From c478f4c43c37968f556de7d310bb3e3d51d764e6 Mon Sep 17 00:00:00 2001 From: Chen Qun Date: Wed, 18 Dec 2019 11:30:12 +0000 Subject: [PATCH] usbredir: Prevent recursion in usbredir_write 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 lang 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 --- ...-Prevent-recursion-in-usbredir_write.patch | 90 +++++++++++++++++++ 1 file changed, 90 insertions(+) create mode 100644 usbredir-Prevent-recursion-in-usbredir_write.patch diff --git a/usbredir-Prevent-recursion-in-usbredir_write.patch b/usbredir-Prevent-recursion-in-usbredir_write.patch new file mode 100644 index 0000000..29eb50f --- /dev/null +++ b/usbredir-Prevent-recursion-in-usbredir_write.patch @@ -0,0 +1,90 @@ +From 30203c01fa1bb2a7b92575683f85695a2d420b38 Mon Sep 17 00:00:00 2001 +From: "Dr. David Alan Gilbert" +Date: Wed, 18 Dec 2019 11:30:12 +0000 +Subject: [PATCH] usbredir: Prevent recursion in usbredir_write + +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 lang 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 +--- + hw/usb/redirect.c | 9 +++++++++ + 1 file changed, 9 insertions(+) + +diff --git a/hw/usb/redirect.c b/hw/usb/redirect.c +index 9764a57987..3cf82589ed 100644 +--- a/hw/usb/redirect.c ++++ b/hw/usb/redirect.c +@@ -109,6 +109,7 @@ struct USBRedirDevice { + /* Properties */ + CharBackend cs; + bool enable_streams; ++ bool in_write; + uint8_t debug; + int32_t bootindex; + char *filter_str; +@@ -286,6 +287,13 @@ 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) { +@@ -296,6 +304,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count) + r = 0; + } + } ++ dev->in_write = false; + return r; + } + +-- +2.27.0 +