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From 8f6311159977b8ee4b78172caa411d3cee4d2ae5 Mon Sep 17 00:00:00 2001
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From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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Date: Tue, 14 Jan 2020 20:23:30 +0000
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Subject: [PATCH 4/5] usbredir: Prevent recursion in usbredir_write
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
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Message-id: <20200114202331.51831-2-dgilbert@redhat.com>
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Patchwork-id: 93344
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O-Subject: [RHEL-AV-8.2.0 qemu-kvm PATCH 1/2] usbredir: Prevent recursion in usbredir_write
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Bugzilla: 1790844
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RH-Acked-by: Peter Xu <peterx@redhat.com>
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RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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RH-Acked-by: Gerd Hoffmann <kraxel@redhat.com>
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From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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I've got a case where usbredir_write manages to call back into itself
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via spice; this patch causes the recursion to fail (0 bytes) the write;
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this seems to avoid the deadlock I was previously seeing.
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I can't say I fully understand the interaction of usbredir and spice;
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but there are a few similar guards in spice and usbredir
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to catch other cases especially onces also related to spice_server_char_device_wakeup
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This case seems to be triggered by repeated migration+repeated
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reconnection of the viewer; but my debugging suggests the migration
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finished before this hits.
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The backtrace of the hang looks like:
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reds_handle_ticket
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reds_handle_other_links
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reds_channel_do_link
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red_channel_connect
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spicevmc_connect
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usbredir_create_parser
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usbredirparser_do_write
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usbredir_write
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qemu_chr_fe_write
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qemu_chr_write
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qemu_chr_write_buffer
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spice_chr_write
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spice_server_char_device_wakeup
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red_char_device_wakeup
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red_char_device_write_to_device
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vmc_write
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usbredirparser_do_write
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usbredir_write
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qemu_chr_fe_write
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qemu_chr_write
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qemu_chr_write_buffer
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qemu_mutex_lock_impl
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and we fail as we land through qemu_chr_write_buffer's lock
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twice.
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Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1752320
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Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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Message-Id: <20191218113012.13331-1-dgilbert@redhat.com>
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Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
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(cherry picked from commit 394642a8d3742c885e397d5bb5ee0ec40743cdc6)
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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hw/usb/redirect.c | 9 +++++++++
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1 file changed, 9 insertions(+)
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diff --git a/hw/usb/redirect.c b/hw/usb/redirect.c
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index e0f5ca6..97f2c3a 100644
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--- a/hw/usb/redirect.c
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+++ b/hw/usb/redirect.c
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@@ -113,6 +113,7 @@ struct USBRedirDevice {
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/* Properties */
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CharBackend cs;
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bool enable_streams;
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+ bool in_write;
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uint8_t debug;
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int32_t bootindex;
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char *filter_str;
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@@ -290,6 +291,13 @@ static int usbredir_write(void *priv, uint8_t *data, int count)
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return 0;
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}
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+ /* Recursion check */
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+ if (dev->in_write) {
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+ DPRINTF("usbredir_write recursion\n");
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+ return 0;
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+ }
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+ dev->in_write = true;
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+
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r = qemu_chr_fe_write(&dev->cs, data, count);
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if (r < count) {
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if (!dev->watch) {
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@@ -300,6 +308,7 @@ static int usbredir_write(void *priv, uint8_t *data, int count)
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r = 0;
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}
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}
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+ dev->in_write = false;
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return r;
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}
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--
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1.8.3.1
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