From 155e3ab0c8a7886781755ba44849c77048815025 Mon Sep 17 00:00:00 2001

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Date: Wed, 15 Jan 2020 12:07:41 +0100

Subject: [PATCH 3/7] usbredir: Prevent recursion in usbredir_write

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RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>

Message-id: <20200115120742.19583-2-dgilbert@redhat.com>

Patchwork-id: 93352

O-Subject: [RHEL-8.2.0 qemu-kvm PATCH 1/2] usbredir: Prevent recursion in usbredir_write

Bugzilla: 1752320

RH-Acked-by: Gerd Hoffmann <kraxel@redhat.com>

RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>

RH-Acked-by: Peter Xu <peterx@redhat.com>

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

I've got a case where usbredir_write manages to call back into itself

via spice; this patch causes the recursion to fail (0 bytes) the write;

this seems to avoid the deadlock I was previously seeing.

I can't say I fully understand the interaction of usbredir and spice;

but there are a few similar guards in spice and usbredir

to catch other cases especially onces also related to spice_server_char_device_wakeup

This case seems to be triggered by repeated migration+repeated

reconnection of the viewer; but my debugging suggests the migration

finished before this hits.

The backtrace of the hang looks like:

  reds_handle_ticket

  reds_handle_other_links

  reds_channel_do_link

  red_channel_connect

  spicevmc_connect

  usbredir_create_parser

  usbredirparser_do_write

  usbredir_write

  qemu_chr_fe_write

  qemu_chr_write

  qemu_chr_write_buffer

  spice_chr_write

  spice_server_char_device_wakeup

  red_char_device_wakeup

  red_char_device_write_to_device

  vmc_write

  usbredirparser_do_write

  usbredir_write

  qemu_chr_fe_write

  qemu_chr_write

  qemu_chr_write_buffer

  qemu_mutex_lock_impl

and we fail as we land through qemu_chr_write_buffer's lock

twice.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1752320

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Message-Id: <20191218113012.13331-1-dgilbert@redhat.com>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

(cherry picked from commit 394642a8d3742c885e397d5bb5ee0ec40743cdc6)

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 hw/usb/redirect.c | 9 +++++++++

 1 file changed, 9 insertions(+)

diff --git a/hw/usb/redirect.c b/hw/usb/redirect.c

index 65a9196..3dcf76e 100644

--- a/hw/usb/redirect.c

+++ b/hw/usb/redirect.c

@@ -107,6 +107,7 @@ struct USBRedirDevice {

     /* Properties */

     CharBackend cs;

     bool enable_streams;

+    bool in_write;

     uint8_t debug;

     int32_t bootindex;

     char *filter_str;

@@ -284,6 +285,13 @@ static int usbredir_write(void *priv, uint8_t *data, int count)

         return 0;

     }

+    /* Recursion check */

+    if (dev->in_write) {

+        DPRINTF("usbredir_write recursion\n");

+        return 0;

+    }

+    dev->in_write = true;

+

     r = qemu_chr_fe_write(&dev->cs, data, count);

     if (r < count) {

         if (!dev->watch) {

@@ -294,6 +302,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count)

             r = 0;

         }

     }

+    dev->in_write = false;

     return r;

 }

-- 

1.8.3.1