From 8f6311159977b8ee4b78172caa411d3cee4d2ae5 Mon Sep 17 00:00:00 2001
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Date: Tue, 14 Jan 2020 20:23:30 +0000
Subject: [PATCH 4/5] usbredir: Prevent recursion in usbredir_write
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RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: <20200114202331.51831-2-dgilbert@redhat.com>
Patchwork-id: 93344
O-Subject: [RHEL-AV-8.2.0 qemu-kvm PATCH 1/2] usbredir: Prevent recursion in usbredir_write
Bugzilla: 1790844
RH-Acked-by: Peter Xu <peterx@redhat.com>
RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
RH-Acked-by: Gerd Hoffmann <kraxel@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: Danilo C. L. de Paula <ddepaula@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 e0f5ca6..97f2c3a 100644
--- a/hw/usb/redirect.c
+++ b/hw/usb/redirect.c
@@ -113,6 +113,7 @@ struct USBRedirDevice {
     /* Properties */
     CharBackend cs;
     bool enable_streams;
+    bool in_write;
     uint8_t debug;
     int32_t bootindex;
     char *filter_str;
@@ -290,6 +291,13 @@ 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) {
@@ -300,6 +308,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count)
             r = 0;
         }
     }
+    dev->in_write = false;
     return r;
 }
 
-- 
1.8.3.1