_overlapped = tap_win32_overlapped_new(handle);
 
-    *phandle = &tap_overlapped;
+    tap_overlapped->thread_handle = CreateThread(NULL, 0,
+        tap_win32_thread_entry, (LPVOID)tap_overlapped, 0, NULL);
+
+    *phandle = tap_overlapped;
 
-    CreateThread(NULL, 0, tap_win32_thread_entry,
-                 (LPVOID)&tap_overlapped, 0, &idThread);
     return 0;
 }
 
+static void tap_win32_close(tap_win32_overlapped_t *overlapped)
+{
+    TerminateThread(overlapped->thread_handle, 0);
+
+    CloseHandle(overlapped->tap_semaphore);
+    CloseHandle(overlapped->free_list_semaphore);
+    CloseHandle(overlapped->output_queue_semaphore);
+
+    DeleteCriticalSection(&overlapped->free_list_cs);
+    DeleteCriticalSection(&overlapped->output_queue_cs);
+
+    CloseHandle(overlapped->write_event);
+    CloseHandle(overlapped->read_event);
+
+    CloseHandle(overlapped->handle);
+
+    g_free(overlapped);
+}
+
 /********************************************/
 
  typedef struct TAPState {
@@ -667,9 +690,8 @@ static void tap_cleanup(NetClientState *nc)
 
     qemu_del_wait_object(s->handle->tap_semaphore, NULL, NULL);
 
-    /* FIXME: need to kill thread and close file handle:
-       tap_win32_close(s);
-    */
+    tap_win32_close(s->handle);
+    s->handle = NULL;
 }
 
 static ssize_t tap_receive(NetClientState *nc, const uint8_t *buf, size_t size)

---
base-commit: ab8008b231e758e03c87c1c483c03afdd9c02e19
change-id: 20250920-fix-win32-multiple-taps-ed16ccefbd17

Best regards,
-- 
Gal Horowitz <galush.horowitz@gmail.com>

Re: [PATCH v2] tap-win32: fix multiple tap support

[Philippe Mathieu-Daudé]

