File main-loop.h-introduce-qemu_in_main_threa.patch of Package qemu.37352

From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 3 Mar 2022 10:15:46 -0500
Subject: main-loop.h: introduce qemu_in_main_thread() (bsc#1229007,
 CVE-2024-7409)

When invoked from the main loop, this function is the same
as qemu_mutex_iothread_locked, and returns true if the BQL is held.
When invoked from iothreads or tests, it returns true only
if the current AioContext is the Main Loop.

This essentially just extends qemu_mutex_iothread_locked to work
also in unit tests or other users like storage-daemon, that run
in the Main Loop but end up using the implementation in
stubs/iothread-lock.c.

Using qemu_mutex_iothread_locked in unit tests defaults to false
because they use the implementation in stubs/iothread-lock,
making all assertions added in next patches fail despite the
AioContext is still the main loop.

See the comment in the function header for more information.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220303151616.325444-2-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 6538692e2802666926e62d9309ddddda5ec9dc3b)
References: bsc#1229007
References: CVE-2024-7409
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
---
 include/qemu/main-loop.h    | 24 ++++++++++++++++++++++++
 softmmu/cpus.c              |  5 +++++
 stubs/iothread-lock-block.c |  8 ++++++++
 stubs/meson.build           |  3 +++
 4 files changed, 40 insertions(+)

diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index d6892fd2081a4774677fdb141e42..0775ead70710eb5655084fa639f8 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -260,9 +260,33 @@ int qemu_add_child_watch(pid_t pid);
  * must always be taken outside other locks.  This function helps
  * functions take different paths depending on whether the current
  * thread is running within the main loop mutex.
+ *
+ * This function should never be used in the block layer, because
+ * unit tests, block layer tools and qemu-storage-daemon do not
+ * have a BQL.
+ * Please instead refer to qemu_in_main_thread().
  */
 bool qemu_mutex_iothread_locked(void);
 
+/**
+ * qemu_in_main_thread: return whether it's possible to safely access
+ * the global state of the block layer.
+ *
+ * Global state of the block layer is not accessible from I/O threads
+ * or worker threads; only from threads that "own" the default
+ * AioContext that qemu_get_aio_context() returns.  For tests, block
+ * layer tools and qemu-storage-daemon there is a designated thread that
+ * runs the event loop for qemu_get_aio_context(), and that is the
+ * main thread.
+ *
+ * For emulators, however, any thread that holds the BQL can act
+ * as the block layer main thread; this will be any of the actual
+ * main thread, the vCPU threads or the RCU thread.
+ *
+ * For clarity, do not use this function outside the block layer.
+ */
+bool qemu_in_main_thread(void);
+
 /**
  * qemu_mutex_lock_iothread: Lock the main loop mutex.
  *
diff --git a/softmmu/cpus.c b/softmmu/cpus.c
index e46ac68ad096fd082c261588e397..9362bb128add52cf84417207a3b5 100644
--- a/softmmu/cpus.c
+++ b/softmmu/cpus.c
@@ -473,6 +473,11 @@ bool qemu_mutex_iothread_locked(void)
     return iothread_locked;
 }
 
+bool qemu_in_main_thread(void)
+{
+    return qemu_mutex_iothread_locked();
+}
+
 /*
  * The BQL is taken from so many places that it is worth profiling the
  * callers directly, instead of funneling them all through a single function.
diff --git a/stubs/iothread-lock-block.c b/stubs/iothread-lock-block.c
new file mode 100644
index 0000000000000000000000000000000000000000..c88ed70462514bf65ca98e9e749f387901f5cbc4
--- /dev/null
+++ b/stubs/iothread-lock-block.c
@@ -0,0 +1,8 @@
+#include "qemu/osdep.h"
+#include "qemu/main-loop.h"
+
+bool qemu_in_main_thread(void)
+{
+    return qemu_get_current_aio_context() == qemu_get_aio_context();
+}
+
diff --git a/stubs/meson.build b/stubs/meson.build
index 82b7ba60abe54bfe0e3069ac17c2..fa2e9658b79b651f7b3dbb115768 100644
--- a/stubs/meson.build
+++ b/stubs/meson.build
@@ -18,6 +18,9 @@ stub_ss.add(files('get-vm-name.c'))
 stub_ss.add(when: 'CONFIG_LINUX_IO_URING', if_true: files('io_uring.c'))
 stub_ss.add(files('iothread.c'))
 stub_ss.add(files('iothread-lock.c'))
+if have_block
+  stub_ss.add(files('iothread-lock-block.c'))
+endif
 stub_ss.add(files('isa-bus.c'))
 stub_ss.add(files('is-daemonized.c'))
 stub_ss.add(when: 'CONFIG_LINUX_AIO', if_true: files('linux-aio.c'))