From 155e3ab0c8a7886781755ba44849c77048815025 Mon Sep 17 00:00:00 2001 From: "Dr. David Alan Gilbert" Date: Wed, 15 Jan 2020 12:07:41 +0100 Subject: [PATCH 3/7] usbredir: Prevent recursion in usbredir_write MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit RH-Author: Dr. David Alan Gilbert 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 RH-Acked-by: Philippe Mathieu-Daudé RH-Acked-by: Peter Xu From: "Dr. David Alan Gilbert" 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 Message-Id: <20191218113012.13331-1-dgilbert@redhat.com> Signed-off-by: Gerd Hoffmann (cherry picked from commit 394642a8d3742c885e397d5bb5ee0ec40743cdc6) Signed-off-by: Miroslav Rezanina --- 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