Hi,

On 23/9/25 21:42, Gal Horowitz wrote:
> Currently when more than one tap is created on Windows, QEMU immediately
> crashes with a null-deref since the code incorrectly uses a static global
> for the tap state.
> 
> Instead, this patch allocates a structure for each tap at startup.
> 
> Signed-off-by: Gal Horowitz <galush.horowitz@gmail.com>
> ---
> Changes in v2:
> - Add cleanup of the structure fields
> - Terminate the thread before freeing the structure
> - Link to v1: https://lore.kernel.org/qemu-devel/20250920-fix-win32-multiple-taps-v1-1-bee41dcc213d@gmail.com
> ---
>   net/tap-win32.c | 44 +++++++++++++++++++++++++++++++++-----------
>   1 file changed, 33 insertions(+), 11 deletions(-)
> 
> diff --git a/net/tap-win32.c b/net/tap-win32.c
> index 38baf90e0b3f121f74eb32f1bff779c84ce03114..1b83423191c5a6f248c771d5eb5582cc80e8abcb 100644
> --- a/net/tap-win32.c
> +++ b/net/tap-win32.c
> @@ -104,6 +104,7 @@ typedef struct tap_win32_overlapped {
>       HANDLE output_queue_semaphore;
>       HANDLE free_list_semaphore;
>       HANDLE tap_semaphore;
> +    HANDLE thread_handle;
>       CRITICAL_SECTION output_queue_cs;
>       CRITICAL_SECTION free_list_cs;
>       OVERLAPPED read_overlapped;
> @@ -114,8 +115,6 @@ typedef struct tap_win32_overlapped {
>       tun_buffer_t* output_queue_back;
>   } tap_win32_overlapped_t;
>   
> -static tap_win32_overlapped_t tap_overlapped;
> -
>   static tun_buffer_t* get_buffer_from_free_list(tap_win32_overlapped_t* const overlapped)
>   {
>       tun_buffer_t* buffer = NULL;
> @@ -402,8 +401,10 @@ static int tap_win32_set_status(HANDLE handle, int status)
>                   &status, sizeof (status), &len, NULL);
>   }
>   
> -static void tap_win32_overlapped_init(tap_win32_overlapped_t* const overlapped, const HANDLE handle)
> +static tap_win32_overlapped_t *tap_win32_overlapped_new(const HANDLE handle)
>   {
> +    tap_win32_overlapped_t *overlapped = g_new0(tap_win32_overlapped_t, 1);
> +
>       overlapped->handle = handle;
>   
>       overlapped->read_event = CreateEvent(NULL, FALSE, FALSE, NULL);
> @@ -454,6 +455,8 @@ static void tap_win32_overlapped_init(tap_win32_overlapped_t* const overlapped,
>       overlapped->tap_semaphore = CreateSemaphore(NULL, 0, TUN_MAX_BUFFER_COUNT, NULL);
>       if(!overlapped->tap_semaphore)
>           fprintf(stderr, "error creating tap_semaphore.\n");
> +
> +    return overlapped;
>   }
>   
>   static int tap_win32_write(tap_win32_overlapped_t *overlapped,
> @@ -604,7 +607,7 @@ static int tap_win32_open(tap_win32_overlapped_t **phandle,
>           unsigned long debug;
>       } version;
>       DWORD version_len;
> -    DWORD idThread;
> +    tap_win32_overlapped_t *tap_overlapped = NULL;
>   
>       if (preferred_name != NULL) {
>           snprintf(name_buffer, sizeof(name_buffer), "%s", preferred_name);
> @@ -645,15 +648,35 @@ static int tap_win32_open(tap_win32_overlapped_t **phandle,
>           return -1;
>       }
>   
> -    tap_win32_overlapped_init(&tap_overlapped, handle);
> +    tap_overlapped = tap_win32_overlapped_new(handle);
>   
> -    *phandle = &tap_overlapped;
> +    tap_overlapped->thread_handle = CreateThread(NULL, 0,
> +        tap_win32_thread_entry, (LPVOID)tap_overlapped, 0, NULL);
> +
> +    *phandle = tap_overlapped;
>   
> -    CreateThread(NULL, 0, tap_win32_thread_entry,
> -                 (LPVOID)&tap_overlapped, 0, &idThread);
>       return 0;
>   }
>   
> +static void tap_win32_close(tap_win32_overlapped_t *overlapped)
> +{
> +    TerminateThread(overlapped->thread_handle, 0);
> +
> +    CloseHandle(overlapped->tap_semaphore);
> +    CloseHandle(overlapped->free_list_semaphore);
> +    CloseHandle(overlapped->output_queue_semaphore);
> +
> +    DeleteCriticalSection(&overlapped->free_list_cs);
> +    DeleteCriticalSection(&overlapped->output_queue_cs);
> +
> +    CloseHandle(overlapped->write_event);
> +    CloseHandle(overlapped->read_event);
> +
> +    CloseHandle(overlapped->handle);
> +
> +    g_free(overlapped);
> +}
> +
>   /********************************************/
>   
>    typedef struct TAPState {
> @@ -667,9 +690,8 @@ static void tap_cleanup(NetClientState *nc)
>   
>       qemu_del_wait_object(s->handle->tap_semaphore, NULL, NULL);
>   
> -    /* FIXME: need to kill thread and close file handle:
> -       tap_win32_close(s);
> -    */
> +    tap_win32_close(s->handle);

This patch is doing various things at once.

Could you implement tap_win32_close() in a preliminary patch,
then convert the tap_overlapped global to an allocation?

Thanks,

Phil.

> +    s->handle = NULL;
>   }
>   
>   static ssize_t tap_receive(NetClientState *nc, const uint8_t *buf, size_t size)
> 
> ---
> base-commit: ab8008b231e758e03c87c1c483c03afdd9c02e19
> change-id: 20250920-fix-win32-multiple-taps-ed16ccefbd17
> 
> Best regards,