On 2025/05/13 23:39, 'Peter Xu' via devel wrote:
On Sun, May 11, 2025 at 03:08:18PM +0900, Akihiko Odaki wrote:
futex(2) - Linux manual page
https://man7.org/linux/man-pages/man2/futex.2.html
Note that a wake-up can also be caused by common futex usage patterns
in unrelated code that happened to have previously used the futex
word's memory location (e.g., typical futex-based implementations of
Pthreads mutexes can cause this under some conditions). Therefore,
callers should always conservatively assume that a return value of 0
can mean a spurious wake-up, and use the futex word's value (i.e.,
the user-space synchronization scheme) to decide whether to continue
to block or not.
I'm just curious - do you know when this will happen?
AFAIU, QEMU uses futex always on private mappings, internally futex does
use (mm, HVA) tuple to index a futex, afaict. Hence, I don't see how it
can get spurious wakeups.. And _if_ it happens, since mm pointer can't
change it must mean the HVA of the futex word is reused, it sounds like an
UAF user bug to me instead.
I checked the man-pages git repo, this line was introduced in:
https://github.com/mkerrisk/man-pages/commit/4b35dc5dabcf356ce6dcb1f949f7b00e76c7587d
I also didn't see details yet in commit message on why that paragraph was
added.
And..
Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>
---
include/qemu/futex.h | 9 +++++++++
tests/unit/test-aio-multithread.c | 4 +++-
util/qemu-thread-posix.c | 28 ++++++++++++++++------------
3 files changed, 28 insertions(+), 13 deletions(-)
diff --git a/include/qemu/futex.h b/include/qemu/futex.h
index 91ae88966e12..f57774005330 100644
--- a/include/qemu/futex.h
+++ b/include/qemu/futex.h
@@ -24,6 +24,15 @@ static inline void qemu_futex_wake(void *f, int n)
qemu_futex(f, FUTEX_WAKE, n, NULL, NULL, 0);
}
+/*
+ * Note that a wake-up can also be caused by common futex usage patterns in
+ * unrelated code that happened to have previously used the futex word's
+ * memory location (e.g., typical futex-based implementations of Pthreads
+ * mutexes can cause this under some conditions). Therefore, callers should
.. another thing that was unclear to me is, here it's mentioning "typical
futex-based implementations of pthreads mutexes..", but here
qemu_futex_wait() is using raw futex without any pthread impl. Does it
also mean that this may not be applicable to whatever might cause a
spurious wakeup?
No. The man-page mentions "unrelated code that happened to have
previously used the futex word's memory location", so it doesn't matter
whether we use pthread here.
libpthread and even this QemuEvent follows the "common futex usage" so
we should do what is written in the man page.
Unfortunately the man page does not describe the "common futex usage
pattern". It looks like as follows:
Assume there are two threads, one atomic variable, and one futex.
Thread A does the following:
A1. Read the atomic variable.
A2. Go A5 if the atomic variable is zero.
A3. Wait using the futex.
A4. Go A1.
A5. Free the atomic variable and the futex.
Thread B does the following:
B1. Set the atomic variable to zero.
B2. Wake up using the futex.
In this example, the execution may happen in the following order:
B1 -> A1 -> A2 -> A5 -> B2
Here, B2 will cause a spurious wake up of QemuEvent if the freed memory
gets reused for QemuEvent.
Regards,
Akihiko Odaki
