Re: [PATCH v2 0/3] futex: Create set_robust_list2

Florian Weimer
2024-11-04T12:37:00Z

* Peter Zijlstra:

> On Sat, Nov 02, 2024 at 10:58:42PM +0100, Florian Weimer wrote:
>
>> QEMU hints towards further problems (in linux-user/syscall.c):
>>
>> case TARGET_NR_set_robust_list:
>> case TARGET_NR_get_robust_list:
>> /* The ABI for supporting robust futexes has userspace pass
>> * the kernel a pointer to a linked list which is updated by
>> * userspace after the syscall; the list is walked by the kernel
>> * when the thread exits. Since the linked list in QEMU guest
>> * memory isn't a valid linked list for the host and we have
>> * no way to reliably intercept the thread-death event, we can't
>> * support these. Silently return ENOSYS so that guest userspace
>> * falls back to a non-robust futex implementation (which should
>> * be OK except in the corner case of the guest crashing while
>> * holding a mutex that is shared with another process via
>> * shared memory).
>> */
>> return -TARGET_ENOSYS;
>
> I don't think we can sanely fix that. Can't QEMU track the robust thing
> itself and use waitpid() to discover the thread is gone and fudge things
> from there?

There are race conditions with munmap, I think, and they probably get a
lot of worse if QEMU does that.

See Rich Felker's bug report:

| The corruption is performed by the kernel when it walks the robust
| list. The basic situation is the same as in PR #13690, except that
| here there's actually a potential write to the memory rather than just
| a read.
|
| The sequence of events leading to corruption goes like this:
|
| 1. Thread A unlocks the process-shared, robust mutex and is preempted
| after the mutex is removed from the robust list and atomically
| unlocked, but before it's removed from the list_op_pending field of
| the robust list header.
|
| 2. Thread B locks the mutex, and, knowing by program logic that it's
| the last user of the mutex, unlocks and unmaps it, allocates/maps
| something else that gets assigned the same address as the shared mutex
| mapping, and then exits.
|
| 3. The kernel destroys the process, which involves walking each
| thread's robust list and processing each thread's list_op_pending
| field of the robust list header. Since thread A has a list_op_pending
| pointing at the address previously occupied by the mutex, the kernel
| obliviously "unlocks the mutex" by writing a 0 to the address and
| futex-waking it. However, the kernel has instead overwritten part of
| whatever mapping thread A created. If this is private memory it
| (probably) doesn't matter since the process is ending anyway (but are
| there race conditions where this can be seen?). If this is shared
| memory or a shared file mapping, however, the kernel corrupts it.
|
| I suspect the race is difficult to hit since thread A has to get
| preempted at exactly the wrong time AND thread B has to do a fair
| amount of work without thread A getting scheduled again. So I'm not
| sure how much luck we'd have getting a test case.

<https://sourceware.org/bugzilla/show_bug.cgi?id=14485#c3>

We also have a silent unlocking failure because userspace does not know
about ROBUST_LIST_LIMIT:

Bug 19089 - Robust mutexes do not take ROBUST_LIST_LIMIT into account
<https://sourceware.org/bugzilla/show_bug.cgi?id=19089>

(I think we may have discussed this one before, and you may have
suggested to just hard-code 2048 in userspace because the constant is
not expected to change.)

So the in-mutex linked list has quite a few problems even outside of
emulation. 8-(

Thanks,
Florian